Geology topics

Charles R. Bacon

Publications and source records attributed to Charles R. Bacon.

At least 37 records · Page 2Linked to original sources

USGS2012

Sixty thousand years of magmatic volatile history before the caldera-forming eruption of Mount Mazama, Crater Lake, Oregon

The well-documented eruptive history of Mount Mazama, Oregon, provides an excellent opportunity to use pre-eruptive volatile concentrations to study the growth of an explosive silicic magmatic system. Melt inclusions (MI) hosted in pyroxene and plagioclase crystals from eight dacitic–rhyodacitic eruptive deposits (71–7.7 ka) were analyzed to determine variations in volatile-element concentrations and changes in magma storage conditions leading up to and including the climactic eruption of Crater Lake caldera. Temperatures (Fe–Ti oxides) increased through the series of dacites, then decreased, and increased again through the rhyodacites (918–968 to ~950 to 845–895 °C). Oxygen fugacity began at nickel–nickel-oxide buffer (NNO) +0.8 (71 ka), dropped slightly to NNO +0.3, and then climbed to its highest value with the climactic eruption (7.7 ka) at NNO +1.1 log units. In parallel with oxidation state, maximum MI sulfur concentrations were high early in the eruptive sequence (~500 ppm), decreased (to ~200 ppm), and then increased again with the climactic eruption (~500 ppm). Maximum MI sulfur correlates with the Sr content (as a proxy for LREE, Ba, Rb, P 2 O 5 ) of recharge magmas, represented by basaltic andesitic to andesitic enclaves and similar-aged lavas. These results suggest that oxidized Sr-rich recharge magmas dominated early and late in the development of the pre-climactic dacite–rhyodacite system. Dissolved H 2 O concentrations in MI do not, however, correlate with these changes in dominant recharge magma, instead recording vapor solubility relations in the developing shallow magma storage and conduit region. Dissolved H 2 O concentrations form two populations through time: the first at 3–4.6 wt% (with a few extreme values up to 6.1 wt%) and the second at ≤2.4 wt%. CO 2 concentrations measured in a subset of these inclusions reach up to 240 ppm in early-erupted deposits (71 ka) and are below detection in climactic deposits (7.7 ka). Combined H 2 O and CO 2 concentrations and solubility models indicate a dominant storage region at 4–7 km (up to 12 km), with drier inclusions that diffusively re-equilibrated and/or were trapped at shallower depths. Boron and Cl (except in the climactic deposit) largely remained in the melt, suggesting vapor–melt partition coefficients and gas fractions were low. Modeled Li, F, and S vapor–melt partition coefficients are higher than those of B and Cl. The decrease in maximum MI CO 2 concentration following the earliest dacitic eruptions is interpreted to result from a broadening of the shallow storage region to greater than the diameter of subjacent feeders, so that greater proportions of reservoir magma were to the side of CO 2 -bearing vapor bubbles ascending vertically from the locus of recharge magma injection, thereby escaping recarbonation by streaming vapor bubbles. The Mazama melt inclusions provide a picture of a growing magma storage region, where chemical variations in melt and magma occur due to changes in the nature and supply rate of magma recharge, the timing of degassing, and the possible degree of equilibration with gases from below.

Oregon↗

USGS2012

The Stanford-U.S. Geological Survey SHRIMP ion microprobe--a tool for micro-scale chemical and isotopic analysis

Answers to many questions in Earth science require chemical analysis of minute volumes of minerals, volcanic glass, or biological materials. Secondary Ion Mass Spectrometry (SIMS) is an extremely sensitive analytical method in which a 5–30 micrometer diameter "primary" beam of charged particles (ions) is focused on a region of a solid specimen to sputter secondary ions from 1–5 nanograms of the sample under high vacuum. The elemental abundances and isotopic ratios of these secondary ions are determined with a mass spectrometer. These results can be used for geochronology to determine the age of a region within a crystal thousands to billions of years old or to precisely measure trace abundances of chemical elements at concentrations as low as parts per billion. A partnership of the U.S. Geological Survey and the Stanford University School of Earth Sciences operates a large SIMS instrument, the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP–RG) on the Stanford campus.

