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A Minoan and a Neolithic tsunami recorded in coastal sediments of Ios Island, Aegean Sea, Greece
Abstract
In this work, we document two distinct tsunami deposits on the coasts of Ios Island, Aegean Sea, Greece. The younger tsunami deposit, dated 1831–1368 cal. BCE, includes both marine sediments and pumices from the ~1600 BCE Minoan eruption of Santorini volcano. This is the first evidence of the Minoan tsunami in the Cycladic Islands North of Santorini. Tsunami waves inundated the Manganari coastal plain, southern coast of Ios, over a distance >200 m (>2 m a.s.l.). The second tsunami deposit reworks pumice from the 22 ka Cape Riva eruption mixed with marine sediment. We assume a Neolithic age for this major tsunami, with a wave runup >13 m a.s.l. on the southern and eastern coasts of Ios. The source of this tsunami - volcanic eruption, landslide, or earthquake - remains unknown. Additionally, we provide the first on-land evidence of Cape Riva deposits outside Santorini, thus questioning previous estimates on the magnitude of this eruption.
... The Minoan eruption is associated with a significant tsunami, whose deposits have been identified on the surrounding islands, on the northern coast of Crete, and as far as Turkey and Israel (Bruins et al., 2008;Novikova et al., 2011;Lespez et al., 2021;Paris et al., 2022). There is an ongoing debate about the tsunami source mechanisms during the Minoan eruption, with caldera collapse and the emplacement of pyroclastic flows being the primary candidates in the publications above. ...
The Minoan eruption of Santorini is one of the largest Holocene volcanic events and produced several cubic kilometers of pyroclastic flows emplaced on the submerged flanks of the volcano. Marine geophysical surveys reveal a multitude of undulating seafloor bedforms (USBs) around Santorini. While similar structures are known from other volcanoes worldwide, Santorini offers the unique opportunity to relate USB formation with volcanic processes during one of the best-studied volcanic eruptions worldwide. In this study, we combine high-resolution seismic reflection data with multibeam echosounder bathymetry to reveal the internal architecture of USBs around Santorini and to relate their morphological characteristics to formational processes. The USBs around Santorini were formed during the Minoan eruption and represent the seafloor expression of mass transport deposits. Three types of deposits differ in composition or origin. (1) Depositional USBs, which can only be found to the north of the island, where Minoan eruption ignimbrites reach their maximum thickness and the undulating topography is the result of thrusting within the deposit. (2) USBs related to slope failures of volcaniclastics from the entire Thera Pyroclastic Formation, which can be found east, south, and west of the island. (3) USBs associated with deep-seated deformation, which occurs on the southwestern flank along an area affected by rift tectonics and extends to a depth of more than 200 m below the seafloor. In cases (2) and (3), the USBs are formed upslope by block rotation and downslope by thrusting. Our study indicates that these processes may have contributed to the generation of the devastating Minoan tsunami. Since Santorini is located in one of the most tectonically active regions in the Mediterranean, capable of producing earthquakes with magnitude M7+, our study has important implications for hazard assessment. A strong earthquake located close to the island may have the potential to reactivate slope instabilities posing a previously undetected but potentially significant tsunami hazard.
On the evening of 15 January 2022, the Hunga Tonga-Hunga Ha’apai volcano1 unleashed a violent underwater eruption, blanketing the surrounding land masses in ash and debris2, 3. The eruption generated tsunamis observed around the world. An event of this type last occurred in 1883 during the eruption of Krakatau4, and thus we have the first observations of a tsunami from a large emergent volcanic eruption captured with modern instrumentation. Here we show the explosive eruption generated waves through multiple mechanisms, including: 1) air-sea coupling with the initial and powerful shockwave radiating out from the explosion in the immediate vicinity of the eruption, 2) the collapse of the water-cavity created by the underwater explosion, and 3) air-sea coupling with the air-pressure pulse that circled the earth multiple times, leading to a global tsunami. In the near-field, the tsunami impacts are strongly controlled by the water-cavity source, while the far-field tsunami, which was unusually persistent, can be largely described by the air-pressure pulse mechanism. Catastrophic damage in some harbors in the far-field was averted by just tens of centimeters, implying that a modest sea level rise combined with a future, similar event would lead to a step-function increase in impacts on infrastructure. Piecing together the complexity of this event has broad implications to the hazard in similar geophysical settings, suggesting a currently neglected source of global tsunamis.
Volcanoes can produce tsunamis through earthquakes, caldera and flank collapses, pyroclastic flows, or underwater explosions1,2,3,4. These mechanisms rarely displace enough water to trigger transoceanic tsunamis. Violent volcanic explosions, however, can cause global tsunamis1,5 by triggering acoustic-gravity waves6,7,8 that excite the atmosphere-ocean interface. The colossal eruption of the Hunga Tonga-Hunga Ha'apai volcano and ensuing tsunami is the first global volcano-triggered tsunami recorded by modern, worldwide dense instrumentation, thus providing a unique opportunity to investigate the role of air-water coupling processes in tsunami generation and propagation. Here we use sea-level, atmospheric and satellite data from across the globe, along with numerical and analytical models, to demonstrate that this tsunami was driven by a constantly moving source in which the acoustic-gravity waves radiating from the eruption excite the ocean and transfer energy into it via resonance. A direct correlation between the tsunami and the acoustic-gravity waves’ arrival times confirms that these phenomena are closely linked. Our models also show that the unusually fast travel times and long duration of the tsunami, as well as its global reach, are consistent with an air-water coupled source. This coupling mechanism has clear hazard implications, since it leads to higher waves along landmasses that rise abruptly from long stretches of deep ocean waters.
Significance
The significance of this study is multi-faceted, touching upon methodological advances in multidisciplinary approaches (earth sciences/geology–archaeology) as well as contributing to the historical and chronological understanding of the Late Bronze Age Thera eruption impacts. Our study presents physical evidence that very large, damaging tsunamis arrived even in the northern Aegean, an area previously assumed to be affected only by ash fallout. The tsunami deposits at Çeşme-Bağlararası contain the first victims (human and dog) ever identified related to the eruption and its immediate consequences. The work also introduces nine radiocarbon ages directly from the event deposit that will be of great interest and cause significant discussion amongst scholars, particularly given their context within a well-constrained, undisturbed, stratigraphic archaeological sequence.
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.
