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Caldera formation on Santorini and the physiography of the islands in the Bronze Age
Abstract
The caldera of Santorini is a composite structure with a subsidence history extending over 100 ka or more. Geomorphological mapping shows that the present-day caldera wall is a complex assemblage of cliff surfaces of different ages, and that collapse at Santorini has repeatedly exhumed earlier caldera cliffs and unconformities. Cliffs bounding the southern, southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the Minoan eruption in the late Bronze Age. The well-scalloped shape of these cliffs is attributed to large-scale rotational landslip around the margins of the Minoan caldera. The deposit from one landslip is preserved subaerially. Minoan landslips in southeast santorini detached along the basement unconformity, exposing a cliff of the prevolcanic island. The caldera wall in the north, northeast and east preserves evidence for three generations of cliff: those of Minoan age and two earlier generations of caldera wall. The two early calderas can be dated relative to a well-established statigraphy of lavas and tuffs. The presence of in situ Minoan tephra plastered onto the present-day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the Minoan eruption. Field relationships permit reconstruction of the physiography of Bronze-Age Santorini immediately before the Minoan eruption. The reconstruction differs from some previously published versions and is believed to be the most accurate to date. Bronze-Age Sa ntorini had a large flooded caldera formed 21 ka ago. This caldera must have acted as an excellent harbour for the Bronze-Age inhabitants of the island. The 3.6 ka Minoan eruption deepened and widened the extant caldera. The volume of Minoan collapse (25 km3) is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of Minoan ignimbrite ponded as intracaldera tuff.
... Finally, as the tuff cone grew, the connection to the sea was sealed off and water evaporated. Large volumes of hot, fluidised pyroclastic flows ran over the island's slopes forming depositional fans (Phase 4), while caldera collapse during or after the eruption formed the Santorini archipelago's present-day topography 3,6,8 . The entrance of pyroclastic flows during Phases 3 and 4 into the sea may have triggered tsunamis that impacted coasts around the Aegean Sea 9,10 . ...
... While Minoan tephra fall deposits (Plinian and co-ignimbrite) have been deposited hundreds of kilometres from Santorini, pyroclastic flows extended only several kilometres from the shoreline and were deposited as thick ignimbrites onshore and in the proximal marine area 21 . Marine seismic reflection profiles reveal the Thera Pyroclastic Formation (TPF 22 ), which was formed by explosive volcanism on Santorini during the last 360 kyrs and has been studied in great detail onshore 5,8 . The Minoan deposits form the shallowest stratigraphic unit on Santorini and cover the Cape Riva eruption (22 ka) deposits in many areas on Santorini 5,8 , including Thera's northwestern cliff (Fig. 2a). ...
... The Santorini caldera has been shaped by at least four caldera-forming eruptions over the last 200,000 years 5 . Some cliffs surrounding the northern caldera basin predate the~1600 BCE eruption, as indicated by a sparse, local cover of Minoan deposits, while the southern and south-eastern cliffs are morphologically fresh and likely formed during the Minoan eruption 8 (Fig. 4a). This is in agreement with cosmic-ray exposure dating, which indicates that the northern caldera wall existed before the Minoan eruption 29 . ...
Despite their global societal importance, the volumes of large-scale volcanic eruptions remain poorly constrained. Here, we integrate seismic reflection and P-wave tomography datasets with computed tomography-derived sedimentological analyses to estimate the volume of the iconic Minoan eruption. Our results reveal a total dense-rock equivalent eruption volume of 34.5 ± 6.8 km³, which encompasses 21.4 ± 3.6 km³ of tephra fall deposits, 6.9 ± 2 km³ of ignimbrites, and 6.1 ± 1.2 km³ of intra-caldera deposits. 2.8 ± 1.5 km³ of the total material consists of lithics. These volume estimates are in agreement with an independent caldera collapse reconstruction (33.1 ± 1.2 km³). Our results show that the Plinian phase contributed most to the distal tephra fall, and that the pyroclastic flow volume is significantly smaller than previously assumed. This benchmark reconstruction demonstrates that complementary geophysical and sedimentological datasets are required for reliable eruption volume estimates, which are necessary for regional and global volcanic hazard assessments.
... The LBA paroxysmal eruption and consequent caldera subsidence destroyed Stronghyle island, leaving a flooded caldera with the ring-islands of Thera, Therasia, and Aspronisi as remnants. Studies of caldera formation (Druitt and Francaviglia 1992;Druitt 2014;Nomikou et al. 2016) suggest the present-day caldera is a complex structure formed by at least four collapses over 180 ky, the last of which was associated with the LBA eruption. ...
... The northern caldera wall was formed from the products of the last major Plinian eruption, the Cape Riva eruption, about 21,800 ± 400 years ago (Druitt 1985;Fabbro et al. 2013;Karatson et al. 2018). This eruption is thought to have collapsed the pre-existing Skaros-Therasia lava shield (Druitt and Francaviglia 1992). Evidence for this ancient caldera is: ...
... (2) LBA Plinian deposits plastered in situ on the present-day caldera wall at some locations, indicating those cliffs existed prior to the LBA eruption (Druitt and Francaviglia 1992; Druitt 2014); ...
The Late Bronze Age (LBA) eruption (ca. 1650 B.C.) of Thera (Santorini) was one of the largest known in history, burying and destroying a thriving Theran culture that occupied the island and surrounding islands. The consequent thick tephra deposit provides detailed information on the eruptive sequence and vent mechanics – it also provides details on that culture through burial of towns, farmsteads, and landscapes, the most prominent being the town of Akrotiri on the south coast of Thera. Here stratigraphic relationships between volcanic and archaeological deposits/constructions clearly indicate this eruption was signaled by seismic and minor eruptive events precursory to the main Plinian eruption. Because no casualties have yet been found beneath the tephra, inhabitants had advance notice of the impending diaster by a precursory eruption whose deposits are well preserved both within the archaeological site and in geological exposures throughout southern Thera island – and escaped by boat. Yet archaeological sites on nearby islands rarely record an influx of new arrivals at the time of the eruption. Accordingly, it is suggested here that those escaping were incinerated at sea by pyroclastic density currents (PDCs) that traversed across the ocean surface during the second phase of the catastrophic eruption.
... Based on isopachs (Bond and Sparks, 1976;Druitt et al., 1999;Cioni et al., 2000), the vent of both the precursory and the Plinian phases was to the south of a pre-existing caldera (see below), which was located in the northern part of the present-day caldera. The caldera was occupied by an intracaldera island similar to the Kameni islands, referred to as 'Pre-Kameni' (Eriksen et al., 1990;Druitt and Francaviglia, 1992;Karátson et al., 2018b). ...
... Few studies have focused so far on the volume of lithic clasts contained in the Minoan tuffs. Pyle (1990) suggested 5-7 km 3 in total, of which~3 km 3 represents the volume of Pre-Kameni (Druitt and Francaviglia, 1992). These figures have remained tentative in the subsequent literature without using a quantitative approach (e.g., Johnston et al., 2014). ...
... Friedrich et al. (1988) and Eriksen et al. (1990), later completed by Anadón et al. (2013), reported the presence of stromatolites and travertines in the Minoan tuffs (in the N part of Thíra and Thirasia) as direct evidence of a shallow, pre-existing flooded caldera. Druitt and Francaviglia (1992) pointed out patches of in situ Minoan pumice adhering to the modern NE caldera, which proved that those cliffs existed prior to the Minoan eruption. More recently, the formation of the northern walls of the present-day caldera was determined by 36 Cl exposure dating, verifying that the inward-looking cliffs of both Thirasia and N Thíra already existed in the Late Bronze Age (Athanassas et al., 2016). ...
One of the best known places on Earth where volcanology meets archaeology and history is the volcanic island of Santorini (Thíra), Greece. It is famous for the cataclysmic Late Bronze Age (Minoan) Plinian eruption which destroyed the Minoan culture that flourished on the island. Hosting a central, flooded caldera bay and, within that, the active islands of Palaea and Nea Kameni, Santorini volcano has been the focus of international research efforts for over one and a half centuries. In this paper, we summarize recent findings and related ideas about the Minoan physiography of the island, also known as Strongyli, from a volcanological, geomorphological and archaeological point of view. As proposed as early as the 1980s, a central caldera bay existed prior to the Late Bronze Age. Probably characterised by a smaller size and located in the northern part of the present-day caldera, this earlier caldera bay was formed during the previous Plinian eruption – called Cape Riva eruption – c. 22,000 years ago. Within the caldera bay, a central island, Pre-Kameni, existed, named after the present-day Kameni Islands. High-precision radioisotopic dating revealed that Pre-Kameni started to grow c. 20,000 years ago. Whereas volcanologists have accepted and refined the caldera concept, archaeologists have generally favoured the theory of an exploded central cone instead of a pre-existing central caldera. However, analysis of the Flotilla Fresco, one of the wall paintings found in the Bronze Age settlement of Akrotiri, reveals the interior of a Late Bronze Age caldera that may be interpreted as a realistic landscape. Approximately 3600 years ago, the island of Strongyli was destroyed during the explosive VEI = 7 Minoan eruption. Pre-Kameni was lost by this eruption, but its scattered fragments, together with other parts of Strongyli, can be recovered as lithic clasts from the Minoan tuffs. On the basis of photo-statistics and granulometry of the lithic clasts contained in the Minoan tuffs, complemented by volumetric assessment of the erupted tephra and digital elevation model (DEM) analysis of alternative models for the pre-eruptive topography, the volume of Pre-Kameni can be constrained between 1.6 and 3.0 km³, whereas the volume of the destroyed portion of the ring island of Strongyli between 9.1 and 17.1 km³. Of these, the larger values are considered more realistic, and imply that most of the destroyed part of Strongyli was incorporated as lithic components in the Minoan tuffs, whereas up to 3 km³ of Strongyli might have been downfaulted and sunken during caldera formation and is not accounted for in the lithics.
... Profitis Ilias. According to [59,60], the southern part of the caldera wall has scallop shapes attributed to rotational landslips occurring during caldera formation that were significant to enlarge it beyond the main collapse faults [25]. For the whole island, 271 debris flows were identified through aerial photointerpretation of the crown; the slide and the invasion areas were also mapped in each case. ...
... Conversely, the gentle areas and the carbonate slopes have respectively no displacements or little ones (0-2 cm). The ashfall distribution from the sub-Plinian eruption scenario in the Nea Kameni volcano is eastward oriented along the dominant winds, as shown in Figure 7 [59]. The contours display thickness values starting from a minimum of 5 cm to a maximum of 50 cm; the highest values are located close to the eruptive center, and do not reach Thira Island. ...
... Generally, the highest values are found in a central sector of the inner rim (MGU II), between Cape Alonaki and Athinios. The ashfall distribution from the sub-Plinian eruption scenario in the Nea Kameni volcano is eastward oriented along the dominant winds, as shown in Figure 7 [59]. The contours display thickness values starting from a minimum of 5 cm to a maximum of 50 cm; the highest values are located close to the eruptive center, and do not reach Thira Island. ...
Under the European FP7 SNOWBALL project (2014–2017), the island of Santorini was used as a case study to validate a procedure to assess the possible multiple cascading effects caused by volcanic eruptions. From January 2011 to April 2012, the area was affected by low to moderate (Mw <3.2) seismic shaking, which caused concern regarding a possible volcanic eruption that ultimately failed to materialize. Assuming the worst-case scenario of a sub-Plinian eruption, this study provides insights into the approach adopted by the SNOWBALL project to identify the most critical areas (hot spots) for slope stability. Geological field surveys, thematic maps, and geomorphological data on aerial photos and landform interpretation were adopted to assess the static susceptibility. The eruption scenario is related to two different phenomena: a pre-eruption earthquake (Mw 5.2) and the subsequent ash fallout deposition following the prevailing winds. Landslide susceptibility in seismic conditions was assessed through the HAZUS approach and the estimate of Newmark displacements (u), while the critical areas for ash fallout mobilization were assessed adopting empirical relationships. The findings are summarized in a scenario map reporting the most critical areas and the infrastructures most vulnerable to such phenomena.
... Two large shield volcanos, following an earlier -but perisherd-shield volcano in the north, (Huijsmans & Barton, 1988), existed later in the northerly, but still central regions of the archipelago, filling up most, if not all of the preexisting ancient caldera there, which had about the same size as the present caldera, (Huijsmans & Barton, 1988), (Druitt, et al., 1999), Table 4 The Cape Riva eruption, ca 18 000 years before the Minoan eruption, largely destroyed these double shield volcano´s edifice, but the location, size or shape of a resulting caldera(s) is unclear (Pichler & Kussmaul, 1972), (Heiken & McCoy, 1984), (Druitt T. H., 1985), (Druitt & Francaviglia, 1992), (Fabbro, Druitt, & Scaillet, 2013), (Athanassas & al., 2016), (Nomikou, et al., 2016), (Fabbro, Gareth, Druitt, & Costa, 2017), (Karátson, Gertisser, Telbisz, & al., 2018), Firm evidence for the existence of the conjectured intra-caldera lagoon came from stromatolithic clasts found in the Minoan debris and -via backpropagation of ballistic trajectories-a location in the north of to-day´s caldera is likely, (Friedrich, et al., 1988), (Eriksen, Friedrich, Buchardt, Tauber, & Thomsen, 1990), (Anadón, Canet, & Friedrich, 2013), Figure 4.3 e, Figure 4.5. ...
... The existence of a central island already in Minoan times and a sole opening of the intra-caldera lagoon towards the south-west had been conjectured tentatively, (Druitt & Francaviglia, 1992) , Figures 4.3 e, f. ...
