Figure 6 - uploaded by Adam J. R. Kent
Content may be subject to copyright.
Geologic map of Mount Hood. Samples locations shown as black dots. Parkdale samples not included in this figure. Modified from Scott, et al. (2000).
Source publication
We have studied the relationship between lava composition and magma chamber processes at Mount Hood, Oregon for the last 475,000 years. Mount Hood is unusual, in comparison to nearby Mount St. Helens and Mount Jefferson, in that it has produced relatively homogeneous lava compositions over this time period. Erupted lavas are mostly crystal rich and...
Contexts in source publication
Context 1
... 2 lists each lava sample along with several other characteristics of each sample. Figure 6 is a simplified geologic map of the Mount Hood region that shows the approximate location of the sample locations (excluding the four Parkdale samples). Explanations of map units are in Table 3. ...
Context 2
... is found in abundance in lavas with silica greater than 62 weight percent comprising approximately 10% of the phenocryst populations and 0% in lavas less than 62 weight percent silica. Very few hornblende crystals are found in lavas with lower silica and when observed have thick oxide reaction rims, and some are completely reacted into oxides ( Figure 16). The hornblende found in more silicic lavas tends to be elongated crystals with frayed ends. ...
Context 3
... have a euhedral tabular shape with no distinct lineation formed by the plagioclase crystals. Figure 36 ...
Similar publications
In volcanically and seismically active rift systems, preexisting faults may control the rise and eruption of magma, and direct the flow of hydrothermal fluids and gas in the subsurface. Using high-resolution airborne imagery, field observations, and CO2 degassing data on Aluto, a typical young silicic volcano in the Main Ethiopian Rift, we explore...
The ability of volatiles to escape rising magma regulates the explosivity of a volcanic system. During silicic lava dome eruptions, strain localization at the conduit margin occurs during magma ascent, creating a damage halo with implications for gas escape. Here we report the first systematic study of permeability network anisotropy across the mar...
The 2008 eruption of Chaitén Volcano was widely cited as the first activity at the volcano for over 9000 years. However, we have identified evidence from proximal pyroclastic deposits for three additional explosive eruptions of Chaitén within the past 5000 years. Chaitén has therefore produced at least five explosive eruptions in the Holocene, maki...
The active Lassen hydrothermal system includes a central vapor-dominated zone or zones beneath the Lassen highlands underlain
by ~240 °C high-chloride waters that discharge at lower elevations. It is the best-exposed and largest hydrothermal system
in the Cascade Range, discharging 41 ± 10 kg/s of steam (~115 MW) and 23 ± 2 kg/s of high-chloride wa...
A database of more than 650 whole-rock analyses on basaltic rocks from the Eastern Goldfields Superterrane has been compiled from the literature and from public-domain datasets. The data fall into three distinct geochemical categories: a High-Th Siliceous Basalt group, a Low-Th Basalt group and an Intermediate-Th Basalt group. The Low-Th Basalt gro...
Citations
... The three most recent eruptive episodes are Polallie (13-20 kyrs), Timberline (1.5 kyrs), and Old Maid (~200 yrs, Koleszar et al., 2012), with the Main Stage preceding this (>29 kyrs, Scott et al., 1997). manuscript submitted to Geochemistry, Geophysics, Geosystems Mineral Compositions: Darr (2006) present N=15 Cpx analyses from the Main Stage (>29kyrs) and Parkdale Flow (7.5-7.7 kyrs, Scott et al., 1997). Cribb, (1997) present N=123 Cpx from the Main Stage Cloud Cap (400-600 kyr, Keith et al., 1985), Main Stage, and Polallie. ...
The iconic volcanoes of the Cascade arc stretch from Lassen Volcanic Center in northern California, through Oregon and Washington, to the Garibaldi Volcanic Belt in British Columbia. Recent studies have reviewed differences in the distribution and eruptive volumes of vents, as well as variations in geochemical compositions and heat flux along strike (amongst other characteristics). We investigate whether these along-arc variations manifest as variations in magma storage conditions. We compile available constraints on magma storage depths from InSAR, geodetics, seismic inversions, and magnotellurics for each major edifice, and compare these to melt inclusion saturation pressures, pressures calculated using mineral-only barometers, and constraints from experimental petrology. The availability of magma storage depth estimates varies greatly along the arc, with abundant geochemical and geophysical data available for some systems (e.g. Lassen Volcanic Center, Mt. St. Helens), and very limited data available for other volcanoes, including many which are classified as “very high threat” by the USGS (e.g., Glacier Peak, Mt. Baker, Mt. Hood, Three Sisters). Acknowledging the limitations of data availability and the large uncertainties associated with certain methods, available data is indicative of magma storage within the upper 15 km of the crust (~2 ± 2 kbar). These findings are consistent with previous work recognising barometric estimates cluster within the upper crust in many arcs worldwide. There are no clear offsets in magma storage between arc segments that are in extension, transtension or compression, although substantially more petrological work is needed for fine scale evaluation of storage pressures.