Sea-level change is thought to influence the frequencies of volcanic eruptions on glacial to interglacial timescales. However, the underlying physical processes and their importance relative to other influences (e.g. magma recharge rates), remain poorly understood. Here we compare a ~360 kyr long record of effusive and explosive eruptions from the flooded caldera volcano at Santorini (Greece) with a high resolution sea-level record spanning the last four glacial-interglacial cycles. Numerical modelling shows that when the sea level falls by 40 m below the present-day level, the induced tensile stresses in the roof of the magma chamber of Santorini trigger dyke injections. As the sea-level continues to fall to -70 or -80 m, the induced tensile stress spreads throughout the roof so that some dykes reach the surface to feed eruptions. Similarly, the volcanic activity gradually disappears after the sea-level rises above -40 m. Synchronising Santorini’s stratigraphy with the sea-level record by using tephra layers in marine sediment cores shows that 208 out of 211 eruptions (both effusive and explosive) occurred during periods constrained by sea level falls (below -40m) and subsequent rises, suggesting a strong absolute sea-level control on the timing of eruptions on Santorini – a result that probably applies to many other volcanic islands around the world.
A geomorphological survey immediately west of the Minoan town of Malia (Crete) shows that a tsunami resulting from the Bronze Age Santorini eruption reached the outskirts of the Palatial center. Sediment cores testify a unique erosional event during the Late Minoan period, followed locally by a high energy sand unit comprising marine fauna. This confirms that a tsunami impacted northern Crete and caused an inundation up to 400 m inland at Malia. We obtained a radiocarbon range of 1744–1544 BCE for the secure pre-tsunami context and an interval 1509–1430 BCE for the post-event layer. Examination of tsunami deposits was used to constrain run-up not exceeding 8 m asl. The results open the field for new research on the Bronze Age Santorini tsunami regarding both impact and consequences for the Minoan civilization.
The concentration of radiocarbon ( ¹⁴ C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their ¹⁴ C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0–55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 ¹⁴ C curve and reconstructed changes in CO 2 obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric ¹⁴ C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric ¹⁴ C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for ΔR , the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/ .
Geological Records of Tsunamis and Other Extreme Waves provides a systematic compendium with concise chapters on the concept and history of paleotsunami research, sediment types and sediment sources, field methods, sedimentary and geomorphological characteristics, as well as dating and modeling approaches. By contrasting tsunami deposits with those of competing mechanisms in the coastal zone such as storm waves and surges, and by embedding this field of research into the wider context of tsunami science, the book is also relevant to readers interested in paleotempestology, coastal sedimentary environments, or sea-level changes, and coastal hazard management.
The effectiveness of paleotsunami records in coastal hazard-mitigation strategies strongly depends on the appropriate selection of research approaches and methods that are tailored to the site-specific environment and age of the deposits. In addition to summarizing the state-of-the-art in tsunami sedimentology, Geological Records of Tsunamis and Other Extreme Waves guides researchers through establishing an appropriate research design and how to develop reliable records of prehistoric events using field-based and laboratory methods, as well as modeling techniques.
X‐ray tomography is used to analyse the grain size and sedimentary fabric of two tsunami deposits in the Marquesas Islands (French Polynesia, Pacific Ocean) which are particularly exposed to trans‐Pacific tsunamis. One site is located on the southern coast of Nuku Hiva Island (Hooumi) and the other one is on the southern coast of Hiva Oa Island (Tahauku). Results are compared with other techniques such as two‐dimensional image analysis on bulk samples (particle analyser) and anisotropy of magnetic susceptibility. The sedimentary fabric is characterised through three‐dimensional stacks of horizontal slices (following a vertical step of 2.5 mm along the cores), while grain‐size distribution is estimated from two‐dimensional vertical slices (following a step of 2 mm). Four types of fabric are distinguished: (a) moderate to high angle (15 to 75°); (b) bimodal low‐angle (<15°); (c) low to high angle with at least two different orientations; and (d) dispersed fabric. The fabric geometry in a tsunami deposit is not only controlled by the characteristics of the flow itself (current strength, flow regime, etc.) but also sediment concentration, deposition rate and grain‐size distribution. There is a notable correlation between unimodal high‐angle fabric – type (a) – and finely‐skewed grain‐size distribution. The two tsunami deposits studied represent two different scenarios of inundation. As demonstrated here, X‐ray tomography is an essential method for characterising past tsunamis from their deposits. The method can be applied to many other types of sediments and sedimentary rocks.
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Caldera-forming eruptions of island volcanoes generate tsunamis by the interaction of different eruptive phenomena with the sea. Such tsunamis are a major hazard, but forward models of their impacts are limited by poor understanding of source mechanisms. The caldera-forming eruption of Santorini in the Late Bronze Age is known to have been tsunamigenic, and caldera collapse has been proposed as a mechanism. Here, we present bathymetric and seismic evidence showing that the caldera was not open to the sea during the main phase of the eruption, but was flooded once the eruption had finished. Inflow of water and associated landsliding cut a deep, 2.0–2.5 km³, submarine channel, thus filling the caldera in less than a couple of days. If, as at most such volcanoes, caldera collapse occurred syn-eruptively, then it cannot have generated tsunamis. Entry of pyroclastic flows into the sea, combined with slumping of submarine pyroclastic accumulations, were the main mechanisms of tsunami production.
Volcanic tsunamis are generated by a variety of mechanisms, including volcano-tectonic earthquakes, slope instabilities, pyroclastic flows, underwater explosions, shock waves and caldera collapse. In this review, we focus on the lessons that can be learnt from past events and address the influence of parameters such as volume flux of mass flows, explosion energy or duration of caldera collapse on tsunami generation. The diversity of waves in terms of amplitude, period, form, dispersion, etc. poses difficulties for integration and harmonization of sources to be used for numerical models and probabilistic tsunami hazard maps. In many cases, monitoring and warning of volcanic tsunamis remain challenging (further technical and scientific developments being necessary) and must be coupled with policies of population preparedness.
The date of the Late Bronze Age Minoan eruption of the Thera volcano has provoked much debate among archaeologists, not least in a recent issue of Antiquity (‘Bronze Age catastrophe and modern controversy: dating the Santorini eruption’, March 2014). Here, the authors respond to those recent contributions, citing evidence that closes the gap between the conclusions offered by previous typological, stratigraphic and radiometric dating techniques. They reject the need to choose between alternative approaches to the problem and make a case for the synchronisation of eastern Mediterranean and Egyptian chronologies with agreement on a ‘high’ date in the late seventeenth century BC for the Thera eruption.