Fact Sheet↗

USGS2012

Under trees and water at Crater Lake National Park, Oregon

Crater Lake partially fills the caldera that formed approximately 7,700 years ago during the eruption of a 12,000-ft-high volcano known as Mount Mazama. The caldera-forming, or climactic, eruption of Mount Mazama devastated the surrounding landscape, left a thick deposit of pumice and ash in adjacent valleys, and spread a blanket of volcanic ash as far away as southern Canada. Prior to the climactic event, Mount Mazama had a 400,000-year history of volcanic activity similar to other large Cascade volcanoes such as Mounts Shasta, Hood, and Rainier. Since the caldera formed, many smaller, less violent eruptions occurred at volcanic vents below Crater Lake's surface, including Wizard Island. A survey of Crater Lake National Park with airborne LiDAR (Light Detection And Ranging) resulted in a digital elevation map of the ground surface beneath the forest canopy. The average resolution is 1.6 laser returns per square meter yielding vertical and horizontal accuracies of ±5 cm. The map of the floor beneath the surface of the 1,947-ft-deep (593-m-deep) Crater Lake was developed from a multibeam sonar bathymetric survey and was added to the map to provide a continuous view of the landscape from the highest peak on Mount Scott to the deepest part of Crater Lake. Four enlarged shaded-relief views provide a sampling of features that illustrate the resolution of the LiDAR survey and illustrate its utility in revealing volcanic landforms and subtle features of the climactic eruption deposits. LiDAR's high precision and ability to "see" through the forest canopy reveal features that may not be easily recognized-even when walked over-because their full extent is hidden by vegetation, such as the 1-m-tall arcuate scarp near Castle Creek.

Oregon↗

USGS2012

Using rocks to reveal the inner workings of magma chambers below volcanoes in Alaska’s National Parks

Alaska is one of the most vigorously volcanic regions on the planet, and Alaska’s national parks are home to many of the state’s most active volcanoes. These pose both local and more distant hazards in the form of lava and pyroclastic flows, lahars (mudflows), ash clouds, and ash fall. Alaska’s volcanoes lie along the arc of the Aleutian-Alaskan subduction zone, caused as the oceanic Pacific plate moves northward and dips below the North American plate. These volcanoes form as water-rich fluid from the down-going Pacific plate is released, lowering the melting temperature of rock in the overlying mantle and enabling it to partially melt. The melted rock (magma) migrates upward, collecting at the base of the approximately 25 mile (40 km) thick crust, occasionally ascending into the shallow crust, and sometimes erupting at the earth’s surface. During volcanic unrest, scientists use geophysical signals to remotely visualize volcanic processes, such as movement of magma in the upper crust. In addition, erupted volcanic rocks, which are quenched samples of magmas, can tell us about subsurface magma characteris-tics, history, and the processes that drive eruptions. The chemical compositions of and the minerals present in the erupted magmas can reveal conditions under which these magmas were stored in crustal “chambers”. Studies of the products of recent eruptions of Novarupta (1912), Aniakchak (1931), Trident (1953-74), and Redoubt (2009) volcanoes reveal the depths and temperatures of magma storage, and tell of complex interactions between magmas of different compositions. One goal of volcanology is to determine the processes that drive or trigger eruptions. Information recorded in the rocks tells us about these processes. Here, we demonstrate how geologists gain these insights through case studies from four recent eruptions of volcanoes in Alaska national parks.