... For the quantitative analysis, this fact has been taken into account as possible, but for the views within the 3D model, it is not really significant. We have however covered the outer island coastal regions, which have been significantly altered in places by direct deposit layers or by re-worked debris, in order to reflect the coast line as it had been in Minoan times, following (Heiken & McCoy, 1984), (Druitt & Francaviglia, 1992), (Heiken, McCoy, & Sheridan, 1988), (Aston & Hardy, 1990), (Doumas C. G., 2017), with the exception of the sections of today´s water straits, which are treated differently in this essay´s model. ...
Localization hypothesis based upon quantitative perspective analysis of landscapes in Bronze Age fresco scenes, which had been discovered in Akrotiri, Santorini, Greece. Creation of a 3D virtual model of the localized landscape and topography of the island of Santorini/Thera 3600 years ago.
... Chemical analysis of the stromatolite lithics show that this caldera was a semi-restricted marine bay with no more than a few meters of water depth (Anadón et al., 2013;Friedrich et al., 1988). Subsequent intra-caldera effusive activity constructed the 2.2-2.5 km 3 Pre-Kameni island inferred from black, glassy andesite lavas found throughout the LBA deposits, but not in the present Santorini edifice (Druitt and Francaviglia, 1992;Karátson et al., 2018). (Hooft et al., 2017) showing the outcrops of pre-existing basement (yellow) including at the summit of Profitis Ilias and the post-caldera volcanism ( Current models for silicic caldera-forming systems involve trans-crustal magmatic systems that evolve significantly over time with rapid final stages of amalgamation preceding the Plinian caldera-forming eruption (Cashman et al., 2017). ...
... The first phase was a Plinian pumice fall indicative of a subaerial eruption. Isopachs of the pumice deposit and the size-distribution of ejected lithics locate the vent on the Pre-Kameni island 1-2 km west of the modern town of Thira (Bond and Sparks, 1976;Druitt and Francaviglia, 1992;Heiken and McCoy, 1984). The second phase is composed of stratified phreatomagmatic base-surges and Plinian deposits showing that variable magma-water interactions occurred at this time (Bond and Sparks, 1976). ...
... ignimbrite fans distributed around the caldera (Bond and Sparks, 1976;Druitt, 2014;Heiken and McCoy, 1984) and a diverse lithology of ejected rock debris (Druitt and Francaviglia, 1992) indicate that the entire topographic caldera subsided during phase 4 and we observe high velocities beneath the remainder of the caldera floor (Fig. 3a-c). We argue that this last observation implies that the formation of the topographic caldera occurred by coherent down-drop of the larger caldera during the last phase of the LBA eruption (Fig. 6c). ...
... Hildreth, 1981;Lipman et al., 1970;Sparks et al., 1985). These caldera-forming events can occur multiple times at the same volcano but typically happen infrequently, separated by tens to hundreds of thousands of years (Bevilacqua et al., 2018;Christiansen, 2001;Druitt & Francaviglia, 1992;Kaneko et al., 2007;Orsi et al., 1996;Wilson et al., 1995). Following large caldera-forming events, eruptions tend to be smaller volume and higher frequency, indicating a significant reduction in chamber size (Degruyter et al., 2016;Forni et al., 2018;Parks et al., 2012;Singer et al., 2014). ...
... Santorini (Thera) is a large volcanic complex within the Hellenic subduction zone, where explosive volcanic activity began at least~360 ka (Druitt et al., 1989). Multiple, nested calderas and well-exposed volcanic stratigraphy indicate that the volcano cycles between large, explosive Plinian-style eruptions and smaller, effusive shield-building eruptions (Druitt & Francaviglia, 1992). The oldest caldera dates back to~180 ka, followed by the Skaros caldera at~70 ka, the Cape Riva caldera et~21 ka, and the caldera associated with the Late-Bronze-Age (Minoan) eruption~3,600 years ago that ejected~40-80 km 3 DRE and devastated Minoan settlements on the island (Druitt & Francaviglia, 1992;Johnston et al., 2014;Sigurdsson et al., 2011). ...
... Multiple, nested calderas and well-exposed volcanic stratigraphy indicate that the volcano cycles between large, explosive Plinian-style eruptions and smaller, effusive shield-building eruptions (Druitt & Francaviglia, 1992). The oldest caldera dates back to~180 ka, followed by the Skaros caldera at~70 ka, the Cape Riva caldera et~21 ka, and the caldera associated with the Late-Bronze-Age (Minoan) eruption~3,600 years ago that ejected~40-80 km 3 DRE and devastated Minoan settlements on the island (Druitt & Francaviglia, 1992;Johnston et al., 2014;Sigurdsson et al., 2011). Since the Minoan eruption, at least 11 eruptions have occurred, giving an average eruption frequency of~3 kyr −1 (Pyle & Elliott, 2006). ...
Crustal magma chambers can grow to be hundreds to thousands of cubic kilometers, potentially feeding catastrophic caldera-forming eruptions. Smaller volume chambers are expected to erupt frequently and freeze quickly; a major outstanding question is how magma chambers ever grow to the sizes required to sustain the largest eruptions on Earth. We use a thermo-mechanical model to investigate the primary factors that govern the extrusive:intrusive ratio in a chamber, and how this relates to eruption frequency, eruption size, and long-term chamber growth. The model consists of three fundamental timescales: the magma injection timescale τ in , the cooling timescale τ cool , and the timescale for viscous relaxation of the crust τ relax . We estimate these timescales using geologic and geophysical data from four volcanoes (Laguna del Maule, Campi Flegrei, Santorini, and Aso) to compare them with the model. In each of these systems, τ in is much shorter than τ cool and slightly shorter than τ relax , conditions that in the model are associated with efficient chamber growth and simultaneous eruption. In addition, the model suggests that the magma chambers underlying these volcanoes are growing at rates between ~10 ⁻⁴ and 10 ⁻² km ³ /year, speeding up over time as the chamber volume increases. We find scaling relationships for eruption frequency and size that suggest that as chambers grow and volatiles exsolve, eruption frequency decreases but eruption size increases. These scaling relationships provide a good match to the eruptive history from the natural systems, suggesting that the relationships can be used to constrain chamber growth rates and volatile saturation state from the eruptive history alone.
... On the other hand, the presence of abundant lithics, especially clasts of a chemically distinctive black glassy andesite, in the pyroclastic deposits of the Minoan eruption (see below) suggested that the central part of the caldera bay was occupied by an intracaldera island 10,[14][15][16] . We use the term 'Pre-Kameni' 14,17 for this pre-Minoan edifice, which may have been similar to the present-day, post-Minoan islands of Palaea and Nea Kameni, but completely destroyed during the Minoan eruption. ...
... In this paper, by applying a photo-statistical analysis of outcrops complemented by granulometric analysis, we present quantitative results for the volume of the lithic clasts included in the Minoan deposits and, after proportioning their amount, determine the dimensions of the destroyed Pre-Kameni island. The timing of island growth is constrained using high-precision K-Ar dating of the black glassy andesite, which is regarded as the most significant island-forming lithic type 9,10,15 . The topography of Santorini just before the Minoan eruption is also shown by integrating previous and our own results using a high-resolution digitial elevation model (DEM) of Santorini 19 . ...
... A synthesis of published dates for the Cape Riva eruption yields a mean age of 21.8 ± 0.4 ka 20 . This eruption is thought to have collapsed the pre-existing Skaros-Therasia lava shield 13,15 . For the subsequent interplinian period which lasted for ~18 ky only minor explosive activity is documented [20][21][22] . ...
During the Late Bronze Age, the island of Santorini had a semi-closed caldera harbour inherited from the 22 ka Cape Riva Plinian eruption, and a central island referred to as ‘Pre-Kameni’ after the present-day Kameni Islands. Here, the size and age of the intracaldera island prior to the Late Bronze Age (Minoan) eruption are constrained using a photo-statistical method, complemented by granulometry and high-precision K-Ar dating. Furthermore, the topography of Late Bronze Age Santorini is reconstructed by creating a new digital elevation model (DEM). Pre-Kameni and other parts of Santorini were destroyed during the 3.6 ka Minoan eruption, and their fragments were incorporated as lithic clasts in the Minoan pyroclastic deposits. Photo-statistical analysis and granulometry of these lithics, differentiated by lithology, constrain the volume of Pre-Kameni to 2.2–2.5 km3. Applying the Cassignol-Gillot K-Ar dating technique to the most characteristic black glassy andesite lithics, we propose that the island started to grow at 20.2 ± 1.0 ka soon after the Cape Riva eruption. This implies a minimum long-term lava extrusion rate of ~0.13–0.14 km3/ky during the growth of Pre-Kameni.
... Santorini is a stratigraphically and chemically complex stratovolcano, made up of many hundreds to thousands of distinct lava and ignimbrite successions (Druitt et al., 1999). Several of the ignimbrites are associated with catastrophic caldera formation (Druitt and Francaviglia, 1992;Druitt and Sparks, 1982;Roche and Druitt, 2001). There have been four well documented caldera forming events in the geologic record (Druitt et al., 1999). ...
... An interesting study would be to compare caldera fault damage with data from well-studied fault systems (Gudmundsson and Brenner, 2003;Mitchell and Faulkner, 2012). At present there is no direct quantification of caldera fault damage zones, the only studies that hypothesize the occurrence of such mechanical situations rely on interpretation of numerical or dynamic wave velocity models (e.g Browning and Gudmundsson, many hundreds or thousands of years following their formation, as shown by repeated seismic unrest associated with the Bardarbunga caldera faults (Fichtner and Tkalčić, 2010;Gudmundsson et al., 2014;Konstantinou et al., 2003;Sigmundsson et al., 2014), and Santorini caldera faults (Druitt and Francaviglia, 1992;Konstantinou et al., 2013) making fault zone evolution is likely an important consideration. ...
Shallow magma chambers influence a range of crustal processes at active volcanoes.For example many and perhaps most dyke fed eruptions originate from shallow magma storage regions. Magma chamber failure, resulting in the initiation of caldera faults or magma filled fractures (dyke) is likely governed by a complex interplay between regional and local mechanical stresses and thermal effects. This study utilises a multitude of techniques to decipher salient thermo-mechanical processes occurring as a result of magma stored at shallow (<10 km) depth. A new model to forecast magma chamber rupture and dyke initiation is proposed. The analytical solutions presented are applied to field data from Santorini, Greece, and combine poro-elastic material constraints with geodetic data to estimate both magma volumes stored beneath the caldera and internal excess pressure generated during periods of magmatic recharge. Predicting the path or propagation of magma once it has left a shallow magma chamber is an important but so far unachievable goal in volcanology. Caldera faults offer pathways for magma and often develop ring-dykes. A previously unreported mechanism for the formation of ring-dykes is the capture of inclined sheets at caldera fault boundaries. Geological field observations of an exceptionally well-exposed ring-fault at Hafnarfjall in Western Iceland were used as input to the finite element method numerical modelling software COMSOL multiphysics to infer a mechanism of principal stress rotation within a fault damage zone. The same modelling technique was then used to estimate the far-field crustal displacements resulting from the failure and collapse of a shallow magma chamber roof. This study is framed within the context of the 2014-15 Bardarbunga-Holuhraun (Iceland) dyke injection and eruptive episode, and hypothesises that significant ice subsidence was not solely associated with crustal subsidence but instead related to ice flow within Bardarbunga caldera generated by the dyke emplacement. Thermal stresses resulting from hot magma emplacement and gradual cooling likely combine to weaken volcanic edifices. For example, field evidence suggests many normal faults nucleate from cooling joints. A suite of thermal stressing experiments finds that cooling and contraction produces larger and more abundant micro-cracks when compared with heating expansion. This is an important result when considering that almost all previous studies concerned with thermal stressing focused on the heating cycle.
... The Santorini volcanic center consists of the islands Thera, Thirasia and Aspronisi that are arranged as a dissected ring around a flooded caldera. The caldera, a 11 × 7 km composite structure, resulted from at least four collapse events over the last 200 kyrs and contains in its center the post-caldera volcanic islands Palea Kameni and Nea Kameni (Druitt & Francaviglia, 1992;Pyle & Elliott, 2006;Nomikou et al., 2014). The oldest dated volcanic rocks are early Quaternary submarine tuffs near Akrotiri in SW Thera, which are speculated to originate from vents at Akrotiri and Christiana (Francalanci et al., 2005). ...
... The pyroclastic sequence emplaced on Santorini during the past ∼360 kyrs contains 13 tephras from major, dominantly silicic and intermediate eruptions (Figure 2), including at least four caldera-forming events (Lower Pumice 2, Upper Scoriae 1, Cape Riva, Minoan; Druitt et al., 1989Druitt et al., , 1999Druitt & Francaviglia, 1992). All Santorini tephras have more or less similar mineral assemblages and minor to moderate crystal contents (5%-20%) consisting of dominant plagioclase and subordinate clino-and orthopyroxene, Fe-Ti oxides and ± pyrrhotite and apatite; some dacitic and rhyodacitic tephras also have rare amphibole whereas andesites also may include olivine (Druitt et al., 1999;Vespa et al., 2006). ...