The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the generation of large volumes of silicic melt, followed by the segregation of that melt and its accumulation in the upper crust. The 21.8 ± 0.4-ka Cape Riva eruption of Santorini discharged >10 km3 of crystal-poor dacitic magma, along with 3 of hybrid andesite, and collapsed a pre-existing lava shield. We have carried out a field, petrological, chemical, and high-resolution 40Ar/39Ar chronological study of a sequence of lavas discharged prior to the Cape Riva eruption to constrain the crustal residence time of the Cape Riva magma reservoir. The lavas were erupted between 39 and 25 ka, forming a ∼2-km3 complex of dacitic flows, coulées and domes up to 200 m thick (Therasia dome complex). The Therasia dacites show little chemical variation with time, suggesting derivation from one or more thermally buffered reservoirs. Minor pyroclastic layers occur intercalated within the lava succession, particularly near the top. A prominent pumice fall deposit correlates with the 26-ka Y-4 ash layer found in deep-sea sediments SE of Santorini. One of the last Therasia lavas to be discharged was a hybrid andesite formed by the mixing of dacite and basalt. The Cape Riva eruption occurred no more than 2,800 ± 1,400 years after the final Therasia activity. The Cape Riva dacite is similar in major element composition to the Therasia dacites, but is poorer in K and most incompatible trace elements (e.g. Rb, Zr and LREE). The same chemical differences are observed between the Cape Riva and Therasia hybrid andesites, and between the calculated basaltic mixing end-members of each series. The Therasia and Cape Riva dacites are distinct silicic magma batches and are not related by shallow processes of crystal fractionation or assimilation. The Therasia lavas were therefore not simply precursory leaks from the growing Cape Riva magma reservoir. The change 21.8 ky ago from a magma series richer in incompatible elements to one poorer in those elements is one step in the well documented decrease with time of incompatibles in Santorini magmas over the last 530 ky. The two dacitic magma batches are interpreted to have been emplaced sequentially into the upper crust beneath the summit of the volcano, the first (Therasia) then being partially, or wholly, flushed out by the arrival of the second (Cape Riva). This constrains the upper-crustal residence time of the Cape Riva reservoir to less than 2,800 ± 1,400 years, and the associated time-averaged magma accumulation rate to >0.004 km3 year-1. Rapid ascent and accumulation of the Cape Riva dacite may have been caused by an increased flux of mantle-derived basalt into the crust, explaining the occurrence of hybrid andesites (formed by the mixing of olivine basalt and dacite in approximately equal proportions) in the Cape Riva and late Therasia products. Pressurisation of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity to explosive eruption and caldera collapse.
Kolumbo submarine volcano, located NE of Santorini caldera in the Aegean Sea, has only had one recorded eruption during historic times (1650 AD). Tsunamis from this event severely impacted the east coast of Santorini with extensive flooding and loss of buildings. Recent seismic studies in the area indicate a highly active region beneath Kolumbo suggesting the potential for future eruptive activity. Multibeam mapping and remotely operated vehicle explorations of Kolumbo have led to new insights into the eruptive processes of the 1650 AD eruption and improved assessments of the mechanisms by which tsunamis were generated and how they may be produced in future events. Principal mechanisms for tsunami generation at Kolumbo include shallow submarine explosions, entrance of pyroclastic flows into the sea, collapse of rapidly accumulated pyroclastic material, and intense eruption-related seismicity that may trigger submarine slope collapse. Compared with Santorini, the magnitude of explosive eruptions from Kolumbo is likely to be much smaller but the proximity of the volcano to the eastern coast of Santorini presents significant risks even for lower magnitude events.
The advection–diffusion model TEPHRA2 has been used in conjunction with the downhill simplex method (DSM) and one-at-a-time (OAT) inversion methods to reconstruct the eruption conditions and seasonality consistent with the deposit patterns from the Bronze Age (‘Minoan’) eruption of Santorini. We investigated three datasets representing different depositional environments (proximal terrestrial, distal terrestrial and deep-sea core), in order to determine source conditions such as plume height, erupted mass and grain-size and recreate the tephra fall deposit from the Plinian, co-ignimbrite and combined eruptive phases. The results of the DSM and OAT method agreed adequately well with each other for erupted mass, plume height and grain-size distribution. Both approaches were able to successfully recreate the Plinian deposit but estimating conditions that created the co-ignimbrite and deep-sea core dataset were less successful. The reduced agreement is the result of the low quantity (6 to 28 deposit points) and quality (inconsistent deposit depths at localities adjacent to each other) of the datasets, and the different dynamics between co-ignimbrite and Plinian columns, with the former not well represented in the model. Different sampling methods between archaeological and volcanological disciplines and post-depositional processes which have acted on the tephra deposits since the Bronze Age can explain the discrepancy between these computed and observed deposits. The seasonality of the Minoan eruption was investigated by using seasonal wind profiles for winter, spring, summer and autumn. We find that the Bronze Age eruption of Santorini is likely to have during the spring and summer months with a main dispersal axis aligned East. Crete would have received very little ash fall, and the eruption would not have caused much disruption to the life of the inhabitants of the island.
Tsunami generated by the Late Bronze Age (LBA) eruption of Thera were simulated using synthetic tide records produced for selected nearshore (˜20 m depths) sites of northern Crete, the Cyclades Islands, SW Turkey and Sicily. Inundation distances inland were also calculated along northern Crete. Modelling was performed by incorporating fully non-linear Boussinesq wave theory with two tsunamigenic mechanisms. The first involved the entry of pyroclastic flows into the sea, assuming a thick (55 m; 30 km3) flow entering the sea along the south coast of Thera in three different directions all directed towards northern Crete, then a thin pyroclastic flow (1 m; 1.2 km3) entering the sea along the north coast of Thera directed towards the Cyclades Islands. Flows were modelled as a solid block that slowly decelerates along a horizontal surface. The second mechanism assumed caldera collapse, of 19 km3 and 34 km3 modelled as a dynamic landslide producing rapid vertical displacements. Calculated nearshore wave amplitudes varied from a few metres to 28 m along northern Crete from pyroclastic flows, and up to 19 m from caldera collapse (34 km3 volume). Inundation distances on Crete were 250-450 m. Waves produced by pyroclastic flows were highly focused, however, as a function of sea entry direction. Smaller volume pyroclastic flows produced nearshore wave amplitudes up to 4 m in the Cyclades islands north of Thera. Wave amplitudes in the Cyclades from smaller volume caldera collapse (19 km3) were up to 24 m, whereas in SW Turkey were as low as 2.1 and 0.8 m (Didim and Fethye where LBA tsunami deposits have been found). Wave amplitudes for the larger volume caldera collapse (34 km3) were generally 2.5-3 times larger than those generated by the smaller volume collapse (19 km3). These results provide estimates for understanding possible consequences of tsunami impact in LBA coastal zones, thus providing criteria at archaeological sites for detecting inundation damage, as well as for contemporary hazard assessment; they also provide additional criteria for deciphering homogenite layers in the abyssal stratigraphy of the Ionian and eastern Mediterranean Seas.