Alaska↗

USGS2011

Partially melted granodiorite and related rocks ejected from Crater Lake caldera, Oregon

Blocks of medium-grained granodiorite to 4 m, and minor diabase, quartz diorite, granite, aplite and granophyre, are common in ejecta of the ∼6,900 yrBP calderaforming eruption of Mount Mazama. The blocks show degrees of melting from 0–50 vol%. Because very few have adhering juvenile magma, it is thought that the blocks are fragments of the Holocene magma chamber's walls. Primary crystallisation of granodiorite produced phenocrystic pl + hyp + aug + mt + il + ap + zc, followed by qz + hb + bt + alkali feldspar (af). Presence of fluid inclusions in all samples implies complete crystallisation before melting. Subsolidus exchange with meteoric hydrothermal fluids before melting is evident in δ 18 O values of −3·4+4·9‰ for quartz and plagioclase in partially melted granodiorites (fresh lavas from the region have δ 18 O values of +5·8−+7·0‰); δ 18 O values of unmelted granodiorites from preclimatic eruptive units suggest hydrothermal exchange began between ∼70 and 24 ka. Before eruption, the granitic rocks equilibrated at temperatures, estimated from Fe-Ti oxide compositions, of up to ∼1000°C for c. 10 2 –10 4 years at a minimum pressure of 100-180 MPa. Heating caused progressive breakdown or dissolution of hb, af, bt, and qz, so that samples with the highest melt fractions have residual pl + qz and new or re-equilibrated af + hyp + aug + mt + il in high-silica rhyolitic glass (75-77% SiO 2 ). Mineral compositions vary systematically with increasing temperature. Hornblende is absent in rocks with Fe-Ti oxide temperatures >870°C, and bt above 970°C. Oxygen isotope fractionation between qz, pl, and glass in partially fused granodiorite also is consistent with equilibration at T≥900°C (Δ 18 O qz.pl = +0·7 ± 0·5‰). Element partitioning between glass and crystals reflects the large fraction of refractory pl, re-equilibration of af and isolation or incomplete dissolution of accessory phases. Ba and REE contents of analysed glass separates can be successfully modelled by observed degrees of partial melting of granodiorite, but Rb, Sr and Sc concentrations cannot. Several samples have veins of microlite-free glass 1–5 mm thick that are compositionally and physically continuous with intergranular melt and which apparently formed after the climactic eruption began. Whole-rock H 2 O content, microprobe glass analysis sums near 100% and evidence for high temperature suggest liquids in the hotter samples were nearly anhydrous. The occurrence of similar granodiorite blocks at all azimuths around the 8 × 10 km caldera implies derivation from one pluton. Compositional similarity between granodiorite and pre-Mazama rhyodacites suggests that the pluton may have crystallised as recently as 0·4 Ma; compositional data preclude crystallisation from the Holocene chamber. The history of crystallisation, hydrothermal alteration, and remelting of the granitic rocks may be characteristic of shallow igneous systems in which the balance between hydrothermal cooling and magmatic input changes repeatedly over intervals of 10 4 -10 6 years.

Oregon↗

USGS2011

Compositional zonation and cumulus processes in the Mount Mazama magma chamber, Crater Lake, Oregon

The 6845 ± 50 BP climactic eruption of Mount Mazama discharged 47 ± 9 km 3 of vertically zoned calc-alkaline magma, affording a virtually complete section through the chamber. Evidence for two andesitic parents with different trace-element (particularly Sr) and water contents is preserved in the ejecta. Prior to eruption, a dominant volume of rhyodacite was underlain successively by high-Sr andesite, high-Sr crystal mushes, and low-Sr crystal mushes. Intergranular liquids in the high-Sr magmas were probably richer in water than those in the low-Sr magmas. Thermal continuity throughout the ejecta favours eruption from a single, zoned reservoir. Insight into chamber development is given by preclimactic rhyodacitic lavas and tephra erupted between about 30,000 BP and the climactic eruption. The oldest of these lavas, contaminated derivatives of low-Sr magma, contain crystal-poor magmatic inclusions of low-Sr andesite; the youngest has inclusions of high-Sr andesite and, like rhyodacitic pumice in the climactic ejecta, is hybrid magma containing an admixed high-Sr component. A model for steady-state growth of the chamber is inferred whereby repeated recharge, first by low-Sr then high-Sr andesite (± basalt), builds up a cumulate succession, while derivative liquid fractionates convectively, segregates, and mixes with an incrementally growing silicic volume. The magma chamber at Mount Mazama may provide insight into the evolution of some granitoid plutons.