The Milos, Christiana‐Santorini‐Kolumbo (CSK) and Kos‐Yali‐Nisyros (KYN) volcanic complexes of the Aegean Volcanic Arc have repeatedly produced highly explosive eruptions from at least ∼360 ka into historic times and still show recent unrest. We present the marine tephra record from an array of 50, up to 7.4 m long, sediment cores along the arc collected in 2017 during RV Poseidon cruise POS513, which complements earlier work on distal to ultra‐distal eastern Mediterranean sediment cores. A unique set of glass‐shard trace element (LA‐ICPMS) compositions complements our major element (EMP) data on 220 primary ash layers and 40 terrestrial samples to support geochemical fingerprinting for correlations with 19 known tephras from all three volcanic complexes and with the 39 ka Campanian Ignimbrite from the Campi Flegrei, Italy. The correlations include 11 eruptions from CSK (Kameni, Kolumbo 1650, Minoan, Cape Riva, Cape Tripiti, Upper Scoriae 1 and 2, Middle Pumice, Cape Thera, Lower Pumice, Cape Therma 3). We identify a previously unknown widespread tephra from a plinian eruption on Milos (Firiplaka Tephra). Near the KYN we correlate marine tephras with the Kos Plateau Tuff, the Yali 1 and Yali 2 tephras, and the Upper and Lower Pumice on Nisyros. Between these two major tephras, we found two tephras from Nisyros not yet observed on land. The four Nisyros tephras form a systematic trend toward more evolved magma compositions. In the companion paper we use the tephrostratigraphic framework established here to constrain new eruption ages and magnitudes as a contribution to volcanic hazard assessment.
... The present paper aims to showcase nine virtual geosites (VGs) belonging to the Santorini volcanic complex, representing a stunning volcanotectonic environment, the result of multiple caldera collapses associated with major explosive activity [24], using both VR applications and web-based GIS technologies. Web-based GIS platforms have succeeded in enhancing data access and dissemination, spatial data exploration, and visualization capabilities and provide additional options for processing, analyzing, and modeling available datasets [25]. ...
... In 2011-2012, the latter experienced unrest, not followed by an eruption [46,47]. The Santorini complex has been active since the Quaternary and formed by numerous pyroclastic and effusive eruptions (Figure 1b), dike-fed volcanism [43], and multiple caldera collapse events [24,48]. This activity generated a plethora of onshore and offshore volcanotectonic morphological structures [49][50][51][52], such as a large caldera, volcanic craters, cinder cones, levees, lava domes, and a local dike swarm in the northern part of the island (Figure 1a) [53]. ...
We document and show a state-of-the-art methodology that could allow geoheritage sites (geosites) to become accessible to scientific and non-scientific audiences through immersive and non-immersive virtual reality applications. This is achieved through a dedicated WebGIS platform, particularly handy in communicating geoscience during the COVID-19 era. For this application, we selected nine volcanic outcrops in Santorini, Greece. The latter are mainly associated with several geological processes (e.g., dyking, explosive, and effusive eruptions). In particular, they have been associated with the famous Late Bronze Age (LBA) eruption, which made them ideal for geoheritage popularization objectives since they combine scientific and educational purposes with geotourism applications. Initially, we transformed these stunning volcanological outcrops into geospatial models—the so called virtual outcrops (VOs) here defined as virtual geosites (VGs)—through UAV-based photogrammetry and 3D modeling. In the next step, we uploaded them on an online platform that is fully accessible for Earth science teaching and communication. The nine VGs are currently accessible on a PC, a smartphone, or a tablet. Each one includes a detailed description and plenty of annotations available for the viewers during 3D exploration. We hope this work will be regarded as a forward model application for Earth sciences' popularization and make geoheritage open to the scientific community and the lay public.
... In 1650 BCE Santorini, Thera island, Greece, erupted ∼ 30 km 3 of volcanic material ejecting ash over the eastern Mediterranean Sea and in Turkey. The eruption caused the collapse of a caldera that generated a tsunami which destroyed Minoan cities in Crete (Druitt and Francaviglia, 1992 (Vitaliano, 1971;Naddaf , 1994). This is one of the world's most famous historic eruptions and is still a great source of inspiration in our modern society (e.g., Sigurdsson and Lopes, 2015) and thus still vibrates to this day (Figure 1.5). ...
... Finally, in this last section of the thesis, I wish to emphasise the contribution of the Nisga'a of some large historical eruptions were conducted using written and archaeological sources (e.g., Stothers and Rampino, 1983;Druitt and Francaviglia, 1992;Branca and Del Carlo, 2004;Chester et al., 2008). Over the past ∼ 15 years, similar studies were conducted in places with strong Indigenous cultures such as in the Pacific Islands or in Hawaii (e.g . ...
Monogenetic volcanoes are the most common volcanic landforms on Earth and usually form isolated small-volume volcanic centres with a wide range of eruptive styles and products. Here, I focus on the case of Tseax volcano (Wil Ksi Baxhl Mihl) in north-western British Columbia, Canada's deadliest volcanic eruption; its ~ 1700 CE eruption killed up to 2,000 people of the Nisga'a First Nation. Tseax is composed by two imbricated volcanic edifices (an outer breached spatter rampart and an inner 70 m high tephra cone) and 4 far-travelled, valley-filling lava flows (2 pahoehoe and 2 'a'a) for a total volume of 0.5 km³ submerging the former Nisga'a villages. All the erupted products are Fe-, Ti-rich, basanite-to-trachybasalts and their geochemical homogeneity suggests the eruption of a single magma batch that was produced by low partial melting of a cpx-poor wehrlite at 52 - 66 km depth. The magma was stalled for > 10³ days in the upper crust and cooled down to 1094 - 1087 °C prior to eruption. The eruption lasted between 1 to 4 months and was divided in two main periods. The first period occurred in a typical Hawaiian-style with lava fountaining, spatter activity and the eruption of long pahoehoe flows. Almost half of the total lava volume was erupted in the first days of the eruption with fluxes > 800 m³/s. The lava may have engulfed the Nisga'a villages in a few tens of hours and thus be one of the cause for the fatalities. A "vog" produced when the lava entered the Nass River may have been also responsible for the Nisga'a deaths. The second period of activity was characterized by low intensity Strombolian explosions with the building of the tephra cone and eruption of the shorter 'a'a lava flows.In high speed channelised lava flows, standing waves are often interpreted as hydraulic jumps, indicating supercritical conditions. Using open channel hydraulic theory for supercritical flows, the geometry of the standing waves to constrain eruption flux and viscosity. I propose that investigating standing waves during ongoing eruption is a powerful tool to help for lava flow modelling and hazard mitigation.
... In 1650 BCE Santorini, Thera island, Greece, erupted ∼ 30 km 3 of volcanic material ejecting ash over the eastern Mediterranean Sea and in Turkey. The eruption caused the collapse of a caldera that generated a tsunami which destroyed Minoan cities in Crete (Druitt and Francaviglia, 1992 (Vitaliano, 1971;Naddaf , 1994). This is one of the world's most famous historic eruptions and is still a great source of inspiration in our modern society (e.g., Sigurdsson and Lopes, 2015) and thus still vibrates to this day ( Figure 1.5). ...
... I propose to implement our model in MatLab or Python (Figure 7.9). This could serve as a powerful tool for the volcanology community for rapid real time monitoring of channel evolution and supply rate as well as providing key parameters for lava flow modelling and hazard assessment. of some large historical eruptions were conducted using written and archaeological sources (e.g., Stothers and Rampino, 1983;Druitt and Francaviglia, 1992;Branca and Del Carlo, 2004;Chester et al., 2008). Over the past ∼ 15 years, similar studies were conducted in places with strong Indigenous cultures such as in the Pacific Islands or in Hawaii (e.g. ...
Monogenetic volcanoes are the most common volcanic landforms on Earth and usually form isolated small-volume volcanic centres with a wide range of eruptive styles and products. Here, I focus on the case of Tseax volcano (Wil Ksi Baxhl Mihl) in north-western British Columbia, Canada’s deadliest volcanic eruption; its 1700 CE eruption killed up to 2,000 people of the Nisga’a First Nation. Tseax is composed by two imbricated volcanic edifices (an outer breached spatter rampart and an inner 70 m high tephra cone) and 4 far-travelled, valley-filling lava flows (2 pahoehoe and 2 ‘a‘a) for a total volume of 0.5 km3 submerging the former Nisga’a villages. All the erupted products are Fe-, Ti-rich, basanite-to-trachybasalts and their geochemical homogeneity suggests the eruption of a single magma batch that was produced by low partial melting of a cpx-poor wehrlite at 52 - 66 km depth. The magma was stalled for > 1000 days in the upper crust and cooled down to 1094 - 1087 degree Celsius prior to eruption. The eruption lasted between 1 to 4 months and was divided in two main periods. The first period occurred in a typical Hawaiian-style with lava fountaining, spatter activity and the eruption of long pahoehoe flows. Almost half of the total lava volume was erupted in the first days of the eruption with fluxes > 800 m3/s. The lava may have engulfed the Nisga’a villages in a few tens of hours and thus be one of the cause for the fatalities. A ‘vog’ produced when the lava entered the Nass River may have been also responsible for the Nisga’a deaths. The second period of activity was characterized by low intensity Strombolian explosions with the building of the tephra cone and eruption of the shorter ‘a‘a lava flows.
In high speed channelised lava flows, standing waves are often interpreted as hydraulic jumps, indicating supercritical conditions. Using open channel hydraulic theory for supercritical flows, the geometry of the standing waves to constrain eruption flux and viscosity. I propose that investigating standing waves during ongoing eruption is a powerful tool to help for lava flow modelling and hazard mitigation.
... Aspronisi (Aston and Hardy 1990;Friedrich, Seidenkrantz and Nielsen 2000, 74, fig. 3;Druitt and Francaviglia 1992;Vougioukalakis 2006;2015, 24-6;Nomikou et al. 2016, 5, fig. 6a;Karátson et al. 2018, fig. ...
... Heiken and McCoy 1984;Eriksen et al. 1990;Druitt and Francaviglia 1992;Friedrich, Seidenkrantz and Nielsen 2000;Vougioukalakis 2006;Karátson et al. 2018, fig. 1for a summary of the evolving models of preand post-eruption caldera morphology. ...
The study of the history of the first excavations on prehistoric Therasia in the nineteenth century, which were carried out in the context of contemporary scientific interest in the volcanic eruptions of Santorini, has led to the systematic archaeological investigation of the island from 2007 onwards. The intensive archaeological surface survey, the geological survey of the geological structure and palaeotopography of Therasia, and geophysical investigations, undertaken in conjunction with the ongoing excavation of the prehistoric settlement at the site of Panaghia Koimisis at the southern end of modern Therasia, have created the conditions for a more comprehensive approach to the archaeological landscape of the island. Based on the results from the excavation trenches in the south and south-east terraces of the Koimisis hill, which have been excavated down to the virgin soil, we present findings on the organisation, architecture and habitation phases of the Koimisis settlement. The site emerges as an important settlement located on the imposing hilltop rising on the west side of the pre-eruption Santorini caldera in the Early Bronze Age, with a long period of habitation to the end of the Middle Cycladic period, when it was definitively abandoned. The excavation of the settlement provides new information on its architecture and spatial organisation during the Early and Middle Bronze Age, completing the picture from Akrotiri, whose early phases are preserved in a piecemeal fashion under the buildings of the Late Cycladic town.
... The episodic introduction of mafic melt diapirs would have provided the excess heat and pressure at the shallow crustal depths that ultimately led to this cataclysmic eruption. In addition, changes observed in chemistry and lithic compositions of both lithic blocks and ignimbrites imply that the volcanic point source of the initial eruption shifted Northward towards the caldera rim post the Plinian phases of the eruption (Druitt et al., 1992). This could help to explain the collapse or the Northern passage visible today, along with the excessive lavas found in that area by ocean bathymetric surveys (Nomikou et al., 2016;Perisorratis et al., 1990). ...
... SiO2 content and a few (<10%) scoria clasts of a more andesitic/dacitic composition, lithic fragments and ash (Sparks et al., 1977;Druitt et al., 1999). Although the pumice is highly vesicular, the eruption was driven by a strong magmatic gas mass fraction with little to no sign of phreatomagmatic interaction (Sparks et al., 1990), indicating the volcanic edifice was subaerial prior to the main Minoan CFE collapse (Druitt et al., 1992). ...
More than 100 kilometres north of Crete lies the volcanic island group Thera (Santorini). In the winter of 1629/28 BC a catastrophic eruption, better known as the Minoan Event, obliterated the island and destroyed much of the Aegean civilisation, leaving behind the 3-part caldera rim visible above the ocean today. Bathymetric studies have found evidence that this has occurred at least 4 more times in Thera’s volcanic history, leading to a complex composite caldera structure with new intra-caldera edifices growing from within its centre, the most recent being the Kameni Islands that are active volcanic cones today. The highly silicic eruption itself took place in four distinct phases, during which the magmatic source shifted towards the North of the island. During the first phase, a large Plinian eruption destroyed much of the island. This allowed ocean water to percolate into the crater, leading to a violent phreatomagmatic phase. Finally, the shallow magma chamber collapsed leading to the catastrophic caldera forming eruption (CFE) that caused shockwaves and tsunamis that wiped out much of the Minoan and Cyclades cultures on neighbouring islands, leaving behind the caldera rim visible today.
... One of the largest volcanic eruptions during Holocene was the eruption of the Santorini (Thera) volcano during the Late Bronze Age (LBA). The LBA Santorini eruption, dated by AMS-radiocarbon dating to 1621-1605 cal B.C. (Friedrich and Heinemeier, 2009;Friedrich, 2013) is associated with a supraregional tsunami that affected the eastern Mediterranean (Hammer et al., 1987;Druitt and Francaviglia, 1992;McCoy and Heiken, 2000). LBA Santorini tsunamites have already been detected at several sites along the coasts of the Aegean Sea and the eastern Mediterranean. ...
... One of the largest volcanic eruptions during the Holocene was the caldera-forming eruption of Santorini during the Late Bronze Age (LBA) (Hammer et al., 1987;Druitt and Francaviglia, 1992;McCoy and Heiken, 2000). Similar to the eruption of Krakatau and Tambora, the LBA Santorini eruption is supposed to have generated strong earthquakes and a catastrophic tsunami that affected not only the surrounding islands but also at least the northeastern coast of Crete (Bruins et al., 2008). ...