The Hellenic arc is a terrane of extensive Quaternary volcanism. One of the main centers of explosive eruptions is located on Thera (Santorini), and the eruption of the Thera volcano in late Minoan time (1600 1300 B.C.) is considered to have been the most significant Aegean explosive volcanism during the late Holocene. The last eruptive phase of Thera resulted in an enormous submarine caldera, which is believed to have produced tsunamis on a large scale. Evidence suggesting seawater inundation was found previously at some archaeological sites on the coast of Crete; however, the cause of the tsunami and its effects on the area have not been well understood. On the Aegean Sea coast of western Turkey (Didim and Fethye) and Crete (Gouves), we have found traces of tsunami deposits related to the Thera eruption. The sedimentological consequences and the hydraulics of a Thera-caused tsunami indicate that the eruption of Thera volcano was earlier than the previous estimates and the tsunami did not have disruptive influence on Minoan civilization.
As many as 20 air-borne tephra layers have been identified in the upper Quaternary sequence of deep-sea cores from the eastern Mediterranean. Petrographical examination based on refractive index, phenocryst content, and chemical composition of the volcanic glass distinguishes the parent magma types: (1) tephritic, (2) alkalic-trachytic, (3) peralkalic-pantelleritic, and (4) calc-alkalic andesitic to rhyodacitic and alkali-basaltic. Tephra layers could be correlated with the following source volcanoes: Somma-Vesuvius, Roman district, Phlegraean Fields and Ischia, Pantelleria, Aeolian Islands, Mount Etna, and Aegean arc. The distribution for single layers has been traced over more than 2,000 km.
Tsunami were generated during the Late Bronze Age (LBA) eruption of the island of Thera, in the southern Aegean Sea, by both caldera collapse, and by the entry of pyroclastic surges/flows and lahars/debris flows into the sea. Tsunami generated by caldera collapse propagated to the west producing deep-sea sedimentary deposits in the eastern Mediterranean Sea known as homogenites; open-ocean wave heights of about 1.917 m are estimated. Tsunami generated by the entry of pyroclastic flows/surges and lahars/debris flows into the sea propagated in all directions around the island; wave heights along coastal areas were about 712 m as estimated from newly identified tsunami deposits on eastern Thera as well as from pumice deposits found at archaeological sites on northern and eastern Crete.
Understanding the nature and impacts of tsunamis within the Aegean Sea region ofGreece is of importance to both the academic community and those organisationsconcerned with tsunami disaster management. In order to determine hazard and riskand consequently pre-plan mitigative strategies, it is necessary to analyse historical(documentary) and geological records of former tsunami events. Therefore, firstlythis paper provides a summary of the written sources of information on Aegeantsunamis paying particular attention to published catalogues. From the availabledata, it is noted that a large number of events have been reported during the last3500 years. Secondly, the paper provides a review of the published on-shore(terrestrial) geological records of tsunamis within the region. From this analysisit is seen that little geological evidence has been identified for the large numberof tsunamis reported in the catalogues. Thirdly, the paper considers the reliabilityof the written and geological records and how problems of accuracy, coverage,extent and reliability, may have potential implications for the estimation of hazardand risk. The paper concludes by making recommendations for disaster managers,geologists and historians to work closely together.
We present the volcanic ash and tsunami record of the Minoan Late Bronze Age Eruption of Santorini (LBAES) in a distal setting in southwestern Turkey. In one of the drilled cores at the Letoon Hellenic antique site on Eşençay Delta, we encountered a 4 cm thick tephra deposit underlain by 46 cm thick tsunami‐deposited sand (tsunamite), and an organic‐rich layer that we 14C dated to 3295 ± 30 bp or 1633 bc. The relationship between Santorini distal volcanic ash and underlying tsunamite is described and interpreted. LBAES occurred in four main phases: (1) plinian; (2) phreatomagmatic; (3) phreatomagmatic with mudflows; and (4) ignimbritic flows and co‐ignimbrite tephra falls. In this study, we aim to understand which eruptive phases generate distal ash during the Minoan eruptive sequence by examining the 3D surface morphology of ash formed by different fragmentation processes. To that end, we used numerous statistical multivariates, 3D fractal dimension of roughness, and a new textural parameter of surface area‐3D/plotted area‐2D to characterise the eruption dynamics. Based on ash surface morphologies and the calculated statistical parameters, we propose that that distal ash is represented by a single layer composed of well‐mixed (coarse to fine) magmatic and phreatomagmatic ash.
The island of Santorini in the Aegean Sea is one of the world’s most violent active volcanoes. Santorini has produced numerous highly explosive eruptions over at least the past ∼360 kyrs that are documented by the island’s unique proximal tephra record. However, the lack of precise eruption ages and comprehensive glass geochemical datasets for proximal tephras has long hindered the development of a detailed distal tephrostratigraphy for Santorini eruptions. In light of these requirements, this study develops a distal tephrostratigraphy for Santorini covering the past ∼360 kyrs, which represents a major step forward towards the establishment of a tephrostratigraphic framework for the Eastern Mediterranean region. We present new EPMA glass geochemical data of proximal tephra deposits from twelve Plinian and numerous Inter-Plinian Santorini eruptions and use this dataset to establish assignments of 28 distal marine tephras from three Aegean Sea cores (KL49, KL51 and LC21) to specific volcanic events. Based on interpolation of sapropel core chronologies we provide new eruption age estimates for correlated Santorini tephras, including dates for major Plinian eruptions, Upper Scoriae 1 (80.8 ± 2.9 ka), Vourvoulos (126.5 ± 2.9 ka), Middle Pumice (141.0 ± 2.6 ka), Cape Thera (156.9 ± 2.3 ka), Lower Pumice 2 (176.7 ± 0.6 ka), Lower Pumice 1 (185.7 ± 0.7 ka), and Cape Therma 3 (200.2 ± 0.9 ka), but also for 17 Inter-Plinian events. Older Plinian and Inter-Plinian activity between ∼310 ka and 370 ka, documented in the distal terrestrial setting of Tenaghi Philippon (NE Greece), is independently dated by palynostratigraphy and complements the distal Santorini tephrostratigraphic record.