... Unit Qphm has reversed-polarity magnetization and is assigned an early Pleistocene age on the basis of stratigraphic position and a K-Ar age of age of 1.5 Ma reported by Scott and Gardner (2017). Table 6-2; Appendix; Darr, 2006;McClaughry and others, 2012). The Parkdale flow (Qr1pk) extends for a distance of ~6.5 km (4 mi) north from a vent area lying ~12 km (7.5 mi) north-northeast of the Mount Hood summit (Figure 2-1; Wise, 1969;Sherrod and Scott, 1995). ...
The Dog River and northern part of the Badger Lake 7.5′ quadrangles encompasses an area of ~201 km2 (77.6 mi2) of the High Cascades of north-central Oregon, lying across the eastern slopes of Mount Hood volcano (Figure 1 1; Plate 1; referred to herein as Dog River–Badger Lake area). Mount Hood, known as Wy’east to Native Americans, is Oregon’s tallest peak (3,427 m [11,241 ft]). The volcano has erupted episodically for the past 500,000 years, experiencing two major eruptive periods during the last 1,500 years (Scott and others, 1997a; Scott and others, 2003; Scott and Gardner, 2017). Cascade Range volcanism and structural development in the area dates back longer, with eruptive activity dating from latest Miocene to recent time; part of that volcano-tectonic record is detailed by new high-resolution geologic mapping presented here.
The geology of the Dog River–Badger Lake area was mapped by the Oregon Department of Geology and Mineral Industries (DOGAMI) between 2017 and 2020, in collaboration with geoscientists from the U. S. Geological Survey Cascade Volcano Observatory (USGS CVO) and Hamilton College, New York. The primary objective of this investigation is to provide an updated and spatially accurate geologic framework for the Dog River–Badger Lake area as part of a multi-year study of the geology of the larger Middle Columbia Basin (Figure 1 1, Figure 1 2). Additional key objectives of this project are to: 1) determine the geologic history of volcanic rocks in this part of the northern Oregon Cascade Range, including lava flows and volcaniclastic deposits erupted from Middle Pleistocene to Holocene Mount Hood volcano; 2) provide significant new details about the structure and fault history along the northern segment of the High Cascades intra-arc graben (Hood River graben); and 3) better understand geologic hazards in the region, related to earthquakes, volcanoes, and landslides. New detailed geologic data presented here also provides a basis for future geologic, geohydrologic, and geohazard studies in the greater Middle Columbia Basin. Detailed geologic mapping in this part of the Middle Columbia Basin is a high priority of the Oregon Geologic Map Advisory Committee (OGMAC), supported in part by grants from the STATEMAP component of the USGS National Cooperative Geologic Mapping Program (G17AC00210, G19AC00160). Additional funds were provided by the State of Oregon.
The core products of this study are this report, an accompanying geologic map and cross sections (Plate 1), an Esri ArcGIS™ geodatabase, and Microsoft Excel® spreadsheets tabulating point data for geochemistry, geochronology, magnetic polarity, orientation points, and well data. The geodatabase presents the new geologic mapping in a digital format consistent with the USGS National Cooperative Geologic Mapping Program Geologic Map Schema (GeMS) (U.S. Geological Survey National Cooperative Geologic Mapping Program, 2020). This geodatabase contains spatial information, including geologic polygons, contacts, structures, geochemistry, geochronology, magnetic observation, orientation points, and well data, as well as data about each geologic unit such as age, lithology, mineralogy, and structure. Digitization at scales of 1:8,000 or better was accomplished using a combination of high-resolution lidar topography and imagery. Surficial and bedrock geologic units contained in the geodatabase are depicted on the Plate 1 at a scale of 1:24,000. Both the geodatabase and geologic map are supported by this report describing the geology in detail.
... The lava also contains microphenocrysts of magnetite and/or ilmenite and rare (<1 percent) phenocrysts and microphenocrysts of augite and hornblende; the latter are characterized by distinctive oxidation rims. Darr (2006) reported numerous plagioclase analyses with compositions ranging from less than An 20 to greater than An 70 with the most common compositions falling between An 40 and An 60 (andesine to labradorite). Two samples range from near An 50 at the core to An 70-72 at the rim with both having minimum values of An 43 . ...