Oregon↗

USGS2011

Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular terrane basement. Discovery of Pennsylvanian gabbro similar in age to Skolai arc plutons 500 km to the northeast indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a Paleoproterozoic continental fragment may be present in the mid-to-upper crust in southern Alaska.

Alaska↗

USGS2011

Bacon receives 1999 Bowen Award

Charles R. Bacon received the 1999 Bowen Award, presented by the Volcanology Geochemistry, and Petrology Section during the 1999 AGU Fall Meeting.

Eos, Transactions, American Geophysical Union↗

USGS2011

Anderson receives 2001 Bowen Award

Alfred T. Anderson, Jr. received the Bowen Award, presented by the Volcanology, Geochemistry, and Petrology Section at the 2001 Fall Meeting in San Francisco, California, last December.

Eos, Transactions, American Geophysical Union↗

USGS2010

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.

Washington↗

USGS2009

Accessory mineral U–Th–Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies.

California↗

USGS2008

Geologic map of Mount Mazama and Crater Lake Caldera, Oregon, including the database for the geologic map of Mount Mazama and Crater Lake Caldera, Oregon

Crater Lake partly fills one of the most spectacular calderas of the world, an 8-by-10-km basin more than 1 km deep formed by collapse of the volcano known as Mount Mazama (fig. 1) during a rapid series of explosive eruptions about 7,700 years ago. Having a maximum depth of 594 m, Crater Lake is the deepest lake in the United States. Crater Lake National Park, dedicated in 1902, encompasses 645 km2 of pristine forested and alpine terrain, including the lake itself, virtually all of Mount Mazama, and most of the area of the geologic map. The geology of the area was first described in detail by Diller and Patton (1902) and later by Williams (1942), whose vivid account led to international recognition of Crater Lake as the classic collapse caldera. Because of excellent preservation and access, Mount Mazama, Crater Lake caldera, and the deposits formed by the climactic eruption constitute a natural laboratory for study of volcanic and magmatic processes. For example, the climactic ejecta are renowned among volcanologists as evidence for systematic compositional zonation within a subterranean magma chamber. Mount Mazama's climactic eruption also is important as the source of the widespread Mazama ash, a useful Holocene stratigraphic marker throughout the Pacific Northwest, adjacent Canada, and offshore. A detailed bathymetric survey of the floor of Crater Lake in 2000 (Bacon and others, 2002) provides a unique record of postcaldera eruptions, the interplay between volcanism and filling of the lake, and sediment transport within this closed basin. Knowledge of the geology and eruptive history of the Mount Mazama edifice, greatly enhanced by the caldera wall exposures, gives exceptional insight into how large volcanoes of magmatic arcs grow and evolve. Lastly, the many smaller volcanoes of the High Cascades beyond the limits of Mount Mazama are a source of information on the flux of mantle-derived magma through the region. General principles of magmatic and eruptive processes revealed by the present study have been incorporated not only in scientific investigations elsewhere, but in the practical evaluation of hazards (Bacon and others, 1997b) and geothermal resources (Bacon and Nathenson, 1996) in the Crater Lake region. In addition to papers in scientific journals, field trip guides, and the hazard and geothermal reports, the major product of this long-term study of Mount Mazama is the geologic map. The map is unusual because it portrays bedrock (outcrop), surficial, and lake floor geology. Caldera wall geology is depicted in detail on the accompanying geologic panoramas.