The Late Bronze Age (LBA) tsunami and the A.D. 365 tsunami are supposed to have affected the northern coasts of Crete. However, near-coast sedimentary archives have been rarely investigated in this area, and sedimentary archives including palaeotsunami fingerprints are still unknown. The main objective of our research was to search for appropriate tsunami sediment traps in order to gain detailed insights into the Holocene palaeotsunami history of northern Crete. We found an excellent fine sediment archive near Pirgos, located to the west of Rethymnon. Based on a multi-electrode geoelectrical survey and an 11-m-deep sediment core, we analysed the event-geochronostratigraphical record by means of sedimentological, geochemical, geochronological, geomorphological, and microfossil investigations. The Pirgos record revealed a thick unit of homogeneous mud of a lagoonal environment starting ~6000 years ago. The lagoon was affected by five high-energy events, leaving layers of allochthonous sand. Microfossil analyses of these layers revealed a mixed foraminiferal assemblage including species from brackish habitats but also displaced and transported species from open marine and deep-water environments. Considering sedimentary characteristics, the local wave climate of the Cretan Sea, and the overall geomorphological setting, we interpret these layers as related to extreme wave events (EWE). Based on a local radiocarbon age-depth-model, we identified one EWE layer as a reliable candidate for the LBA Santorini tsunami. Another EWE layer is most probably associated with the A.D. 365 tsunami. This EWE ended abruptly the lagoonal conditions at Pirgos because of to the 1.64 m coseismic uplift at the site. The Pirgos lagoon existed between the first half of the 6th mill. B.C. and (at least) the end of the 2nd mill. B.C. We found that the area around Pirgos has continuously subsided over 3000 or so years, reflecting constant tectonogeomorphological conditions dominated by the nearby subduction zone between the Aegean and African plates.
... The seismic profile shown in Fig. 3c strikes parallel to the caldera wall in the southern basin (Fig. 3c). The locations of distinct ridges of the acoustic basement in the seismic data ( Fig. 3c) correspond to the edges of morphologically fresh landslide scars in the caldera walls of the southern basin (Fig. 3f), formed by rotational landslides during or soon after the Minoan eruption 23 . In the northern basin, the acoustic basement forms a deep subcircular depression (Figs. ...
Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Kameni’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km³ of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas.
... Santorini caldera is a complex 11 km × 7 km structure caused by at least four collapse events over the last ~200,000 y, the last of which was the LBA eruption (Druitt and Francaviglia, 1992). It has a northern basin 390 m deep, a southern basin 280 m deep, and is connected to the sea via three breaches . ...
... Following the LBA, volcanic activity in Santorini continued but with diminishing intensity, characterized by several less intense eruptions. Over the past 600,000 years, the region has witnessed at least seven significant explosive eruptions [11,17,19,20]. To this day, Santorini remains the most active volcanic field in the HVA [21], with the most recent eruption occurring early in 1950. ...
The presence of active hydrothermal vent fields near residential areas and their possible link to volcanic activity poses a potential hazard to the environment, society, and the economy. By capitalizing on Autonomous Underwater Vehicle sampling methodologies and applying the Generalized Moments Method model for geological and physical processes in these environments, we shed light on the underlying dynamics shaping the physicochemical characteristics of the vents. In this study, we focus on the Northern Caldera of Santorini and, more specifically, on the recorded CTD data (Conductivity, Temperature, Depth). The data sets were collected in 2017 in Santorini using an Autonomous Underwater Vehicle during the GEOMAR POS510 mission. Our research shows that the active vent field within the caldera probably follows a multifractal behavior and exhibits a weak memory effect. Depth Profiles and Time Series show similar behavior among conductivity and temperature. The variance and moments of both parameters underline the existence of two different mechanisms governing the behavior of the vent field. Finally, the structure function shows that changes in the time series are described by a Cauchy–Lorentz distribution.
... With dimensions of 8 x 11 km, it is one of the largest calderas globally, a result of many Plinian eruptions that have occurred from different eruptive centers during the volcanic history of the region (650-0 Ky) (Druitt, Francaviglia, 1992). Its steep and imposing walls portray layers of explosive products including lavas and pumice, intercalated by paleosols that represent a cease of activity. ...
... Yokoyama (2016) listed Aso, Aira (both are in Kyushu, Japan), and Toba (Sumatra, Indonesia) calderas as composite calderas. In addition to Aso, Aira, and Toba, Santorini caldera in the Aegean Sea, Greece (Druitt and Francaviglia, 1992), and Okataina caldera in the Taupo Volcanic Zone (TVZ), North Island, New Zealand (Jurado-Chichay and Walker, 2000), have been called "composite" due to their complex geometries. Toba and Okataina are also referred to as a "caldera complex" (Spinks et al., 2005;Chesner, 2012). ...
Some calderas are geometrically complex that may be related not to a single eruption, magma body, or structure. In order to reveal their forming processes, multidisciplinary methods should be applied. Akan volcano has E-W elongated and irregular-shaped caldera (24 × 13 km), implying a complex mechanism of formation. Akan caldera results from successive explosive eruptions from 1.4 to 0.1 Ma. On the basis of duration of dormancy and petrological features (mainly whole-rock major element compositions) of juvenile materials, these eruptions have been grouped into 17 eruptive groups (Ak1–Ak17), each of which consists of a single or a sequential phase. In order to investigate the processes of caldera formation, we focus on the younger eruptive groups (Ak1–Ak7: 0.8 to 0.2 Ma) that have relatively large magnitudes (>10 km³) and likely control the present caldera shape. We performed component analysis of lithic fragments from pyroclastic fallout deposits, whole-rock trace element analysis of juveniles, and gravitational survey of the caldera. We grouped Ak1–Ak7 into three types, namely, type A (Ak1, Ak2), type B (Ak3–Ak5), and type C (Ak6, Ak7), based on the lithic componentry, most of which are accessary and accidental fragments from vent and conduit areas. The characteristic lithic component in each type is as follows: altered rock in type A, aphyric dacite in type B, and pyroxene andesite in type C. These data suggest that explosive eruptions of each type are derived from distinct sources. The whole-rock composition of juvenile materials of each type also shows distinct trends on Harker diagrams. These three chemical trends are nearly parallel, suggesting that three different magma systems were active. This is consistent with the lithic componentry showing three spatially distinct vent sources. The geological and petrological evidence is supported by a Bouguer anomaly map. Akan caldera is characterized by three circular closed minima, indicative of three depressed segments that correspond to the source regions, each of which possibly discharged the three types of magma. Caldera-forming eruptions of Akan caldera occurred from at least three distinct sources with distinct magma systems. In conclusion, Akan caldera is a composite caldera, and its shape reflects the distribution of multiple source regions. The case study of Akan caldera shows a possible time-space evolutionary pathway for a caldera complex where several smaller calderas are nested.
... This is a scenario that has been evidenced in Santorini by Druitt and Francaviglia (1992) and suggests that an unconformity related to caldera collapse has been already reported on the island. Scoria cones are not only commonly fed by volcanic fissures eruptions but are also commonly located at the periphery of a ring fault (Gudmundsson 2011). ...
Volcanic and tectonic activities in the Aegean region have controlled the evolution of Santorini volcano, including changes in the shape and size of the island through time. Previous studies associate much of the island’s volcanic activity with the presence of regional faults, but a comprehensive volcanotectonic study that clarifies the relationship between dyking and faulting in the island has not been made. Here we present a detailed structural analysis focused on the northern caldera wall of Santorini, where numerous dykes and faults outcrop and can be studied in the mesoscale. To augment our discussion of dyke and fault interactions, we combine previous volcanological and geophysical observations with our structural analysis to report the volcanotectonic evolution of the northern part of the island and design a conceptual spatial-temporal model. We mapped 91 dyke segments and 15 faults and classified the latter, where possible, with respect to their observed or recorded kinematics, their size, and the active stress field under which they were formed based on prior geophysical data. We relate our observations to a mechanical unconformity within the northern caldera wall. Our field observations, coupled with previous numerical, geophysical, and volcanological studies, offer insights on the interaction between dykes and faults and indicate the conditions under which the faults facilitated magma emplacement, or not, during the volcano’s activity. Our analysis attempts to answer an essential question: under what conditions do crustal faults facilitate or inhibit magma propagation to the surface, with application to the island of Santorini.
... However, within a given system, it is likely that recurring cycles change in duration as a function of the state of the emptied reservoir and the thermal regime of the surrounding crust (Degruyter et al., 2016;Townsend et al., 2019). For example, the duration of a caldera cycle changed from ~6.5 ka to ~3 ka pre-and post-Memorial Ignimbrite at Rabaul (Fig. 5), from ~225 ka to ~280 ka preand post-Matahina Ignimbrite at Okataina (Fig. 6), from 110 ka, to 49 ka, to ~18 ka between the oldest caldera collapse and the Minoan eruption at Santorini (Druitt and Francaviglia, 1992), and from 125 ka, to 18 ka, to 33 ka from the Aso-1 to Aso-4 caldera-forming eruptions at Aso caldera in Japan (Kaneko et al., 2007). The relative durations of the various phases (recovery versus maturation versus fermentation), however, may be expected to remain constant from one cycle to the next and from one system to the next, as they are essentially controlled by the thermo-mechanical evolution of the sub-caldera reservoir(s). ...
Silicic calderas globally tend to record a cyclic magmatic, structural, and eruptive evolutionary progression. Some calderas are polycyclic, involving multiple catastrophic collapses in the same approximate location. Here we discuss five examples from well-studied, geologically-young and active magmatic systems: The Kos-Nisyros Volcanic Complex (Greece), Long Valley (USA), Campi Flegrei (Southern Italy), Rabaul (Papua New Guinea), and Okataina (New Zealand) in order to gain insights on the inner workings of caldera systems during the build up to and recovery from large explosive eruptions. We show that the sub-caldera magmatic system evolves through a series of processes, here collectively termed “caldera cycle”, that are common to monocyclic and polycyclic calderas. In the case of polycyclic calderas, they accompany the transition from one caldera-forming eruption to the next. The caldera cycle comprises (1) the period of pre-collapse activity (incubation, maturation, widespread presence of a magmatic volatile phase), (2) the catastrophic caldera-forming (CCF) eruption, and (3) post-collapse recovery (resurgence, renewed eruptions, subsequent maturation) or the possible cessation of the cycle. The incubation phase corresponds to a period of thermal maturation of the crust, during which eruptions are frequent and of small volume due to the limited capability of reservoirs to grow. During the maturation phase, magma reservoirs gradually grow, coalesce, homogenize, magmas differentiate, and eruption frequency decreases. The system transitions into the fermentation phase once an exsolved magmatic volatile phase is continuously present in the reservoir, thereby increasing the compressibility of the magma and instigating a period of runaway growth of the reservoir. A CCF eruption at the end of the fermentation phase could be the concatenated result of multiple magmatic processes (including magma recharge, volatile exsolution, and crystal mush remelting) pressurizing the reservoir, while external factors (e.g., tectonic processes) can also play a role. Postcaldera eruptions, subvolcanic intrusions, and hydrothermal activity typically continue, even if the magma supply wanes. If, however, magma supply at depth remains substantial, the system may recover, initially erupting the remobilized remains of the CCF reservoir and/or new recharging magmas until a shallow reservoir starts to grow and mature again. Placing other calderas worldwide within this framework would enable to test the robustness of the proposed framework, deepen the understanding of what controls the duration of a cycle and its individual phases, and refine the petrologic, geophysical, and unrest symptoms that are characteristic of the state of a system.
... At the top of Unit D, a lithic lag breccia is observed at locality D3S2/NP4 (Fig. 5b), and at D2S2/U23 there are intensely fractured metre-scale glassy blocks (Fig. 5c). These blocks are reminiscent of the large lava blocks found in phase 3 of 4 of the Late Bronze Age (Minoan) eruptive sequence in Santorini (Druitt and Francaviglia, 1992;Sparks and Wilson, 1990). By analogy, lava blocks originating from the volcanic island may have been entrained by PDCs. ...
Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions.
... The central SAVA is composed of the Santorini Volcanic Complex, Milos Island and Christiana (islets SW of Santorini) volcanic fields, all associated with tholeiitic and calc-alkaline magmatism. Featuring a high temperature geothermal field, Milos is a presently dormant stratovolcano; the most recent significant activity there was related to a series of phreatic eruptions between 380 and 90 Ka ago [49,50], although other studies propose younger ages [51]. Phreatic explosions, commonly producing overlapping craters of usually sub-kilometric diameter, continued from the late-Pleistocene to recent times. ...
Knowledge and visualization of the present-day relationship between earthquakes, active tectonics and crustal deformation is a key to understanding geodynamic processes, and is also essential for risk mitigation and the management of geo-reservoirs for energy and waste. The study of the complexity of the Greek tectonics has been the subject of intense efforts of our working group, employing multidisciplinary methodologies that include detailed geological mapping, geophysical and seismological data processing using innovative methods and geodetic data processing, involved in surveying at various scales. The data and results from these studies are merged with existing or updated datasets to compose the new Seismotectonic Atlas of Greece. The main objective of the Atlas is to harmonize and integrate the most recent seismological, geological, tectonic, geophysical and geodetic data in an interactive, online GIS environment. To demonstrate the wealth of information available in the end product, herein, we present thematic layers of important seismotectonic and geophysical content, which facilitates the comprehensive visualization and first order insight into seismic and other risks of the Greek territories. The future prospect of the Atlas is the incorporation of tools and algorithms for joint analysis and appraisal of these datasets, so as to enable rapid seismotectonic analysis and scenario-based seismic risk assessment.