... Two samples range from near An 50 at the core to An 70-72 at the rim with both having minimum values of An 43 . Darr (2006) also reported a mean orthopyroxene composition of En 68 with a range of En 49-80 , Fs 19-49 , and Wo 0-12 . Samples obtained from this flow have an andesitic chemical composition with 58.28 to 58.79 weight percent SiO2, 18.00 to 18.34 weight percent Al2O3, 1.12 to 1.17 weight percent TiO2, and 3.37 to 3.48 weight percent MgO (Table 3; Appendix; Gardner, unpublished data). ...
This report and accompanying thematic geologic map and digital geodatabase for the Hood River Valley, Hood River and Wasco Counties, Oregon was prepared to provide an updated and spatially accurate and detailed geologic database for the area (Figure 1; Plate 1). The project was a high priority of the Oregon Geologic Map Advisory Committee and was supported in part by the U.S. Geological Survey (USGS) STATEMAP component of the National Cooperative Geologic Mapping Program under assistance award G11AC20275 (2011). Matching funds were provided by the Oregon Department of Geology and Mineral Industries (DOGAMI). The core product of this study is an Esri ArcGIS™ ArcMap™ 10 format geodatabase that combines new and existing mapping in a digital format consistent with the digital statewide Oregon Geologic Data Compilation database (OGDC-5, Ma and others, 2009). The geodatabase contains detailed spatial information about geologic polygons and structures, and basic data about each geologic unit such as age, lithology, mineralogy, structure, and texture. The geodatabase is supported by this report describing the geology in detail, and digital appendices with geochemical, geochronological, structural, and water well data. This publication also includes a thematic map (Plate 1) at a scale of 1:36,000 to show the distribution of key lithologies in the study area. This thematic geologic map refines our understanding of geologic conditions in the Hood River area that control the distribution, quantity, and quality of groundwater resources, the distribution of terrain susceptible to landslides, seismic hazards, and delineates areas of potential aggregate and other mineral resources. The database also provides a basis for future geologic, geohydrologic, and geohazard studies in the Hood River Valley.
... The following section and Table 1 summarize a number of previous studies (e.g. Cribb & Barton 1997;Scott et al. 1997;Woods 2004;Darr 2006;Kent et al. 2010;Koleszar 2011;Eppich et al. 2012;Koleszar et al. 2012). ...
Abstract: Andesitic magmas (herein defined as having c. 55–65 wt% SiO2) are abundant in con- vergent margin volcanoes. Although andesites and other intermediate magmas can plausibly be produced by a number of mechanisms, a lack of evidence for andesitic liquids relative to the abun- dance of andesite volcanic rocks, together with widespread evidence for hybridization and recharge, suggests that many erupted andesites form by mixing between relatively mafic magmas that ultimately derive from the mantle wedge, and more felsic magmas or mush zones derived from differentiation and crustal melting.
A model of recharge filtering accounts for the high abundance of andesitic volcanic rocks and reflects the simple idea that the processes that create hybridized andesitic magmas in the shallow crust can also initiate volcanic activity, resulting in preferential eruption of andesitic magmas rela- tive to the parental magmas that mix to produce them. This occurs via mafic recharge – intrusion of a mafic magma into a silicic magma or mush within the shallow crust. The overall abundance and variability from volcano to volcano of andesitic volcanism at convergent margins also suggests important roles for hydrous mafic magma in promoting mixing and eruption during mafic recharge, and the crust for modulating the compositions of volcanic outputs.
... Fig. 1), and the presence of crystals in individual lavas with compositions showing that they derived from multiple magmatic sources (e.g. Cribb and Barton, 1997;Darr, 2006;Eichelberger, 1978;Kent et al., 2010;Woods, 2004). ...
... Magma mixing hybridized the liquid components of the endmember magmas without substantially altering the textural and geochemical characteristics of the two plagioclase populations (Kent et al., 2010). Late-stage syn-and post-mixing plagioclase growth also occurred, resulting in the crystallization of 25-50 μm An-rich rims on An-poor Population 2 crystals (and in some cases, b5 μm An-poor rims on Population 1 crystals; Darr, 2006). Diffusion modeling of Mg distributions across Population 2 rims demonstrate that they crystallized within days to weeks of eruption, suggesting a clear causeand-effect relationship between mixing events and eruptions (Kent et al., 2010), consistent with eruption driven by recharge of a mafic magma into a cooler silicic magma reservoir. ...
Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report here 230Th–226Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations and 238U–230Th–226Ra disequilibria in four plagioclase size fractions from the Timberline (1500 a) and Old Maid (215 a) eruptive sequences. After correction for groundmass and apatite contamination, average 230Th–226Ra model ages of large (> 500 μm) plagioclase are > 4.5 ka (Timberline) and > 5.5 ka (Old Maid), with ages of cores that are > 10 ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (< 500 μm) plagioclase, derived from mafic magmas, have high (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading to 230Th–226Ra model ages that are <~3 ka for Old Maid and undefined for Timberline separates. However, the preservation of significant 230Th–226Ra disequilibria require that the majority of crystals in the separate are young (<<10 ka). The high (226Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with evidence of rapid crystallization of rims. Rapid crystallization of mafic intrusions may trigger eruption at Mount Hood by producing a partially-crystalline mafic magma capable of mixing with a reheated silicic crystal mush.
... We examine andesite petrogenesis with reference to Mount Hood, Oregon, a volcano from the Cascadia subduction zone 7 . Lavas erupted from Mount Hood over the past ∼500,000 years are remarkably restricted in composition, with 95% having SiO 2 contents between 58 and 66 wt% (Fig. 1b,c; Supplementary Table S1), and are also typical of subduction-zone andesites as they are crystal-rich (∼20-45 volume% crystals) and dominated by plagioclase, with lesser amounts of pyroxene, amphibole and oxides [7][8][9] . Evidence also suggests that magma mixing plays an important role in petrogenesis at Mount Hood 2,8-10 : disequilibrium mineral textures and assemblages are ubiquitous, quenched mafic inclusions are common and bulk lava compositions describe simple linear trends on bivariate plots of major oxide composition [8][9][10] . ...
... Lavas erupted from Mount Hood over the past ∼500,000 years are remarkably restricted in composition, with 95% having SiO 2 contents between 58 and 66 wt% (Fig. 1b,c; Supplementary Table S1), and are also typical of subduction-zone andesites as they are crystal-rich (∼20-45 volume% crystals) and dominated by plagioclase, with lesser amounts of pyroxene, amphibole and oxides [7][8][9] . Evidence also suggests that magma mixing plays an important role in petrogenesis at Mount Hood 2,8-10 : disequilibrium mineral textures and assemblages are ubiquitous, quenched mafic inclusions are common and bulk lava compositions describe simple linear trends on bivariate plots of major oxide composition [8][9][10] . ...
Andesitic compositions dominate the output of many subduction zone volcanoes. In this environment most andesites are produced by magma mixing, typically between mafic magmas, ultimately derived from the underlying mantle wedge, and felsic magmas produced by crustal melting or extensive differentiation. The high relative abundance of andesitic magmas in arcs require that they erupt in preference to the mafic and felsic magmas that mix to produce them, although the factors that control this remain less well understood. We investigate this issue through studies of Mount Hood, Oregon, which represents a class of intermediate volcanoes characterized by long-term outputs of compositionally monotonous andesitic magmas, and where recharge and magma mixing play a dominant role in petrogenesis. At Mount Hood 95% of magmas erupted over the last ~500,000 years have SiO2 contents between 58-66 wt.%, and textural and petrological evidence of magma mixing is ubiquitous. Estimates of the composition of mafic and felsic magmas involved in mixing at Mount Hood can be made by the combination of textural (CSD) and compositional data, and suggest that erupted magmas result from the mixing of mafic (50.7 ± 4.3 wt.% SiO2) and felsic (70.9 ± 2.1 wt.% SiO2) endmembers in approximately subequal proportions. These endmember compositions appear to have remained broadly constant through time but are virtually absent from the spectrum of erupted lavas. Mineral zoning and diffusion modeling shows that mafic and felsic endmember magmas evolve separately, and that mafic recharge and efficient mixing occurs weeks to months prior to eruption. Petrological estimates of pressure and temperature, melt inclusions measurements of volatile abundances and mineral ages from U-series, CSD and additional diffusion modeling also provide additional constraints on the dynamics of the system. The dependence on recharge for eruption also suggests that crustal and or magmatic conditions beneath Mount Hood prevent eruption of mafic or felsic endmember magmas by themselves, although these are demonstrably involved in magma genesis. We speculate that this relates to difficulties that these magmas have in surmounting density and viscosity barriers, but other mechanisms may also be important. Recharge appears to be the only means by which the volcano can erupt. Because recharge also results in mixing between mafic and felsic endmember magmas, this effectively filters erupted magmas to be mixed andesitic compositions, a process that we term ``recharge filtering''. We suggest that recharge filtering behavior is important at many subduction zone volcanoes, including those which appear highly dependent on recharge to initiate eruptions (Mount Unzen, Soufriére Hills, Mont Peleé) as well as those where recharge and mixing is still common (e.g. Mount St. Helens, Mount Shasta), but where more diverse magma compositions also erupt. This implies that local crustal conditions play an important role in dictating the compositions of erupted magmas at a given volcano, and that the range and proportions of erupted magma compositions may differ significantly from those present within the magmatic system beneath a given volcano.