New bathymetric and seismic reflection data from the Santorini–Amorgos Tectonic Zone in the southern Cyclades have been analysed and a description of the morphology and tectonic structure of the area has been presented. The basins of Anhydros, Amorgos and Santorini–Anafi have been distinguished together with the intermediate Anhydros Horst within the NE-SW oriented Santorini–Amorgos Tectonic Zone which has a length of 60–70 km and a width of 20–25 km. The basins represent tectonic grabens or semi-grabens bordered by the active marginal normal faults of Santorini–Anafi, Amorgos, Ios, Anhydros and Astypalaea. The Santorini–Anafi, Amorgos and Ios marginal faults have their footwall towards the NW where Alpine basement occurs in the submarine scarps and their hangingwall towards the southeast, where the Quaternary sediments have been deposited with maximum thickness of 700 m. Six sedimentary Units 1–6 have been distinguished in the stratigraphic successions of the Santorini–Anafi and the western Anhydros Basin whereas in the rest area only the upper four Units 3–6 have been deposited. This shows the expansion of the basin with subsidence during the Quaternary due to ongoing extension in a northwest-southeast direction. Growth structures are characterized by different periods of maximum deformation as this is indicated by the different sedimentary units with maximum thickness next to each fault. Transverse structures of northwest-southeast direction have been identified along the Santorini–Amorgos Tectonic Zone with distinction of the blocks/segments of Santorini, Anhydros/Kolumbo, Anhydros islet and Amorgos. Recent escarpments with 7–9 m offset observed along the Amorgos Fault indicate that this was activated during the first earthquake of the 7.5 magnitude 1956 events whereas no recent landslide was found in the area that could be related to the 1956 tsunami.
Tephra from the Cape Riva (Y-2) eruption of Santorini has been found across the eastern Mediterranean. It presents an important link between marine and terrestrial records. A Poisson process (P Sequence) age-depth prior, with model averaging, is used to model individual previously published radiocarbon sequences, cross-linked with an exponential phase model parameter to obtain a robust age. Multiple sequences and ¹⁴ C determinations from 3 eastern Mediterranean data sets (Seymour et al. 2004; Margari et al. 2009; Müller et al. 2011; Roeser et al. 2012) are used in the model. The modeled age of the Y-2 tephra produced within this study is 22,329–21,088 cal BP at 95.4% probability.
The 184 ka Lower Pumice 1 eruption sequence records a complex history of eruption behaviours denoted by two significant eruptive phases: (1) a minor precursor (LP1-Pc) and (2) a major Plinian phase (LP1-A, B, C). The precursor phase produced 13 small-volume pyroclastic fallout, surge and flow deposits, which record the transition from a dominantly magmatic to a phreatomagmatic eruptive style, and exhibit a normal (dacite to andesitic-dacite) to reverse (andesitic-dacite to dacite) compositional zonation of juvenile pyroclasts in the stratigraphy. Incipient bioturbation and variability in unit thickness and lithology, reflects multiple time breaks and highlights the episodic nature of volcanism prior to the main Plinian eruption phase. The Plinian magmatic eruption phase is defined by three major stratigraphic divisions, including a basal pumice fallout deposit (LP1-A), an overlying valley-confined ignimbrite (LP1-B) and a compositionally zoned (rhyodacite to basaltic andesite) lithic-rich lag breccia (LP1-C), which caps the sequence. This sequence records the initial development of a buoyant convective eruption column and the transition to eruption column and catastrophic late-stage caldera collapse events. Similarities in pyroclast properties (i.e., chemistry, density), between the Plinian fallout (LP1-A1) and pyroclastic flow (LP1-B4) deposits, indicate that changes in magma properties exerted no influence on the dynamics and temporal evolution of the LP1 eruption. Conversely, lithic breccias (LP1-B3) at the base of the ignimbrite (LP1-B4) suggest the transition from a buoyant convective column to column collapse was facilitated by mechanical erosion of the conduit system and/or the initiation of caldera collapse, leading to vent widening, an increase in magma discharge rate and the increased incorporation of lithics into the eruption column, causing mass overload. Lithic-rich lag breccia deposits (LP1-C), which cap the eruption sequence, record incremental, high-energy caldera collapse events, whereby downfaulting occurred in discrete jumps, resulting in variable magma discharge rates and the development of a fissure vent system.
The 1650 AD explosive eruption of Kolumbo submarine volcano (Aegean Sea, Greece) generated a destructive tsunami. In this paper we propose a source mechanism of this poorly documented tsunami using both geological investigations and numerical simulations. Sedimentary evidence of the 1650 AD tsunami was found along the coast of Santorini Island at maximum altitudes ranging between 3.5 m a.s.l. (Perissa, southern coast) and 20 m a.s.l. (Monolithos, eastern coast), corresponding to a minimum inundation of 360 and 630 m respectively. Tsunami deposits consist of an irregular 5 to 30 cm thick layer of dark grey sand that overlies pumiceous deposits erupted during the Minoan eruption and are found at depths of 30–50 cm below the surface. Composition of the tsunami sand is similar to the composition of the present-day beach sand but differs from the pumiceous gravelly deposits on which it rests. The spatial distribution of the tsunami deposits was compared to available historical records and to the results of numerical simulations of tsunami inundation. Different source mechanisms were tested: earthquakes, underwater explosions, caldera collapse, and pyroclastic flows. The most probable source of the 1650 AD Kolumbo tsunami is a 250 m high water surface displacement generated by underwater explosion with an energy of ~ 2 × 1016 J at water depths between 20 and 150 m. The tsunamigenic explosion(s) occurred on September 29, 1650 during the transition between submarine and subaerial phases of the eruption. Caldera subsidence is not an efficient tsunami source mechanism as short (and probably unrealistic) collapse durations (< 5 min) are needed. Pyroclastic flows cannot be discarded, but the required flux (106 to 107 m3 · s− 1) is exceptionally high compared to the magnitude of the eruption.
The structural evolution of South Aegean Sea is little explored due to the lack of marine seismic data. Our present day understanding is mainly based on some island outcrops and GPS measurements. In this study we discuss the rather incremental opening of the Anydros Basin in the Pliocene during six major tectonic pulses and the subsequent basin fill processes by interpreting seismic data and derived time isochore maps. Between the active pulses basin floor tilting persisted on a much lower rate. Seismic data illustrate the depositional processes in the emerging Anydros Basin. The observation of onlap fill strata, divergent reflection patter, moat channels and contourite drifts imply that deposition was controlled by turbiditiy and contour currents as well as the tilting basin floor. The metamorphic Attico-Cycladic basement shows a rise that aligns along an NW-SE directed axis crossing Anydros island. This axis marks a structural change of the Santorini-Amorgos Ridge and thus represents a major structural boundary. Dip angles of NE–SW trending major faults like the Santorini-Amorgos Fault indicate normal faulting to be the superior mechanism, forming the present horst and graben environment. Hence, the area is likely to be in a state of NW-SE directed extensional stresses forming the asymmetric graben structure of Anydros. Secondary fault clusters strike the same direction but show much steeper dip angles, possibly indicating strike-slip movement or resulting from deformational stresses along the hinge zones of the normal faults. A majority of the faults we discovered are located in the area of earthquake clusters, which is another indication of recent faulting. Ring faults around Kolumbo submarine volcano, result from caldera collapse and mark the diameter of the magma chamber approximately to 20 km.