... Bathymetric and available tectonic data of the SATZ most recently published by Nomikou, Hübscher et al. (2019) and Hooft et al. (2017) reveal a system of ridges and basins which has been interpreted as an extensional complex of tectonic grabens and horsts. To the southwest, the SATZ is characterized by the volcanic centers of Christiana, Santorini, and Kolumbo, which are responsible for numerous volcanic eruptions, including the well-known Minoan Eruption of Santorini approximately 3,600 years ago (Druitt & Francaviglia, 1992;Druitt et al., 1999;Hooft et al., 2019;Nomikou, Druitt et al., 2016). The remarkably linear alignment of the volcanic edifices highlights the fundamental control that crustal structure and tectonics have on the location of volcanic activity Hooft et al., 2019;Nomikou, Hübscher et al., 2019;Nomikou et al., 2013). ...
A vast majority of marine geological research is based on academic seismic data collected with single‐channel systems or short‐offset multichannel seismic cables, which often lack reflection moveout for conventional velocity analysis. Consequently, our understanding of Earth processes often relies on seismic time sections, which hampers quantitative analysis in terms of depth, formation thicknesses, or dip angles of faults. In order to overcome these limitations, we present a robust diffraction extraction scheme that models and adaptively subtracts the reflected wavefield from the data. We use diffractions to estimate insightful wavefront attributes and perform wavefront tomography to obtain laterally resolved seismic velocity information in depth. Using diffraction focusing as a quality control tool, we perform an interpretation‐driven refinement to derive a geologically plausible depth‐velocity model. In a final step, we perform depth migration to arrive at a spatial reconstruction of the shallow crust. Further, we focus the diffracted wavefield to demonstrate how these diffraction images can be used as physics‐guided attribute maps to support the identification of faults and unconformities. We demonstrate the potential of this processing scheme by its application to a seismic line from the Santorini Amorgos Tectonic Zone, located on the Hellenic Volcanic Arc, which is notorious for its catastrophic volcanic eruptions, earthquakes, and tsunamis. The resulting depth image allows a refined fault pattern delineation and, for the first time, a quantitative analysis of the basin stratigraphy. We conclude that diffraction‐based data analysis has a high potential, especially when the acquisition geometry of seismic data does not allow conventional velocity analysis.
... Santorini volcanic group is a ring of three islands (Thera, Therasia, and Aspronisi) around a flooded caldera containing the islands of Palea and Nea Kameni, which postdate caldera collapse (3.6ka) and are the subaerial expressions of an intracaldera, largely submarine lava shield (Druitt, 2014). The caldera is a 11x7km composite structure resulting from at least four collapses over the last 200ky (Druitt and Francaviglia, 1992), the last of which took place during, and immediately following, the ~1630 BCE 'Minoan' eruption (Friedrich et al., 2006). It consists of three flat-floored basins: a large northern 390m deep, and two smaller ones (western: 320m and southern: 270m respectively, Nomikou et al., 2013). ...
... The intervals between the twelve Plinian eruptions vary between 17 and 40 Ka, averaging to 30 Ka. The explosive activity triggered at least four caldera collapses and resulted in the formation of the present-day composite caldera structure (Druitt & Francaviglia 1992), which is bordered by cliffs as high as 300 m and extends to at least 400 m below sea level. The last caldera-forming explosion was the renowned Minoan eruption of the late Bronze Age (1645-1500 BCE), which ejected about 30 km 3 of dense-rock equivalent material according to Pyle (1990) and over 60 km 3 according to Sigurdsson et al. (2006); the vent was located in the vicinity of the Kammeni Islets (Bond & Sparks 1976). ...
Tectonic activity is very difficult to study in the Santorini volcanic complex (SVC) as it comprises a cluster of small/awkwardly shaped islands covered by pyroclastic deposits from which tell-tale markers are swiftly erased, while seismicity is generally absent. We address the problem by combining geophysical exploration methods to evaluate the long-term effects of tectonic deformation and time-lapse differential GPS to directly evaluate the magnitude and kinematics of present-day deformation. The former comprise 3-D gravity modelling to investigate the footprint of tectonics on the pre-volcanic Alpine basement and natural-field EM induction to map conductivity anomalies epiphenomenal to fluid circulation in faults. Our analysis identified the following principal tectonic elements:
The Trans-Santorin Divide (TSD), a segmented NNW–SSE dextral strike-slip fault splitting the SVC sideways of the line joining Cape Exomytis, the Kammeni Islets and the Oia–Therassia Strait. It is collocated with a major vertical conductive zone and forms a series of dents and depressions in the basement. The Columbo Fault Zone (CFZ) is a pair of parallel NE–SW subvertical normal-sinistral faults straddling the northern SVC and terminating against the TSD; it may be associated with fluid injection into the shallow crust but appears to have limited effect on crustal conductivity (compared to TSD). The Anhydros Fault Zone (AFZ) is detected by its footprint on the basement, as a set of parallel northerly dipping NE–SW faults between the Athinios–Monolithos line and Fira. If it has any heave, it is left-lateral. It does not have distinguishable electrical signature and does not contribute to present-day horizontal deformation. The CFZ and AFZ are antithetic and form a graben containing the volcanic centre of Kammeni Islets.
E–W extension was identified lengthwise of a zone stretching from Cape Exomytis to Athinios and along the east flank of the caldera to Imerovigli. N–S normal faulting confirmed therein, may have contributed to the localization of the east caldera wall. NNE–SSW compression was observed at SW Thera; this may have produced E-W failure and contributed to the localization of the south caldera wall. The footprint of the caldera on the basement is a parallelogram with N–S long and WNW–ESE short dimensions: if the east and south flanks collapsed along N–S normal and E–W inverse failures, then the west and north flanks may have formed analogously. Present-day deformation is localized on the TSD and CFZ: this can only be explained if the former is the synthetic (dextral) Riedel-R shear and the latter the antithetic (sinistral) Riedel-R′ shear, generated by N–S σ1 and E–W σ3 principal stress axes. Accordingly, NW–SE right-lateral shearing of the broader area is expected and indicated by several lines of indirect evidence. The geographic extent of this shearing and its role in the regional tectonics of the south Aegean remains to be confirmed and appraised by future research.
Contemporary volcanic centres develop at the interface of the TSD with the CFZ/AFZ graben; volcanism appears to be controlled by tectonics and the SVC to be shaped by tectonic rather than volcanic activity.
... This assessment is supported by the observation that the majority of currently active continental arcs are, contrary to the Andean paradigm, characterized by sequences of rocks erupted and deposited within subsiding basins, not volcanoes sitting on high-standing thick crust (Busby-Spera, 1988;Hildebrand and Bowring, 1984). Recent examples include: (a) both the modern and ancestral Miocene Cascades, where the volcanoes and their debris aprons subsided in grabens (Williams and McBirney, 1979); (b) the low-standing Alaskan Peninsula (Burk, 1965), where volcanoes such as Augustine are located in Cook Inlet (Power et al., 2010); (c) the Kamchatka Peninsula of easternmost Russia, where stratovolcanoes erupt in extensive fault-bounded troughs close to sea level (Levin et al., 2002); (d) the North Island of New Zealand, where the Taupo zone sector of the arc is actively extending as calderas and stratocones erupt (Harrison and White, 2006); (e) the Central American arc, where volcanoes are aligned in a long, linear, low-standing depression (Williams and McBirney, 1979); and (f) the Hellenic arc, where volcanoes form islands in the Aegean Sea (Druitt and Francaviglia, 1992). ...
Although widely used, classification schemes for granitic rocks have until now failed to separate slab failure from arc magmatism in collisional orogens. Our previous work showed that Cretaceous Cordilleran-type batholiths are composed of a pre-collisional belt of mesozonal to epizonal arc plutons, commonly emplaced into their own cover, and a post- accretion tract of what we interpret as “slab failure” plutons, emplaced into the tectonically thickened collisional hinterland. Histograms based on new geochemical compilations of pre- and post- collisional igneous rocks were employed to derive trace element criterion to distinguish between these two plu- tonic types. Using rocks over the SiO2 range of 55–70 wt. % and aluminum saturation index of b1.1, slab failure rocks are readily separated from arc rocks based on Sr/Y N 20, Nb/Y N 0.4, Ta/Yb N 0.3, La/Yb N 10, Gd/Yb N 2, and Sm/Yb N 2.5. Also, we found that A-type magmas can be distinguished from other post-collisional igneous rocks based on Ta + YbN6 ppm and Nb + Y N 60 ppm. These trace element differences between our pre- collisional arc and post-collisional slab failure plus A-type groups allowed us to build composite discrimination diagrams for destructive margin igneous rocks. Although most S-type granites are also post-collisional, they should not be plotted on our diagrams, for some of their geochemical features overlap those of our slab failure group. Unlike our arc and slab failure groups, A-type granites are formed in diverse tectonic settings such that their classification is more related to geochemical signature and magma sources than to tectonic setting. Lastly, we show how our discrimination diagrams delineate examples of slab failure magmatism through time and sug- gest links with porphyry Cu-Au deposits.348-349
... This assessment is supported by the observation that the majority of currently active continental arcs are, contrary to the Andean paradigm, characterized by sequences of rocks erupted and deposited within subsiding basins, not volcanoes sitting on high-standing thick crust (Busby-Spera, 1988;Hildebrand and Bowring, 1984). Recent examples include: (a) both the modern and ancestral Miocene Cascades, where the volcanoes and their debris aprons subsided in grabens (Williams and McBirney, 1979); (b) the low-standing Alaskan Peninsula (Burk, 1965), where volcanoes such as Augustine are located in Cook Inlet (Power et al., 2010); (c) the Kamchatka Peninsula of easternmost Russia, where stratovolcanoes erupt in extensive fault-bounded troughs close to sea level (Levin et al., 2002); (d) the North Island of New Zealand, where the Taupo zone sector of the arc is actively extending as calderas and stratocones erupt (Harrison and White, 2006); (e) the Central American arc, where volcanoes are aligned in a long, linear, low-standing depression (Williams and McBirney, 1979); and (f) the Hellenic arc, where volcanoes form islands in the Aegean Sea (Druitt and Francaviglia, 1992). ...
Trace element discrimination of arc, slab failure, and A-type granitic rocks
Joseph B. Whalen and Robert S. Hildebrand
Abstract
Although widely used, classification schemes for granitic rocks have until now failed to separate slab failure from arc magmatism in collisional orogens. Our previous work showed that Cretaceous Cordilleran-type batholiths are composed of a pre-collisional belt of mesozonal to epizonal arc plutons, commonly emplaced into their own cover, and a post- accretion tract of what we interpret as “slab failure” plutons, emplaced into the tectonically thickened collisional hinterland. Histograms based on new geochemical compilations of pre- and post-collisional igneous rocks were employed to derive trace element criterion to distinguish between these two plutonic types. Using rocks over the SiO2 range of 55-70 wt. % and aluminum saturation index of <1.1, slab failure rocks are readily separated from arc rocks based on Sr/Y>20, Nb/Y>0.4, Ta/Yb>0.3, La/Yb>10, Gd/Yb>2, and Sm/Yb>2.5. Also, we found that A-type magmas can be distinguished from other post-collisional igneous rocks based on Ta + Yb>6 ppm and Nb + Y>60 ppm. These trace element differences between our pre-collisional arc and post-collisional slab failure plus A-type groups allowed us to build composite discrimination diagrams for destructive margin igneous rocks. Although most S-type granites are also post-collisional, they should not be plotted on our diagrams, for some of their geochemical features overlap those of our slab failure group. Unlike our arc and slab failure groups, A-type granites are formed in diverse tectonic settings such that their classification is more related to geochemical signature and magma sources than to tectonic setting. Lastly, we show how our discrimination diagrams delineate examples of slab failure magmatism through time and suggest links with porphyry Cu-Au deposits Keywords: Slab failure, I-types, A-types, Trace elements, Discrimination diagrams, Tectonic settings
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... By the end of the aforementioned volcanic phase, the Akrotiri volcanic center was reactivated (Druitt and Francaviglia 1992). The main characteristic of this activity was the creation of a ''Strombolian''-type volcano as a result of the creation of cinder cone volcanoes in the areas of Cape Balos, Kokkinopetra and Mavrorachidi (Red Beach). ...
... I do not question the existence of an ancient caldera at Santorini (probably associated with an eruption 22 ky ago). It was proposed based on the presence of stromatolite fragments in the UPS [2], and was confirmed first by geomorphological mapping [3], then by 36 Cl cosmic-ray exposure dating [4], of the caldera cliffs. Evidence published to date places this ancient structure largely in the northern basin of the present-day caldera; any extension further south is as yet unconstrained. ...
... By the end of the aforementioned volcanic phase, the Akrotiri volcanic center was reactivated (Druitt and Francaviglia 1992). The main characteristic of this activity was the creation of a ''Strombolian''-type volcano as a result of the creation of cinder cone volcanoes in the areas of Cape Balos, Kokkinopetra and Mavrorachidi (Red Beach). ...
The outstanding Red Beach in Santorini, a famous volcanic island in the Aegean Sea in the territory of Greece, exhibits extended rockfall instabilities along its cliffs, placing its highly frequented touristic zones at high risk. This study aimed to generate an engineering geological interpretation of these instabilities and to evaluate the degree of the rockfall potential in relation to the natural evolution of the beach. A detailed field survey of the engineering geological conditions and thorough scanning of the cliffs using terrestrial scanning (LiDAR) combined with accurate geodetic survey façade plans, enabled the detection of cinematically unstable sections. A semiautomated qualitative method for evaluating the site-specific landslide potential was also performed here using an unmanned aerial vehicle, which executed several flights around the research area 3.5 years after LiDAR scanning. The resulting products, namely a digital surface model, an orthophotograph and point clouds, were compared with the LiDAR data to evaluate the behavior of the volcanic formations at the cliff and the rockfall potential. The Red Beach cliff, mainly composed of volcanic scoria cones, was found to be very challenging case for determining a feasible engineering geological solution. Its numerous requirements with respect to rockfall control and stabilization in conjunction with the necessity of maintaining the landscape’s natural beauty and preserving the adjacent archeological site further complicated the problem. However, stabilization to prevent rock falls will provoke a disruption of the balance between the sea-induced erosion and the supply of material that originates from rock falls of the natural slopes toward the beach that could put the existence of the Red Beach in significant danger. A feasible engineering geological solution for stabilizing the cliffs was investigated by evaluating all the possible protective and prevent measures. However, whether any of these measures are acceptable is doubtful.