... We examine andesite petrogenesis with reference to Mount Hood, Oregon, a volcano from the Cascadia subduction zone 7 . Lavas erupted from Mount Hood over the past ∼500,000 years are remarkably restricted in composition, with 95% having SiO 2 contents between 58 and 66 wt% (Fig. 1b,c; Supplementary Table S1), and are also typical of subduction-zone andesites as they are crystal-rich (∼20-45 volume% crystals) and dominated by plagioclase, with lesser amounts of pyroxene, amphibole and oxides [7][8][9] . Evidence also suggests that magma mixing plays an important role in petrogenesis at Mount Hood 2,8-10 : disequilibrium mineral textures and assemblages are ubiquitous, quenched mafic inclusions are common and bulk lava compositions describe simple linear trends on bivariate plots of major oxide composition [8][9][10] . ...
... Lavas erupted from Mount Hood over the past ∼500,000 years are remarkably restricted in composition, with 95% having SiO 2 contents between 58 and 66 wt% (Fig. 1b,c; Supplementary Table S1), and are also typical of subduction-zone andesites as they are crystal-rich (∼20-45 volume% crystals) and dominated by plagioclase, with lesser amounts of pyroxene, amphibole and oxides [7][8][9] . Evidence also suggests that magma mixing plays an important role in petrogenesis at Mount Hood 2,8-10 : disequilibrium mineral textures and assemblages are ubiquitous, quenched mafic inclusions are common and bulk lava compositions describe simple linear trends on bivariate plots of major oxide composition [8][9][10] . ...
Andesitic volcanic rocks are common in subduction zones and are argued to play an important role in the formation and evolution of the continental crust at convergent margins. Andesite formation is dominated by mixing between iron- and magnesium-rich (mafic) magmas and silica-rich (felsic) magmas. The abundance of andesites in many subduction zones suggests they erupt in preference to the magmas that mix to produce them; however, the reasons for this remain unclear. Here we use textural and geochemical analyses of andesites from Mount Hood, Oregon, to show that eruptions are closely linked with episodes of mafic recharge-the intrusion of mafic magma into a shallow felsic magma reservoir. The felsic and mafic magmas involved rarely erupt by themselves, probably because the former are too viscous and the latter too dense. Mafic recharge overcomes these barriers to eruption, and, as it also promotes efficient mixing, results in preferential eruption of mixed andesitic magmas. The abundance of andesites therefore relates to local crustal conditions and the ability of magmas to erupt. We suggest that volcanoes, such as Mount Hood, that erupt homogeneous andesitic compositions through time are those that are the most reliant on mafic recharge to initiate eruptions.
The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage--that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.
The processes involved in the formation and storage of magma within the Earth's upper crust are of fundamental importance to volcanology. Many volcanic eruptions, including some of the largest, result from the eruption of components stored for tens to hundreds of thousands of years before eruption. Although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes, they remain relatively poorly known. Eruptions of crystal-rich magma are often suggested to require the mobilization of magma stored at near-solidus conditions; however, accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures. What has been lacking in this debate is clear observational evidence linking the thermal (and therefore physical) conditions within a magma reservoir to timescales of storage-that is, thermal histories. Here we present a method of constraining such thermal histories by combining timescales derived from uranium-series disequilibria, crystal sizes and trace-element zoning in crystals. At Mount Hood (Oregon, USA), only a small fraction of the total magma storage duration (at most 12 per cent and probably much less than 1 per cent) has been spent at temperatures above the critical crystallinity (40-50 per cent) at which magma is easily mobilized. Partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near-solidus temperatures is common. Magma storage at low temperatures indicates that, although thermobarometry calculations based on mineral compositions may record the conditions of crystallization, they are unlikely to reflect the conditions of most of the time that the magma is stored. Our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption.
The factors that control the explosivity of silicic volcanoes are critical for hazard assessment, but are often poorly constrained for specific volcanic systems. Mount Hood, Oregon, is a somewhat atypical arc volcano in that it is characterized by a lack of large explosive eruptions over the entire lifetime of the current edifice (~ 500,000 years). Erupted Mount Hood lavas are also compositionally homogeneous, with ~ 95% having SiO2 contents between 58 and 66 wt.%. The last three eruptive periods in particular have produced compositionally homogeneous andesite–dacite lava domes and flows.