This paper presents a review of the Japanese policies for tsunami countermeasures before and after the 2011 off the Pacific coast of Tohoku Earthquake and its consequent tsunami. The current status of tsunami geology under the new policies is also discussed. The 2011 event was regarded as an unexpected hazard. Such a large hazard had not been considered for Japanese tsunami countermeasures before, although geological studies have indicated their potential occurrence. Based on lessons learned from the 2011 event, the Japanese government changed policies related to tsunami disaster countermeasures. The salient change is that estimation of the maximum possible earthquake and tsunami along the each coast of Japan is now required. Following this policy change, the maximum possible earthquake and tsunami have been estimated along several coasts of Japan, such as the Nankai Trough region based on seismology, irrespective of past occurrence. Tsunami geology is regarded as an important research field for estimating the maximum possible earthquake and tsunami strength because paleotsunami histories of several thousand years are crucially important for future tsunami risk assessment. However, many issues remain to be resolved to respond to the rapid change of policy. Acceleration of studies, sharing knowledge with governors, and development of schemes for outreach to the public (e.g. a database system) should be considered for improvement of future tsunami countermeasures in Japan.
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.
Here we show for the first time the 3D-structural evolution of an explosive submarine volcano by means of reflection seismic interpretation. Four to five vertically stacked circular and cone-shaped units consisting mainly of volcaniclastics build the Kolumbo underwater volcano which experienced its first eruption > 70 ka ago and its last explosive eruption 1650 AD, 7 km NE of Santorini volcano (southern Aegean Sea). The summed volume of volcaniclastics is estimated to range between 13–22 km3. The entire Kolumbo volcanic complex has a height of ≥ 1 km and a diameter of ≥ 11 km. All volcaniclastic units reveal the same transparent reflection pattern strongly suggesting that explosive underwater volcanism was the prevalent process. Growth faults terminate upwards at the base of volcaniclastic units, thus representing a predictor to an eruption phase. Similarities in seismic reflection pattern between Kolumbo and near-by volcanic cones imply that the smaller cones evolved through explosive eruptions as well. Hence, the central Aegean Sea experienced several more explosive eruptions (≥ 23) than previously assumed, thus justifying further risk assessment. However, the eruption columns from the smaller volcanic cones did not reach the air and– consequently – no sub-aerial pyroclastic surge was created. The Anydros basin that hosts Kolumbo volcanic field opened incrementally NW to SE and parallel to the Pliny and Strabo trends during four major tectonic pulses prior to the onset of underwater volcanism.
The well-documented 1883 eruption of Krakatau volcano (Indonesia) offers an opportunity to couple the eruption’s history with the tsunami record. The aim of this paper is not to re-analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions. Indeed, though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits, they may include juvenile material (e.g. fresh pumice) or be interbedded with distal pyroclastic deposits (ash fall, surges), due to their simultaneity with the eruption. Five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of Java and Sumatra: (1) bioclastic tsunami sands and (2) pumiceous tsunami sands, deposited respectively before and during the Plinian phase (26–27 August); (3) rounded pumice lapilli reworked by tsunami; (4) pumiceous ash fall deposits and (5) pyroclastic surge deposits (only in Sumatra). The stratigraphic record on the coasts of Java and Sumatra, which agrees particularly well with observations of the 1883 events, is tentatively linked to the proximal stratigraphy of the eruption.
The origin of tsunamis in the Mediterranean region and its connected seas, including the Marmara Sea, the Black Sea and the SW Iberian Margin in the NE Atlantic Ocean, is reviewed within the geological and seismotectonic settings of the region. A variety of historical documentary sources combined with evidence from onshore and offshore geological signatures, geomorphological imprints, observations from selected coastal archaeological sites, as well as instrumental records, eyewitnesses accounts and pictorial material, clearly indicate that tsunami sources both seismic and non-seismic (e.g. volcanism, landslides) can be found in all the seas of the region with a variable tsunamigenic potential. Local, regional and basin-wide tsunamis have been documented. An improved map of 22 main tsunamigenic zones and their relative potential for tsunami generation is presented. From west to east, the most important tsunamigenic zones are situated offshore SW Iberia, in the North Algerian margin, in the Tyrrhenian Calabria and Messina Straits, in the western and eastern segments of the Hellenic Arc, in the Corinth Gulf of Central Greece, in the Levantine Sea offshore the Dead Sea Transform Fault and in the eastern side of the Marmara Sea. Important historical examples, including destructive tsunamis associated with large earthquakes, are presented. The mean recurrence of strong tsunamis in the several basins varies greatly but the highest event frequency (1/96 yrs) is observed in the east Mediterranean basin. For most of the historical events it is still unclear which was the causative seismic source and if the tsunami was caused by co-seismic slip, by earthquake-triggered submarine landslides or by a combination of both mechanisms. In pre-historical times, submarine volcanic eruptions (i.e. caldera collapse, massive pyroclastic flows, volcanogenic landslides) and large submarine landslides caused important tsunamis although little is known about their source mechanisms. We conclude that further investigation of the tsunami generation mechanisms is of primary importance in the Mediterranean region. Inputs from tsunami numerical modeling as well as from empirical discrimination criteria for characterizing tsunami sources have been proved particularly effective for recent, well-documented, aseismic landslide tsunamis (e.g., 1963 Corinth Gulf, 1979 Côte d’Azur, 1999 Izmit Bay, 2002 Stromboli volcano). Since the tsunami generation mechanisms are controlled by a variety of factors, and given that the knowledge of past tsunami activity is the cornerstone for undertaking tsunami risk mitigation action, future interdisciplinary research efforts on past tsunamis are needed.
The late-seventeenth century BC Minoan eruption of Santorini discharged 30-60 km3 of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE-SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption—a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite-diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it.
We have simulated the impact of the tsunami generated by the Late Bronze
Age (LBA) volcanic eruption of Santorini on the Eastern Mediterranean.
Two different tsunami triggering mechanisms were considered: a caldera
collapse and pyroclastic flows/surges entering the sea. Simulations
include the "worst" input conditions in order to evaluate the maximum
possible impacts, but also "lighter" input conditions, compatible with
the lack of any tsunami trace on the Northern coasts of Crete. In all
the simulations, tsunami propagation is mainly confined to the Southern
Aegean. Outside the Aegean, the tsunami impact was negligible and not
responsible for the slide-slumping of fine-grained pelagic and/or
hemipelagic sediments considered the sources of the sporadically located
sea-deposits in the Ionian Sea and of the widespread megaturbidite
deposits localized in the Ionian and Sirte Abyssal Plains.