... This type of calderas, named underpressure calderas by , are characterized in the field by the presence of relatively thick Plinian deposits underlying the caldera-forming ignimbrites, and would correspond to those calderas in which the initiation of caldera collapse requires a substantial decompression of the magma chamber (Druitt and Sparks, 1984). Examples of such calderas are Crater Lake (e.g., Bacon, 1983), Katmai (e.g., Hildreth, 1991), Santorini (e.g., Druitt and Francaviglia, 1992). The other end-member corresponds to those calderas in which caldera collapse starts at the beginning of the eruption, without any Plinian phase preceding it. ...
... Santorini ve yakın çevresindeki birkaç irili ufaklı adayı oluşturan volkanik etkinlik ise yaklaşık 1 milyon yıl önce denizaltı volkanizması olarak başlamıştır, bu etkinlik daha sonraları bir ada oluşturmuş ve son püskürme tarihi olan 1950 yılına kadar aralıklarla devam etmiştir. Yapılan araştırmalara göre son 200 000 yıl içerisinde en az 12 büyük püskürme gerçekleşmiştir (Druitt, Francaviglia, 1992). İÖ 1628 yılındaki püskürme (Tarih konusunda bazı tartışmalar vardır, o yüzden farklı kaynaklarda ± 36 yıllık bir fark bulunmaktadır, karışıklığa yol açmamak için bu püskürme genel olarak Minoan Püskürmesi olarak adlandırılır) Santorini'nin en tanınmış ve hem adayı hem de çevresini en fazla etkileyen püskürmedir. ...
Tephra layers provide an opportunity to determine the age of an eruption, the extent of ash dispersal, and its impact on natural and human ecosystems. One such tephra from the seventeenth-century BC eruption of Thera on the Aegean island of Santorini is well known in deep-sea sediment cores from the eastern Mediterranean Sea. It has also been discovered in terrestrial sediments on the Aegean islands. There is also layer of volcanic ash in the sediments of lakes in the western Anatolia. Geochemical analysis of the volcanic ash from lake sediments together with radiocarbon datings that the provenance of this tephra is the Minoan eruption of Santorini. The presence of tehpra mixed sediment layers in the Western Anatolia lakes indicates that tephra deposits may also be found in the other proper places of the same region
... As the Minoan eruption was progressing the vent abandoned its subaerial environment and migrated into the pre-existing flooded caldera (Druitt and Francaviglia 1992;Athanassas et al. 2016) lending a phreatomagmatic character to the eruption (phase 2). Phase 2 produced large-scale base surges throughout Santorini (Bond and Sparks 1976;Heiken and McCoy 1984) and it was energetic enough to destroy any piece of infrastructure by catapulting volcanic bombs into Akrotiri (Hédervari 1990;Pichler and Friedrich 1980) and completely burying the settlement underneath the pyroclastic deposits. ...
This study is a step forward in understanding the palaeoenvironmental effects of the Minoan eruption of Santorini (1627–1600 BCE). We employ geostatistics to produce a prediction map for the thickness of the tephra fallout over the Eastern Mediterranean, and we reconstruct the effects by comparisons with recent eruption analogues. Based on the geostatistical map, the amount of environmental disruption over so far undocumented areas is estimated by comparison with archaeological sites where emplaced Minoan tephra has been recorded before. Nevertheless, independent field evidence suggest that the environment responded differently in places, occasionally posing challenges to the presented interpolation. A second line of evidence coming from contemporaneous fluvial archives provides clues for a widespread ‘Minoan flood’ over a large part of the Eastern Mediterranean, associated with the eruption itself. This simultaneous hydrological event may have had a counterbalancing effect on the impacts of the Minoan tephra cover, and could explain the sporadic discrepancies between the predicted effects and the palaeoenvironmental evidence. Traces of the effects of this extraordinary volcanic event are also sought in the regional Late Bronze Age literature.
The 20th century eruptions of the Santorini volcano in Greece are the most recent activity of the volcano’s long lifespan. While the different eruptions taking place between 1925 and 1950 have traditionally been considered to exhibit similar eruptive styles, aspects of their evolution and precise information related to the individual eruption dynamics were poorly constrained. This study collates field reports and historical accounts, mainly from the Greek national scientific committee, which was assigned to study the volcanic activity in Nea Kameni Island with recent field campaigns. This analysis provides further insight into these eruptions and attempts to unravel the timing and style of explosive and effusive episodes that took place. Reconstruction of the recent geological evolution and of the eruptive history allow a more complete description of the eruption dynamics and associated unrest. These include fumarolic behaviour, explosion intensity, direction and volume of the lava flows, eruption duration, vent morphological changes (such as craters, domes, and horseshoe ramparts), textural characteristics and lava morphologies, as well as surface fracturing. Specific features related to first-hand accounts of the eruptions and associated products, in conjunction with our in situ post-eruptive geological study, allow an improved reconstruction of activity, both prior to and during the historical eruptions, which contributes to understanding the development of the eruption and enhances the forecast of potential future eruptions from patterns of precursory activity.
The destructive tsunami on 22 December 2018 due to the flank collapse of the Anak Krakatau volcano was a bitter reminder of large tsunami risks and of the shortcomings of the existing tsunami warning systems for atypical sources (tsunamis generated by non-seismic and complex sources). In the Mediterranean, several tsunamis were generated by landslides associated with volcanic systems in the past.The volcanic unrest experienced in 2011–2012 on the Santorini volcanic island in the Southern Aegean Sea pointed out the need to identify and quantify tsunami hazard and risk due to possible flank instability which may be triggered as a result of volcanic unrest or nearby seismotectonic activities. Inspired from this need, in this study we examined three possible landslide scenarios in Santorini Island with tsunamigenic potential. The results show that the scenarios considered in our study are able to generate significant local tsunamis impacting Santorini and the nearby islands, as well as producing significant impact along the coasts of the Southern Aegean Sea. While maximum tsunami amplitudes/arrival time ranges are 1.2 m/30-90 min for locations in the Greek-Turkish coasts in the far field, they are in the order of ≈60 m/1-2 min for some locations at the Santorini Island. The extreme tsunami amplitudes and short arrival times for locations inside the Santorini Island is a major challenge in terms of tsunami hazard warning and mitigation. As an effort to address this challenge, a discussion on the requirements for local tsunami warning system addressing atypical sources in the context of multi-hazard disaster risk reduction is also provided.
We use the tephrostratigraphic framework along the Aegean Volcanic Arc established in Part 1 of this contribution to determine hemipelagic sedimentation rates, calculate new tephra ages, and constrain the minimum magnitudes of (sub)plinian eruptions of the last 200 kyrs. Hemipelagic sedimentation rates range from ∼0.5 cm/kyr up to ∼40 cm/kyr and vary laterally as well as over time. Interpolation between dated tephras yields an eruption age of ∼37 ka for the Firiplaka tephra, showing that explosive volcanism on Milos is ∼24 kyrs younger than previously thought. The four marine Nisyros tephras (N1 to N4) identified in Part 1 (including the Upper (N1) and Lower (N4) Pumice) have ages of ∼57 ka, ∼63 ka, ∼69 ka, and ∼76 ka, respectively. Eruption ages for the Yali‐1 and Yali‐2 tephras are ∼55 ka and ∼34 ka, respectively. The Yali‐2 tephra comprises two geochemically and laterally distinct marine facies. The southern facies is identical to the Yali‐2 fall deposit on land but the western facies has slightly less evolved glass compositions. Overall, erupted plinian and co‐ignimbrite fall tephra volumes range from <1 to 56 km³ (excluding possible caldera fillings and ignimbrite volumes), and 80% of the eruptions had magnitude 5.5 < M ≤ 7.2 (M = log(m)‐7; m = erupted magma mass in kg). Twenty percent of the tephras represent 3.2 < M < 5.5 eruptions. The long‐term average tephra magma mass flux through highly explosive eruptions of Santorini is estimated at ∼40 kg/s. The analogous data for the Kos‐Yali‐Nisyros volcanic complex is less‐well constrained but similar to Santorini.
Dykes and inclined sheets are known occasionally to exploit faults as parts of their paths, but the conditions that allow this to happen are still not fully understood. In this paper, we report field observations from a swarm composed of 91 segments of dykes and inclined sheets, the swarm being particularly well-exposed in the mechanically layered caldera walls of the Santorini volcano, Greece. Here the focus is on dykes and sheets in the swarm that are seen deflected into faults and the mechanical conditions that encourage such deflections. In particular, we present new analytical and numerical models to explain the mechanical principles of dyke/sheet deflections into faults. The numerical models are applied to a normal-fault dipping 65° with a damage zone composed of parallel layers or zones of progressively more compliant rocks with increasing distance from the fault rupture plane. We model a sheet-intrusion, dipping from 0° to 90° and with an overpressure of alternatively 1 MPa and 5 MPa, approaching the fault. We further tested the effects of changing (1) the thickness of the sheet-intrusion, (2) the fault-zone thickness, (3) the fault-zone dip-dimension (height), and (4) the loading by, alternatively, regional tension and compression. We find that the stiffness of the fault core, where a compliant core characterises recently active fault zones, has pronounced effects on the orientation and magnitudes of the local stresses and, thereby, on the likelihood of dyke/sheet deflection into the fault zone. Similarly, the analytical models, focusing on the fault-zone tensile strength and energy conditions for dyke/sheet deflection, indicate that dykes/sheets are most likely to be deflected into and use steeply dipping recently active (zero tensile-strength) normal faults as parts of their paths.
Integration and harmonization of the most recent seismological, geological, tectonic, geophysical and geodetic datasets, with the aim of capturing the potential of ground deformation towards a more reliable evaluation of seismic risk at a national level.
A vast majority of marine geological research is based on academic seismic data collected with single-channel systems or short-offset multi-channel seismic cables, which often lack reflection moveout for conventional velocity analysis. Consequently, our understanding of earth processes often relies on seismic time sections, which hampers quantitative analysis in terms of depth, formation thicknesses, or dip angles of faults. In order to overcome these limitations, we present a robust diffraction extraction scheme that models and adaptively subtracts the reflected wavefield from the data. We use diffractions to estimate insightful wavefront attributes and perform wavefront tomography to obtain laterally resolved seismic velocity information in depth. Using diffraction focusing as a quality control tool, we perform an interpretation-driven refinement to derive a geologically plausible depth-velocity-model. In a final step, we perform depth migration to arrive at a spatial reconstruction of the shallow crust. Further, we focus the diffracted wavefield to demonstrate how these diffraction images can be used as physics-guided attribute maps to support the identification of faults and unconformities. We demonstrate the potential of this processing scheme by its application to a seismic line from the Santorini-Amorgos Tectonic Zone, located on the Hellenic Volcanic Arc, which is notorious for its catastrophic volcanic eruptions, earthquakes, and tsunamis. The resulting depth image allows a refined fault pattern delineation and, for the first time, a quantitative analysis of the basin stratigraphy. We conclude that diffraction-based data analysis is a decisive factor, especially when the acquisition geometry of seismic data does not allow conventional velocity analysis.
Hydrothermal mineralization processes are investigated along the Hellenic Volcanic Arc (HVA) and associated environments in order to describe the factors that control the spatial and time variability in the hydrothermal intensity and the composition of the mineral deposits formed. Santorini is the most active hydrothermal center of the HVA. Two Fe-rich submarine hydrothermal deposits were formed. One in the Palaea Kameni (PK) embayment and the other in the Nea Kameni (NK) embayment. Hydrothermally introduced elements such as Fe, Mn, As and Si are more enriched in these metalliferous sediments than in any other similar deposit of the arc. Over a hundred volcanic events occurred in Santorini and this volcano is considered as one of the most violent and most famous in the world. The detailed study of these two deposits in combination with the tectonic regime provided an excellent opportunity to understand the hydrothermal mineralization processes in a convergent plate environment. The Santorini magma below the thinner continental crust and nearest to the greatest lithospheric extension prevents the early crystallization of Fe-Ti oxides, leading to extreme Fe oxide enrichments in the final hydrothermal fluids. The physicochemical conditions of formation of the two deposits are deduced on the basis of their mineralogical and geochemical features. The greater average values of Zn compared to those of Cu in PK deposit, suggest higher temperature of formation than that of NK deposit, where Cu concentrations are higher than those of Zn. This is in consistence with the abundance of pyrite in PK. The greater Mo and V values found in NK compared to those of PK Fe-rich sediments are indicative of the less oxygenated environment, favoring the biological activity of bacteria, known to be involved in the diagenetic transformation of Fe-oxyhydroxides to nontronite. This is in accordance with the plotting of the NK deposits in the nontronite field on the Fe2O3- Al2O3- MgO diagram. The high Si and low Al levels and the stagnant conditions are also in favor of the nontronite formation. Similar deposits were described from Aeolean Arc and from EPR 18 o N. Applying the Dill et al. (1994) model on the PK and NK mineralogical, lithological and geochemical data, all processes involved during the formation of the final deposits and their fate are deduced. An examination of sediment geochemical data from the HVA fore-arc areas in relation to their tectonic setting, the seismic activity and geophysical data led to the production of a map showing the areas of potential presence of metal sulfides on the rock basement. Maleas basin is one of these areas which is characterized by high seismic activity and high heat flow measurements, associated with major fault planes along which magmatic material was injected. The sediment thickness there is very small. Other sites are the crossing points of the faulting lines with volcanic intrusive bodies and/or with a volcanic ridge of a NW-SE direction, near sites of high rates of lithospheric extension. A southward movement of the arc front is deduced.