Oregon↗

USGS2003

Crater Lake revealed

Around 500,000 people each year visit Crater Lake National Park in the Cascade Range of southern Oregon. Volcanic peaks, evergreen forests, and Crater Lake’s incredibly blue water are the park’s main attractions. Crater Lake partially fills the caldera that formed approximately 7,700 years ago by the eruption and subsequent collapse of a 12,000-foot volcano called Mount Mazama. The caldera-forming or climactic eruption of Mount Mazama drastically changed the landscape all around the volcano and spread a blanket of volcanic ash at least as far away as southern Canada. Prior to the climactic event, Mount Mazama had a 400,000 year history of cone building activity like that of other Cascade volcanoes such as Mount Shasta. Since the climactic eruption, there have been several less violent, smaller postcaldera eruptions within the caldera itself. However, relatively little was known about the specifics of these eruptions because their products were obscured beneath Crater Lake’s surface. As the Crater Lake region is still potentially volcanically active, understanding past eruptive events is important to understanding future eruptions, which could threaten facilities and people at Crater Lake National Park and the major transportation corridor east of the Cascades. Recently, the lake bottom was mapped with a high-resolution multibeam echo sounder. The new bathymetric survey provides a 2m/pixel view of the lake floor from its deepest basins virtually to the shoreline. Using Geographic Information Systems (GIS) applications, the bathymetry data can be visualized and analyzed to shed light on the geology, geomorphology, and geologic history of Crater Lake.

Oregon↗

USGS2002

Structure and physical characteristics of pumice from the climactic eruption of Mount Mazama (Crater Lake), Oregon

The vesicularity, permeability, and structure of pumice clasts provide insight into conditions of vesiculation and fragmentation during Plinian fall and pyroclastic flow-producing phases of the ~7,700 cal. year B.P. climactic eruption of Mount Mazama (Crater Lake), Oregon. We show that bulk properties (vesicularity and permeability) can be correlated with internal textures and that the clast structure can be related to inferred changes in eruption conditions. The vesicularity of all pumice clasts is 75–88%, with >90% interconnected pore volume. However, pumice clasts from the Plinian fall deposits exhibit a wider vesicularity range and higher volume percentage of interconnected vesicles than do clasts from pyroclastic-flow deposits. Pumice permeabilities also differ between the two clast types, with pumice from the fall deposit having higher minimum permeabilities (~5×10 –13 m 2 ) and a narrower permeability range (5–50×10 –13 m 2 ) than clasts from pyroclastic-flow deposits (0.2–330×10 –13 m 2 ). The observed permeability can be modeled to estimate average vesicle aperture radii of 1–5 µm for the fall deposit clasts and 0.25–1 µm for clasts from the pyroclastic flows. High vesicle number densities (~10 9 cm –3 ) in all clasts suggest that bubble nucleation occurred rapidly and at high supersaturations. Post-nucleation modifications to bubble populations include both bubble growth and coalescence. A single stage of bubble nucleation and growth can account for 35–60% of the vesicle population in clasts from the fall deposits, and 65–80% in pumice from pyroclastic flows. Large vesicles form a separate population which defines a power law distribution with fractal dimension D=3.3 (range 3.0–3.5). The large D value, coupled with textural evidence, suggests that the large vesicles formed primarily by coalescence. When viewed together, the bulk properties (vesicularity, permeability) and textural characteristics of all clasts indicate rapid bubble nucleation followed by bubble growth, coalescence and permeability development. This sequence of events is best explained by nucleation in response to a downward-propagating decompression wave, followed by rapid bubble growth and coalescence prior to magma disruption by fragmentation. The heterogeneity of vesicle sizes and shapes, and the absence of differential expansion across individual clasts, suggest that post-fragmentation expansion played a limited role in the development of pumice structure. The higher vesicle number densities and lower permeabilities of pyroclastic-flow clasts indicate limited coalescence and suggest that fragmentation occurred shortly after decompression. Either increased eruption velocities or increased depth of fragmentation accompanying caldera collapse could explain compression of the pre-fragmentation vesiculation interval.