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 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
Pyroclastic fall and flow deposits occupy two distinct fields on an $Md_{\phi}/\sigma_{\phi}$ plot (Inman parameters), and a contoured diagram is given based on 1,600 samples to facilitate comparison of mechanical analyses. Analyses which plot where the fields overlap include rain-flushed ashes and thin flow deposits. Among factors influencing $\sigma_{\phi}$ of fall deposits is the wind: a strong wind will reduce its value. Another is the characteristics of the initial population-the entire assemblage of fragments coming from the vent-which is quite different for crystals than for pumice or lithic components. Each component in a polycomponent deposit has a different grain-size distribution due to this and subsequent air sorting. Histograms or cumulative curves where the weight percentages are plotted against the fall velocity are shown to be more meaningful than those against the grain size, and a quantity V is defined analogous to $\phi$. Ignimbrites are remarkably homogeneous, but two departures are he...
We conduct a comprehensive study of the Amorgos, Greece earthquake and tsunami of 1956 July 09, the largest such event in the Aegean Sea in the 20th century. Systematic relocation of the main shock and 34 associated events defines a rupture area measuring 75 × 40 km. The use of the Preliminary Determination of Focal Mechanism algorithm resolves the longstanding controversy about the focal geometry of the event, yielding a normal faulting mechanism along a plane dipping to the southeast, which expresses extensional tectonics in the back arc behind the Hellenic subduction zone. The seismic moment of 3.9 × 1027 dyn cm is the largest measured in the past 100 yr in the Mediterranean Basin.
A quantitative database of 68 values of tsunami run-up was built through the systematic interview, over the past 5 yr, of elderly eyewitness residents of 16 Aegean islands and the Turkish coast of Asia Minor. It confirms values of up to 20 m on the southern coast of Amorgos, 10 m on Astypalaia, and up to 14 m on the western coast of Folegandros, 80 km to the west of the epicentre. These values, largely in excess of the inferred seismic slip at the source, and their concentration along isolated segments of fault, are incompatible with the generation of the tsunami by the seismic dislocation, and require an ancillary source, in the form of a series of landslides triggered by the earthquake and/or its main aftershocks, a model confirmed by hydrodynamic simulations using both the dislocation source and models of landslide sources.
Inundation of coastal areas by tsunamis during the 1883 eruption of Krakatau volcano led to the deposition of unusual pumice- enriched deposits. Fractal analysis of pumice shapes and lithologic characterization of the deposits suggest that the source of the abundant pumiceous material was widespread pumice rafts on the surface of the Sunda Straits that formed by fallout and pyroclastic flow activity. The rafts contained pumices rounded by particle-to- particle abrasion and were strongly depleted in dense components, such as lithics and crystals, by differential settling. Stranding of the floating pumice is inferred to have occurred during the receding phase of tsunamis after they had inundated low-lying coastal areas. Other pumice-bearing tsunami deposits contain significant amounts of coral fragments and nonvolcanic beach sediment. These units represent redeposition of beach and shallow-water sediments that were mixed with varying proportions of primary pyroclastic material. The Krakatau example illustrates the great diversity of lithofacies that may occur in deposits formed from volcanogenic tsunamis. Recognition of such deposits in coastal areas near centers of active explosive volcanism may provide an additional criterion with which to assess volcanic hazards.
A discrete tephra layer has been discovered in three marine sediment cores from the Sea of Marmara, eastern Mediterranean. The rhyodacitic glass chemistry and the stratigraphical position suggest a Santorini provenance and, in particular, a correlation with the marine Y-2 tephra that is known from the southern Aegean Sea and eastern Levantine Basin. This tephra represents the distal facies of the Cape Riva eruption of Santorini, which has been dated by 14C on land at 21 950 cal. yr BP. Hitherto, the Y-2 tephra has been detected only in marine sediment cores recovered south to southeast of its volcanic source. The new occurrence in the Sea of Marmara approximately 530 km NNE of the Santorini eruptive centre suggests a more north-easterly dispersal of fallout products of the Cape Riva eruption than previously supposed.
New field observations of the seismic intensity distribution of the large (Ms = 7.4) South Aegean (Amorgos) earthquake of 9 July 1956 are presented. Interpretations based on local ground conditions, structural properties of buildings and peculiarities of the rupture process lead to a re-evaluation of the macroseismic field configuration. This, together with the aftershock epicentral distribution, quite well defines the earthquake rupture zone, which trends NE-SW and coincides with the Amorgos Astypalea trough. The lateral extent of the rupture zone, however, is about 40% smaller than that predicted for Aegean earthquakes of Ms = 7.4. This discrepancy could be attributed to sea-bottom topography changes, which seem to control the rupture terminations, and to relatively high stressdrop with respect to other Aegean earthquakes.Fault plane solutions obtained by several authors indicate either mainly normal faulting with a significant right-lateral strike-slip component or predominantly strike-slip motion. The neotectonism of Amorgos Island, based on new field observations, aerial photograph analysis and fault mechanisms, is consistent with the dip-slip interpretation. The neotectonic master fault of Amorgos and the 1956 seismic faulting appear to belong to the same tectonic phase (NE-SW strike and a southeasterly dip). However, the significant right-lateral strike-slip component supports the idea that the Amorgos region deviates from the simple description for pure extension in back-arc conditions.
Zusammenfassung Die Kykladeninsel Ios besteht zur Hauptsache aus einem Gneisdom. Der Kern dieses Doms ist ein Augengneiskomplex, welcher von Granat-Muskovit-Schiefern umhüllt wird. Auf dieses Grundgebirge ist eine Marmor-Schiefer-Serie überschoben worden, eine tektonisch bedingte Wechselfolge von Metasedimenten und Metavulkaniten, wahrscheinlich mesozoischen Alters.Die petrologischen Gegebenheiten und Isotopendatierungen bezeugen Polymetamorphose. Es konnten zwei metamorphe Phasen alpidischen Alters nachgewiesen werden, M1 und M2, sowie die Spuren einer hochgradigen, metamorphen oder magmatischen Phase Mo, welche nur die Basis betroffen hat.Radiometrische Alterbestimmungen bestätigen die Deutung, daß das Grundgebirge voralpidisches Alter hat. Die Phasen M1 und M2 wurden datiert auf 43 Mj. und 25 Mj. Die Druck-Temperatur-Bedingungen der M1 und M2 wurden abgeschätzt auf 9–11 Kb und 350–400° C für die M1 und auf 5–7 Kb und 380–420° C im Falle der M2. Der metamorphe Werdegang der Kykladen wird diskutiert sowie die mutmaßliche Existenz eines vergleichbaren voralpidischen Grundgebirges auf den Nachbarinseln Sikinos und Naxos.