During a volcanic unrest period with magma-chamber rupture, fluid-driven fractures (dykes) are injected either from deep reservoirs or shallow magma chambers. Subsequently, the dykes follow propagation paths towards the surface, some eventually reaching the surface to erupt while others become arrested. Here we study dyke paths resulting in eruption or arrest in an excellent 5-km wide exposure of the northern caldera wall of the Santorini volcano in Greece. Mapping of >90 dyke segments shows that they were emplaced in a host rock consisting of layers (of breccia, tuff, scoria, and lava) with a wide variety of mechanical properties. At the contacts, some dykes are arrested or deflected and hence change their propagation paths. Here we combine the field data with numerical models to explore dyke paths resulting in (1) arrest and (2) eruption. We investigate the effect of different host-rock mechanical properties, magmatic overpressures, and tectonic loading on dyke paths. We find that layers with unfavorable local stresses for dyke propagation, namely stress barriers, result from layer stiffness (Young's modulus) contrast and thickness variations and are a common cause of dyke arrest. The study also shows how the details of the dyke path, and eventually dyke-fed eruptions, depend on the mechanical layering and local stresses in volcanoes. The results are of great importance for understanding dyke-propagation paths, and the likelihood of eruption, during unrest periods, particularly in stratovolcanoes fed by shallow chambers, such as Santorini.
This is the first attempt to analyse vascular plant diversity patterns regarding the seven vegetated islands of the Santorini archipelago (Aegean Sea, Greece) as a whole. Hitherto unpublished floristic records, combined with critical use of taxonomic and chorological information from previous and most recent literature, resulted in a total of 696 infrageneric taxa (species and subspecies) occurring in the area. Detailed qualitative and quantitative phytodiversity spectra per individual island are presented, and floristic dissimilarity (beta-diversity) between islands is considered. Spatial distribution of 28 chorological, ecological, vegetative and reproductive traits for each recorded taxon have been calculated in order to detect regional and fundamental patterns in functional biogeography beyond traditional species-based approaches, based on both taxonomic and functional components of diversity. Mediterranean species constitute the most abundant chorological element and therophytes the most abundant life-form element in the region. Surface area is the most influential variable contributing to species richness; very strong relationships in (1) species per area, (2) functional richness per area and (3) functional richness per species richness are revealed for the Santorini archipelago. Floristic cross-correlations revealed an overall high floristic heterogeneity among the individual islands. The phytodiversity assessment presented is undoubtedly of documentary value in consideration of expected future eruptive events in the area which may damage the plant cover at least on some of the involved islands to a yet unpredictable extent.
Citation: Raus Th., Karadimou E. & Dimopoulos P. 2019: Taxonomic and functional plant diversity of the Santorini-Christiana island group (Aegean Sea, Greece). – Willdenowia 49: 363–381. doi: https://doi.org/10.3372/wi.49.49308
Version of record first published online on 2 December 2019 ahead of inclusion in December 2019 issue.
The disastrous volcanic eruption of Thera in the Aegean that happened in late seventeenth century BC (Late Bronce Age, LBA) buried under a thick mantle of volcanic ash the thriving city harbor of Akrotiri, situated at the southern edge of the island. The archaeological excavations at the site have witnessed the city’s wealth and flourish; numerous, luxurious, and outlandish finds clearly indicate well-established maritime contacts between Akrotiri and eastern Mediterranean lands. Such maritime operations and overseas trade required, apparently, adequate harbor facilities. This paper deals with geoarchaeological and geophysical studies aiming at the localization of the buried harbor, a so far unrealized ambitious aim, in spite of the repeated intense attempts undertaken in the last decades. Preference for the relevant investigation was given to three small littoral valleys situated in small distances at both sides of the settlement, suggesting shallow bays before the Minoan eruption, hence probably having hosted the searched harbor(s). Prior to the fieldwork undertaken, all available geological and other related data were cartographically outlaid by means of GIS. Afterwards, in situ geomorphologic studies were conducted, followed by one littoral drilling and geophysical investigations including seismic refraction and electrical resistivity tomography (ERT). Both geophysics and drilling have shown that the hard pre-Minoan basement (consisting of dense andesitic lavas) is situated at depths of ca. 25 m in the Mavrorachidi valley.
The rhyodacitic 172 ka Lower Pumice 2 (LP2) eruption terminated the first magmatic cycle at Santorini (Greece), producing a proximal < 50 m thick succession of pyroclastic fall deposits, diffusely-stratified to massive ignimbrites and multiple lithic breccias. The eruption commenced with the development of a short-lived precursory eruption column, depositing a < 15 cm blanket of 1–2 cm sized pumice fragments at near vent localities (LP2-A1). The precursor deposits are conformably overlain by a < 30 m thick sequence of reversely-graded/ungraded pumice fall deposits that reflect opening and widening of a point-source vent, increasing mass discharge rates up to 10⁸ kg s− 1, and the development of a 36 km high eruption column (LP2-A2, A3). The progressive increase in maximum vesicle number density (NVF) in rhyodacitic pumice, from 3.2 × 10⁹ cm− 3 in the basal fall unit of LP2-A2-1 to 9.2 × 10⁹ cm− 3 in LP2-A3, translates to an increase in magma decompression rate from 18 to 29 MPa s− 1 over the course of LP2-A. This is interpreted to be a consequence of progressive vent widening and a deepening of the fragmentation surface. Such interpretations are supported by the increase in lithic clast abundance vertically through LP2-A, and the occurrence of basement-derived (deep) lithic components in LP2-A3. The increasing lithic clast content and the inability to effectively entrain air into the eruption column, due to vent widening, resulted in column collapse and the development of pyroclastic density currents (PDCs; LP2-B). A major vent excavation event or the opening of new vents, possibly associated with incipient caldera collapse, facilitated the ingress of water into the magmatic system, the development of widespread PDCs and the deposition of a < 20m thick massive phreatomagmatic tuff (LP2-C). The eruption cumulated in catastrophic caldera collapse, the enlargement of a pre-existing flooded caldera and the discharge of lithic-rich PDCs, depositing proximal < 9 m thick lithic lag breccias (LP2-D).
In subduction zone backarcs extensional deformation and arc volcanism interact and these processes, together with mass wasting, shape the seafloor morphology. We present a new bathymetric map of the Santorini-Christiana-Amorgos backarc region of the Hellenic subduction zone by merging high-resolution multibeam swath data from the R/V Langseth PROTEUS seismic experiment with existing maps. The map together with Knudsen subbottom echosounding profiles reveal that recent tectonism, volcanism, and mass wasting are more prevalent in the Santorini-Amorgos region on the east side of Santorini than in the Christiana Basin on the west side. In the Santorini-Amorgos region, large normal faults form the Anydros and Anafi Basins. Where normal fault segments overlap, two nearby accommodation zones generate a relay ramp and the adjoining Anydros synclinal horst with associated complex faulting and elevated seismicity. The ongoing normal faulting in the Santorini-Amorgos region is accompanied by potentially tsunamigenic submarine landsliding; we identified a large submarine landslide along the Santorini-Amorgos Fault and a smaller landslide with an overlying debris chute along the Amorgos Fault. Volcanic activity is also focused in this eastern region along the Kolumbo lineament within the Anydros Basin. Within the Christiana Basin we discovered the Proteus Knoll and adjacent buried edifice. We infer that this is an older volcanic edifice formed along the Hellenic Volcanic Arc between Santorini and Milos. Around Santorini itself, features formed during, and immediately after, the Late Bronze Age eruption dominate the seafloor morphology such as the northwestern strait and wrinkled seafloor pyroclastic flow deposits. This topography is continually reshaped at a smaller scale by ongoing mass wasting. We infer that the earthquake, volcanic, and tsunami activity of the Santorini-Amorgos region is a consequence of focused northwest-southeast extension as the southeastern Aegean moves away from the Attico-Cycladic complex in response to slab breakup and rollback.
We use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at Santorini (associated with 184 ka Lower Pumice 1 eruption), to study the shallow conduit dynamics at the peak of caldera collapse. The main phase of the Lower Pumice 1 eruption commenced with the development of a sustained buoyant eruption column, producing a clast-supported framework of rhyodacitic white pumice (LP1-A). The clasts have densities of 310–740 kg m⁻³, large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities (1.2 × 10⁹–1.7 × 10⁹ cm⁻³). Eruption column collapse, possibly associated with incipient caldera collapse, resulted in the development of pyroclastic flows (LP1-B). The resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range (250–620 kg m⁻³) and moderate to high vesicle number densities (1.3 × 10⁹–2.1 × 10⁹ cm⁻³), comparable to clasts from LP1-A. An absence of deep, basement-derived lithic clast assemblages, together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases, suggests shallow fragmentation depths, a prolonged period of bubble nucleation and growth, and moderate rates of decompression prior to fragmentation (7–11 MPa s⁻¹). Evacuation of magma during the pyroclastic flow phase led to under-pressurisation of the magma reservoir, the propagation of faults (associated with the main phase of caldera collapse) and the formation of 20 m thick lithic lag breccias (LP1-C). Rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density (330–990 kg m⁻³), higher vesicle number densities (4.5 × 10⁹–1.1 × 10¹⁰ cm⁻³) and higher calculated magma decompression rates of 15–28 MPa s⁻¹ than pyroclasts from the pre-collapse eruptive phases. In addition, the abundance of lithic clasts, including deeper, basement-derived lithic assemblages, records the opening of new vents and a deepening of the fragmentation surface. These data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse.
Data for 1053 coastal archaeological sites in the Mediterranean are analysed for age and relative vertical displacement. 335 sites produce accurate data from several periods, creating a reliable data set of 406 records for the last 10,000 years and with a vertical range from -11m to +8.5m relative to present sea level. The variance of rate of displacement is closely related to geographical location. The best fit eustatic curves for each region, after removal of the geographical component, are combined into a single best fit curve for the whole Mediterranean.
A tephra layer originating from the mid-second millennium BC (3300 14C yr BP) ‘Minoan’ eruption of Santorini (or Thera) in the Aegean has been found in lake sediments at G6lhisar in southwest Turkey. Microstratigraphic analyses of tephra shard concentration (TSC), pollen, diatoms, sponge spicules and nonsiliceous microfossils in sediments from Gtlhisar permit the impact of this major volcanic eruption on terrestrial and aquatic biota to be investigated quantitatively. Partial redundancy analysis and associated Monte Carlo permutation tests suggest that TSC alone cannot be shown to have had a demonstrable independent and statistically significant effect on terrestrial pollen, non-siliceous microfossil or diatom assemblages. The lack of any clear, discernible change in the terrestrial pollen composition following tephra deposition suggests that there was minimal impact on regional vegetation over decadal-to-century timescales. However, evidence that the deposition of Santorini tephra may have had an impact on the lake system comes from the combined effect of lithology and TSC (which significantly covary) that explains a significant amount of variance in the aquatic data sets. In particular, diatoms and non-siliceous algae show increases in concentration following tephra deposition, exhibiting what appear to be decadal response times to perturbation. These imply enhanced lake productivity due to accelerated input of silica and other nutrients following tephra dissolution.
THE explosive eruption of Santorini volcano in the Aegean Sea about 3,300 years ago is of considerable archaeological and volcanological significance1–5. Here we report the discovery of tephra from this Minoan event in laminated sediments of the Black Sea. This finding provides constraints on the distribution of debris from the eruption. We estimate a minimum fallout area of 2×l06 km2 extending from the Black Sea in the north to the southeastern Mediterranean Sea. The main dispersal axis trends through southern Turkey, in agreement with other studies of Minoan tephra6,7. The tephra deposits should provide a useful reference horizon for assessing the chronology of Black Sea sediments, which has been much debated8–15.
The origin of reverse grading in air-fall pyroclastic deposits has been ascribed to: (1) changing conditions at an erupting vent; (2) deposition in water; or (3) rolling of large clasts over smaller clasts on the surface of a steep slope. Structural features in a deposit of air-fall pumice lapilli in the Coso Range, California, indicate that reverse grading there formed by a fourth mechanism during flow of pumice. Reverse-graded beds in this deposit occur where pumice lapilli fell on slopes at or near the angle of repose and formed as parts of the blanket of accumulating pumice became unstable and flowed downslope. The process of size sorting during such flow is probably analogous to that which sorts sand grains in a reverse fashion during avalanching on the slip faces of sand dunes, attributed by Bagnold (1954a) to a grain-dispersive pressure acting on particles subjected to a shear stress. In view of the several ways in which air-fall pyroclastic debris may become reverse graded, caution is advised in interpretation of the origin of this structure both in modern and in ancient deposits.
In the long history of Minoan civilization two great catastrophes are discernible, of which the famous Cretan palaces themselves provide the chief source of our knowledge. Everywhere the catastrophes are seen to be contemporaneous. We can distinguish a period of the first palaces (MM) and a subsequent period of the second palaces (LM). There is no perceptible break in the development of the civilization as a result of these catastrophes. For this reason, the theories that the palaces were overthrown by invaders from abroad aroused opposition from the first. Usually the Achaeans—and even the Hyksos—were suggested as the destroyers. By this theory, however, it was not possible to explain two facts : the decorative arts continue on their way undisturbed, and the second palaces are built at once on the ruins of the first and are still unfortified. The Cretans would not have been so foolish as gratuitously to provide easy loot for fresh invaders.