Oregon↗

USGS2002

Mount Mazama and Crater Lake: Growth and destruction of a Cascade volcano

For more than 100 years, scientists have sought to unravel the remarkable story of Crater Lake’s formation. Before Crater Lake came into existence, a cluster of volcanoes dominated the landscape. This cluster, called Mount Mazama (for the Portland, Oregon, climbing club the Mazamas), was destroyed during an enormous explosive eruption 7,700 years ago. So much molten rock was expelled that the summit area collapsed during the eruption to form a large volcanic depression, or caldera. Subsequent smaller eruptions occurred as water began to fill the caldera to eventually form the deepest lake in the United States. Decades of detailed scientific studies of Mount Mazama and new maps of the floor of Crater Lake reveal stunning details of the volcano’s eruptive history and identify potential hazards from future eruptions and earthquakes.

Oregon↗

USGS2000

Rhyolite themobarometry and the shallowing of the magma reservoir, Coso volcanic field, California

The compositionally bimodal Pleistocene Coso volcanic field is located at the western margin of the Basin and Range province ∼60 km north of the Garlock fault. Thirty-nine nearly aphyric high-silica rhyolite domes were emplaced in the past million years: one at 1 Ma from a transient magma reservoir, one at ∼0·6 Ma, and the rest since ∼0·3 Ma. Over the past 0·6 My, the depth from which the rhyolites erupted has decreased and their temperatures have become slightly higher. Pre-eruptive conditions of the rhyolite magmas, calculated from phenocryst compositions using the two-oxide thermometer and the Al-in-hornblende barometer, ranged from 740°C and 270 MPa (2·7 kbar; ∼10 km depth) for the ∼0·6 Ma magma, to 770°C and 140 MPa (1·4 kbar; ∼5·5 km) for the youngest (∼0·04 Ma) magma. Results are consistent with either a single rhyolitic reservoir moving upward through the crust, or a series of successively shallower reservoirs. As the reservoir has become closer to the surface, eruptions have become both more frequent and more voluminous.

California↗

USGS1999

Gas-driven filter pressing in magmas

Most silicic and some mafic magmas expand via second boiling if they crystallize at depths of about 10 km or less. The buildup of gas pressure due to second boiling can be relieved by expulsion of melt out of the region of crystallization, and this process of gas-driven filter pressing assists the crystallization differentiation of magmas. For gas-driven filter pressing to be effective, the region of crystallization must inflate slowly relative to buildup of pressure and expulsion of melt These conditions are satisfied in undercooled magmatic inclusions and in thin sheets of primitive magma underplating cooler magma reservoirs. Gas-driven filter pressing thereby adds fractionated melt to magma bodies. Gas-driven filter pressing is probably the dominant process by which highly evolved melts segregate from crystal mush to form aplitic dikes in granitic plutons; this process could also account for the production of voluminous, crystal-poor rhyolites.

Geology↗

USGS1997

Volcano and earthquake hazards in the Crater Lake region, Oregon

Crater Lake lies in a basin, or caldera, formed by collapse of the Cascade volcano known as Mount Mazama during a violent, climactic eruption about 7,700 years ago. This event dramatically changed the character of the volcano so that many potential types of future events have no precedent there. This potentially active volcanic center is contained within Crater Lake National Park, visited by 500,000 people per year, and is adjacent to the main transportation corridor east of the Cascade Range. Because a lake is now present within the most likely site of future volcanic activity, many of the hazards at Crater Lake are different from those at most other Cascade volcanoes. Also significant are many faults near Crater Lake that clearly have been active in the recent past. These faults, and historic seismicity, indicate that damaging earthquakes can occur there in the future. This report describes the various types of volcano and earthquake hazards in the Crater Lake area, estimates of the likelihood of future events, recommendations for mitigation, and a map of hazard zones. The main conclusions are summarized below.

Oregon↗