An unusual stratigraphic unit (nicknamed 'homogenite') fills topographic lows in the complex ridge and trough bathymetry at two survey sites on the W Mediterranean Ridge and the Calabrian Ridge. On near-bottom 4-kHz seismic-reflection profiles, this unit is an acoustically transparent, near-surface, flat-lying layer, whereas in cores, it is a homogeneous gray marl as much as 7m thick. Grain size decreases upcore within the unit, implying that it was deposited in a single event controlled by gravitational settling. The stratigraphic position of the homogenite relative to a firmly dated sapropel bed suggests emplacement between 4400 and 3100 yr B.P. The source of the homogenite is inferred to be the nearby basin walls and a Santorini tsunami mechanism is discussed.-Authors
A sedimentary deposit on the continental shelf off Caesarea Maritima, Israel, is identified, dated, and attributed to tsunami waves produced during the Late Bronze Age (ca. 1630ג€“1550 B.C.E.) eruption of Santorini, Greece. The sheet-like deposit was found as a layer as much as 40 cm thick in four cores collected from 10 to 20 m water depths. Particle-size distribution, planar bedding, shell taphoecoensis, dating (radiocarbon, optically stimulated luminescence, and pottery), and comparison of the horizon to more recent tsunamigenic layers distinguish it from normal storm and typical marine conditions across a wide (>1 km2) lateral area. The presence of this deposit is evidence that tsunami waves from the Santorini eruption radiated throughout the Eastern Mediterranean Sea, affecting the coastal people living there. Dates for the tsunami deposit bracket both the so-called ג€highג€ and ג€lowג€ chronology for the Santorini eruption. In addition to resolving the question of the extent of tsunami impact from the Santorini eruption, the research presented also provides a new means of discovering, identifying, and studying continuous records of paleotsunami deposits in the upper shelf coastal environment. The latter is key to understanding past events, better interpreting sedimentological records, and creating stronger models for understanding tsunami propagation, coastal management, and hazard preparation worldwide.
ABSTRACT A series of experiments was designed to investigate grain orientation in flows with concentrations of granular solids in the range from 0.5 to 15%. Fourteen experiments were carried out in which sand-sized material was dispersed in liquid plaster of paris and the dispersion allowed to flow down a slope until it came to rest. The preferred orientation of the grains was then estimated by measuring the anisotropy of magnetic susceptibility of specimens cut from the solidified flows.
Five of the experiments showed signs of deformation, both in the gross characteristics of the flow and in the grain alignment. The remaining nine had grain alignment dependent on grain concentration. At concentrations of 1.2% and less and 11.1% and more, by volume, alignment of long axes was parallel to flow. These observations were consistent with the existing theories for low concentrations, in which preferred alignment is shown to result from the varying rate of rotation of grains in a shearing flow, and for high concentrations, in which preferred alignment results from the transfer of angular momentum between colliding grains.
At some intermediate concentrations transverse alignment was observed. A theoretical explanation combining the theories of the extreme cases is suggested. The observation of transverse alignment is compared with a similar observation in some turbidites.
The Minoan eruption of Santorini resulted in deposition of pyroclastic material over a large area of the Aegean Sea and Eastern Mediterranean. The eruptive activity commenced with a plinian phase, which was followed by a phreatomagmatic phase, and ended with numerous pyroclastic flows. We present a sedimentological study of the Minoan ash deposits found in cores taken throughout the region. The cores are divided into three groups: (1) those from the sub-marine slopes immediately surrounding Santorini; (2) those from the Aegean Sea where the topography consists of many steep-sided basins; and (3) those from the Eastern Mediterranean where the topography is relatively flat and smooth. This last group is designated the Distal Ash Zone. The deposits from the Aegean Sea have been remobilised as turbidity currents. Grain-size distribution analysis reveals that these deposits are bimodal, the Mdø of the coarse population increasing away from source, whilst that of the fine population remains uniform irrespective of distance from source. The bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co-ignimbrite deposits within the turbidity current.
The timing and scale of the 1994 Rabaul tsunamis accompanying the eruption of Vulcan and Tavurvur volcanoes were estimated
from the temporal and spatial distribution of tsunami deposits. The deposits are identified as sand layers or characteristic
pumiceous sand layers (mixtures of pumice and sand) sandwiched by tephras from the two volcanoes. The tephras appear to play
an important role in preserving the original structures of the tsunami deposits. According to chronological data from both
tephra and tsunami deposits, the tsunamis were not generated by the first eruption of Vulcan volcano that occurred close to
the coast, but major tsunamis were excited several times by larger pyroclastic flows and base surges during the climactic
stage of the eruption. Tsunami run-up heights, estimated from distribution of the tsunami deposits, are about 8 m near Sulphur
Creak and more than 3.5 m around western to southern shore of Matupit Island.
Known tsunamis of volcanic origin are reviewed and classified according to their causes. Earthquakes accompanying eruptions (excluding tectonic events which apparently triggered eruptions), pyroclastic flows, and submarine explosions have each accounted for about 20% of cases. Ten causes of volcanic tsunamis are discussed. From the risk point of view, those due to landslides are particularly dangerous. Eruptions at calderas are more likely to generate tsunamis than eruptions elsewhere. Of those killed directly by volcanic eruptions, nearly a quarter have died as a result of tsunamis. By transfer of energy to sea waves, a violent eruption, which would be comparatively harmless on land, extends greatly the radius over which destruction occurs.
Krakatoa, 1883, is the only eruption sequence for which sufficient data exist for a detailed study of tsunamis. The times at which air and water waves generated by this sequence were recorded have been reread, and new origin times have been calculated and compared with observations made at the time. Origin times of successive pairs of air and water waves agree closely, except in some cases in which the tsunami arrived up to 15 minutes early, thus giving an apparent origin time 15 minutes before that of the corresponding air wave. This is explained by postulating that these tsunamis did not originate at the focus of the explosions, but at distances along the path towards the tide gauge, equivalent to those which would be covered by a tsunami in the time interval observed.
The calculated point at which the largest recorded tsunami originated coincides with the outer edge of a bank of volcanic debris laid down during the eruption. This is interpreted as part of an unwelded ignimbrite deposit, the violent emplacement of which, within a minute or so of the explosion, generated the tsunami. A satisfactory correlation is established between explosions and deposits laid down by the eruptions, as described from a geological section close to the source vent.
An outline is given of a proposed numerical index to define tsunamigenic potential at a given volcano. Such an index could be used to calculate the expected amplitudes of tsunamis at particular places in the vicinity, and hence could serve as a basis for tsunami risk contingency planning.