Regional distributions of tephra deposits within Upper Quaternary pelagic sediments of the eastern Mediterranean and southern Aegean Seas have been determined from cores of deep-sea sediments. Reconstructions of the extent and thickness of individual ash layers required an evaluation of physical and biological redepositional processes, as well as possible mixing effects during sampling. Slumping or other mass-wasting processes on the sea-floor have significantly altered tephra distributions, as illustrated by regional variations in isopachs for the Z-2 Minoan and Y-5 Campanian Tuff ash layers: local variability can be equally significant as indicated by thickness variations of the Y-5 Campanian Tuff ash layer in a small area of the Mediterranean Ridge. Additional dispersal or mixing due to oceanic currents and biological activity, the latter primarily by benthic burrowing organisms, have also been important physical processes in both pre- and post-burial modification of ash layers.-Author
The latest pottery evidence from Thera indicates that the Akrotiri settlement was not abandoned until ca. 1470 B.C. The pottery assemblages in the so-called LM IB destruction horizon in Crete (at Mallia, Gournia, Pseira, etc.) are also consistent with a date ca. 1470 B.C. It is therefore possible to attribute both destructions to a "single-phase" eruption of the Thera volcano. The layer of humus over the Akrotiri ruins represents decayed mud-brick and roofing clay, and is not the product of a period of twenty to thirty years during which the town lay uninhabited prior to the great tephra eruption. The nature of the destructions at various Cretan sites is more consistent with the hypothesis of devastation by natural forces (shock waves, tsunamis, ash fallout) than by invaders. This position is strengthened by the recent finds of Thera ash in LM IB levels in Crete.
The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 BC, Thira consisted of three lava shields in the N half of the island and a flooded depression surrounded by tuff deposits in the S. The Minoan eruption of about 1400 BC had four distinct phases, each reflecting a different vent geometry and eruption mechanism. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff. -from Authors
The Toba ash occurs extensively in the Indian subcontinent and marks a ca. 74,000-yr-old event. In the Bay of Bengal and Indian Ocean it is about 10 cm thick, whereas in several alluvial basins, it is usually 1-3 m thick. The latter occurs in a partly reworked state but as nearly chemically pure first-cycle sediments, The ash has a broad northwesterly dispersal pattern. Samples of ash from the Indian subcontinent compare closely with the youngest (74,000 yr B.P.) Toba Tuff and the deep-sea Toba ash in bulk chemical composition, REE signature, and bubble-wall shard morphology. However, a more proximally located and thicker (2-5 m) ash-bed, from the alluvial basins in the gneissic area and close to east coast, has a lower magnitude negative Eu anomaly, possibly because of minor contamination by feldspathic silt. Quaternary sediments in the central Narmada and middle Son basins contain rich late and middle Pleistocene mammalian and cultural records. Based on the presence of the ash layer marker and stratigraphic relations, late Pleistocene sediments within the subcontinent can be correlated with those from central India and the deep sea.
The 18 500 yr BP Cape Riva (CR) eruption of Santorini vented several km3 or more of magma, generating 4 eruption units, each of which is discussed. The eruption sampled a zoned magma chamber containing rhyodacite overlying andesite, and leaks of these magmas were manifested as the Skaros-Therasia lavas preceding the CR eruption. Plinian and initial ignimbrite stages occurred while the magma chamber was overpressured; subsequent underpressuring, due to magma discharge, caused fracturing of the chamber roof, caldera collapse, and eruption of pyroclastic flows from multiple vents. Activation and widening of new conduits during collapse resulted in the rapid escalation of discharge rate favoring the formation of lag breccias by: 1) promoting erosion of lithic debris at the surface vent; and 2) raising surface exit pressures, thereby resulting in a dramatic increase in the grain size of the ejecta.-from Author
The Minoan eruption of Santorini produced the following sequence of deposits- a plinian pumice fall deposit, interbedded surtseyan-type ash fall and base surge deposits, mud-flow deposits and ignimbrite interbedded with very coarse, well-sorted flood deposits. The variation of thickness and grain size in the plinian deposit indicates a vent x km west of Thera town. The base surges and surtseyan-type activity is interpreted as the result of sea water entering the magma chamber. The poorly sorted mud-flow deposits and ignimbrite are distinguished on their grain size, temperature and morphological characteristics, which indicate substantial rheological contrasts in the mass flows which produced them. Grain size analyses show wide ranges in the lithic contents of the different types of deposit: ignimbrite (35-60%), mudflows (20-30%) and the pyroclastic fall and base surge deposits (4-15 %). The ignimbrite is enriched in crystals, complemented by depletion in fine air-fall ash beds that interstratify with the ignimbrite. The gas velocity of the plinian phase is estimated as 550 m/s, the eruption column height as greater than 20 km and it is shown that only particles of 2 mm could have reached Minoan Crete.
Determining a reliable calendrical age of the Santorini (Minoan) eruption is necessary to place the impact of the eruption into its proper context within Bronze Age society in the Aegean region. The high-resolution record of the deposition of volcanically produced acids on polar ice sheets, as available in the SO42−time series from ice cores (a direct signal), and the high-resolution record of the climatic impact of past volcanism inferred in tree rings (a secondary signal) have been widely used to assign a 1628/1627BCage to the eruption. The layer of ice in the GISP2 (Greenland) ice core corresponding to 1623±36BC, which is probably correlative to the 1628/1627BCevent, not only contains a large volcanic-SO42−spike, but it contains volcanic glass. Composition of this glass does not match the composition of glass from the Santorini eruption, thus severely challenging the 1620sBCage for the eruption. Similarly, the GISP2 glass does not match the composition of glass from other eruptions (Aniakchak, Mt. St. Helens, Vesuvius) thought to have occurred in the 17th centurybcnor does it match potential Icelandic sources. These findings suggest that an eruption not documented in the geological record is responsible for the many climate-proxy signals in the late 1620sBC. Although these findings do not unequivocally discount the 1620sBCage, we recommend that 1628/1627BCno longer be held as the “definitive” age for the Santorini eruption.
Investigations on tephra layers of deep-sea cores from near the island of Santorini showed that only the pumice clasts from the west of the island can be attributed to the Minoan eruption. The tephra layers within the caldera are of uncertain origin.
Additional comment on the new high-precision calibration curves and tables published after this article (Archaeometry 29 (1), 1987, 45–49) was in press.
It is well recognised that radiocarbon dating is a powerful tool capable of providing a detailed chronostratigraphy for areas that have been subjected to recurrent volcanic activity during Late Pleistocene and Holocene time. In such geological contexts carbon-bearing materials suitable for 14C dating are, for the most part, refractory components of pedosphere (e.g. paleosol humic matter) and organic detritus (e.g. charred/uncharred remains of higher plants) which occur within the tephra suites. Since such materials derive initially from atmospheric carbon dioxide, as well as record significant lapses in volcanic activity, it is then often assumed that these conditions comply ideally with the fundamental requirements of the radiocarbon dating theory. However, two recent 14C dating programmes based on materials from the volcanic districts of the Phlegrean Fields and the island of Procida (Campania, southern Italy) yielded evidence to the contrary and, in turn, gave rise to concern over the validity of the 14C dating method when applied in volcanic regions.In particular, it has been found that the conventional radiocarbon ages recorded by modern tree-leaves from the caldera of Solfatara volcano range from ‘modern’ to ca. 5000yr BP. Furthermore, the magnitude of the age anomaly is determined by the pattern of localised dilution of the mean atmospheric 14C concentration by juvenile (14C-free and 13C-enriched) carbon dioxide issuing from nearby volcanic vents. In the case of paleosols, a series of 14C measurements on different chemical fractions isolated from the humic matter in a suite of four paleosol horizons entrained in tephra deposits at Procida island yielded, for each paleosol horizon, a systematic ca. 2000yr scatter of 14C ages. It is shown that such intra-soil age differences, which reflect the origin and history of humic constituents, can be used to afford a better definition of the true paleosol age.
New data about climatically-effective volcanic eruptions during the past several thousand years may be contained in frost-damage zones in the annual rings of trees. There is good agreement in the timing of frost events and recent eruptions, and the damage can be plausibly linked to climatic effects of stratospheric aerosol veils on hemispheric and global scales. The cataclysmic proto-historic eruption of Santorini (Thera), in the Aegean, is tentatively dated to 1628-26 BC from frost-ring evidence.
A simple method is described for detecting annual stratification of ice cores, and layers of high acidity due to violent volcanic eruptions in the past. The method is based on a relationship between the H 3O + concentration (pH) of melted samples and the electrical current between two brass electrodes moved along the cleaned ice-core surface. Acidity and current profiles are shown through layers deposited soon after historically well-known volcanic eruptions, such as Katmai, AD 1912, Tambora, AD l8l5, Laki, AD 1973, Hekla, AD 1104, and Thera (Santorin) c.l400 B.C. High-acidity layers seem to be the cause for the internal radio- echo layers in polar ice sheets.-from Author
Analyses of tephra in abyssal piston cores collected during cruises of
R/V Trident show that the Minoan eruption produced at least 28
km3 of tephra (13 km3 dense rock equivalent). A
layer up to 5 cm thick must have been deposited on eastern Crete.
Dispersal of volcanic ash from the violent Bronze Age (Minoan) eruption of the Santorini volcano in the southern Aegean has been the subject of much research1–6. A sediment core taken from a small mountain lake in western Turkey contains a layer of tephra from the Minoan eruption. The Santorini origin is established by refractive index, major glass chemistry, and stratigraphic position. The lake is located north of the recognized dispersal area for the tephra. This deposit provides evidence for dispersal of the Minoan tephra well to the northeast of Santorini, and implies that the west coast of Asia Minor must have experienced heavy tephra fallout from the Bronze Age eruption. The Minoan tephra has not previously been identified from sites in Turkey.
The eruption on Santorini (Thera: 36.40° N, 25.40° E) in the Aegean Sea during Late Minoan time is considered the most violent volcanic event in the Mediterranean in the second millennium BC. The eruption buried a number of developing Bronze Age settlements on the island (one of which is presently being excavated at the village Akrotiri1) and spread huge amounts of tephra over the eastern Mediterranean and adjacent lands2–7. This event is therefore an important time marker both for archaeologists and Earth scientists. A dating of the eruption has previously been attempted by archaeological inference1,8 and by radiocarbon dating, but the two methods have tended to give ages that deviate by up to 150 years. Here we present a new ice-core dating of the eruption, which suggests an age of 1645 BC based on variations in acid fallout in the annual ice layers in a core drilled at the site Dye 3 (65.18° N, 43.49° W) in South Greenland.
THE caldera walls of the ring-islands Thera and Therasia, belonging to the Santorini Group (Fig. 1) in the southern Aegean, are built up of lavas and pyroclastics, among which pumice layers are particularly striking (Fig. 2). The top layer (Bo, abbreviated from the German term “oberer Bimsstein”1) comprises air-fall pumice and ashes, overlain by thick pyroclastic flows2,3. It was produced by the catastrophic outburst of the Thera volcano in late-Minoan time, which culminated in the collapse of the Santorini caldera4. The decline of the Minoan civilisation around 1500 BC has been attributed to these volcanic events5. The Upper Pumice Series buried late-Minoan settlements on Thera and Therasia, and an important settlement near Akrotiri in the southern part of Thera is still being excavated6. The Upper Pumice Series rests on an old soil horizon (black layer in Fig. 2) in which remnants of late-Minoan houses were found in 1869 (ref. 7). Wood from such houses found at the base of the pumice quarries south of the town of Fira has provided radiocarbon dates of 3,370±100 yr b.p. (ref. 8). This age coincides exactly with archaeological data for the destruction of late-Minoan settlements on Santorini6.
Two volcanic ash layers have been correlated in deep sea piston cores from the eastern Mediterranean1,2. The lower ash layer (n = 1.521) occurs between late Würm carbonate sediment and originated in an eruption more than 25,000 years ago. The upper ash layer (n = 1.509) occurs in postglacial carbonate sediment less than 5,000 yr old. Patterns of distribution indicate that both beds of volcanic ash originated in eruptions of Santorini (Aegean Sea).
Archaeomagnetic dating on the Minoan ash horizons of the Santorini volcano and on fired destruction levels at late Minoan sites on Crete demonstrates that the basal (Plinian) air-fall ash of the first ‘Minoan’ pumice is contemporaneous with the destruction levels on central Crete, while the higher ‘Minoan’ ashes are contemporaneous with the destruction levels in extreme eastern Crete. These destruction levels were almost certainly caused by seismic activity, rather than the ash fall The determination of a time gap between these events lead to a reappraisal of the archaeological evidence and is important volcanologically.
Abstrct
Santorini volcanic field has had 12 major (1–10 km ³ or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption.
Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.
Five deposits of Santorini tephra have been found in the excavations currently under way at the site of Mochlos which lies on the north coast of Crete about 140 km to the south of Santorini. This paper provides refractive index and trace element analyses for the largest of these deposits, examines the stratigraphy of all five deposits, and notes the chronological implication of this stratigraphy for the Late Minoan IB period, the floruit of Minoan civilization.
A program of geophysical research was carried out as a preliminary stage of study of the Santorini volcanic group. This area is of remarkable geothermal and volcanological interest, and the definition of a volcanological structural model is the starting point for an understanding of the local geodynamic processes.Gravity, magnetic and geoelectrical data proved that:(i)
the core of the volcanic edifice consists of a sedimentary-metamorphic basement;
(ii)
the basement is tectonically disturbed and a linear tectonic system produces a graben-type structure in the middle part of the area.