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At least 433 records · Page 24Linked to original sources

USGS1996

Geology of 243 Ida

The surface of 243 Ida is dominated by the effects of impacts. No complex crater morphologies are observed. A complete range of crater degradation states is present, which also reveals optical maturation of the surface (darkening and reddening of materials with increasing exposure age). Regions of bright material associated with the freshest craters might be ballistically emplaced deposits or the result of seismic disturbance of loosely-bound surface materials. Diameter/depth ratios for fresh craters on Ida are ∼1:6.5, similar to Gaspra results, but greater than the 1:5 ratios common on other rocky bodies. Contributing causes include rim degradation by whole-body “ringing,” relatively thin ejecta blankets around crater rims, or an extended strength gradient in near-surface materials due to low gravitational self-packing. Grooves probably represent expressions in surface debris of reactivated fractures in the deeper interior. Isolated positive relief features as large as 150 m are probably ejecta blocks related to large impacts. Evidence for the presence of debris on the surface includes resolved ejecta blocks, mass-wasting scars, contrasts in color and albedo of fresh crater materials, and albedo streaks oriented down local slopes. Color data indicate relatively uniform calcium abundance in pyroxenes and constant pyroxene/olivine ratio. A large, relatively blue unit across the northern polar area is probably related to regolith processes involving ejecta from Azzurra rather than representing internal compositional heterogeneity. A small number of bluer, brighter craters are randomly distributed across the surface, unlike on Gaspra where these features are concentrated along ridges. This implies that debris on Ida is less mobile and/or consistently thicker than on Gaspra. Estimates of the average depth of mobile materials derived from chute depths (20–60 m), grooves (≥30 m), and shallowing of the largest degraded craters (20–50 m minimum, ∼100 m maximum) suggest a thickness of potentially mobile materials of ∼50 m, and a typical thickness for the debris layer of 50–100 m.

Icarus↗

USGS1998

Large impact features on Europa: Results of the Galileo Nominal Mission

The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) “classic” impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all <30 km diameter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Callisto, probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on Europa. Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement. Callanish and Tyre do not appear to be larger and even more viscously relaxed versions of the classic craters; rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ∼10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that ∼10–15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<10 8 years). Hence, it seems likely that our preliminary conclusions about the subsurface structure of Europa apply to the current day.

Icarus↗

USGS2017

Dendrochronology and late Holocene history of Bering piedmont glacier, Alaska

Fluctuations of the piedmont lobe of Bering Glacier and its sublobe Steller Glacier over the past two millennia are reconstructed using 34 radiocarbon dates and tree-ring data from 16 sites across the glaciers' forelands. The general sequence of glacial activity is consistent with well-dated fluctuations of tidewater and land-terminating glaciers elsewhere along the Gulf of Alaska. Extensive forested areas along 25 km of the Bering ice margin were inundated by glacio-lacustrine and glacio-fluvial sediments during a probable ice advance shortly before 500 cal yr A.D. Regrowth of forests followed the retreating ice as early as the 7th century A.D., with frequent interruptions of tree growth due to outwash aggradation. Forests overrun by ice and buried in outwash indicate readvance about 1080 cal yr A.D. Retreat followed, with ice-free conditions maintained along the distal portions of the forefield until the early 17th century after which the ice advanced to within a few kilometers of its outer Neoglacial moraine. Ice reached this position after the mid-17th century and prior to 200 yr ago. Since the early 20th century, glacial retreat has been punctuated by periodic surges. The record from forests overrun by the nonsurging Steller Lobe shows that this western ice margin was advancing by 1250 A.D., reaching near its outer moraine after 1420 cal yr A.D. Since the late 19th century, the lobe has dominantly retreated.

Quaternary Research↗

USGS2021

Polar bear foraging behavior

Polar bears forage in the marine environment, primarily on the sea ice over the shallow waters of the continental shelf. They are solitary, ambush hunters that catch ringed and bearded seals when they surface to breathe in ice holes or haul out on the ice to rest and molt. In most parts of their range, polar bears experience dramatic seasonal variability in their ability to catch seals, with foraging success peaking in late spring and early summer when seal pups are weaned. During this time, the body mass of polar bears can nearly double, especially in pregnant females, such that body composition may reach 49% body fat. The accumulation of body fat is vital for these bears to survive through the autumn and winter when seals are less accessible or when pregnant adult female bears enter dens and fast. When the sea ice retreats in summer, some bears exhibit a temporary switch to omnivory, feeding on a variety of terrestrial food. However, the energetic benefit of most terrestrial food is small relative to their marine mammal prey and, in some regions, increased land use has been associated with declines in body condition. Reduced accessibility of seal prey to polar bears as a result of global climate change threatens the long-term sustainability of this Arctic predator.

Book chapter↗

USGS1996

The age and thermal history of Cerro Rico de Potosi, Bolivia

Cerro Rico de Potosi, Bolivia, is the world's largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8 ± 0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a 40 Ar/ 39 Ar date of 13.76 ± 0.10Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5 ± 1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and wide-spread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of 39 Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system.

Cerro Rico de Potosi↗

USGS1990

Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intrusions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and provided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sulfate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, intermediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system. ?? 1990 Springer-Verlag.

Mineralium Deposita↗

USGS1992

The Galileo Solid-State Imaging experiment

The Solid State Imaging (SSI) experiment on the Galileo Orbiter spacecraft utilizes a high-resolution (1500 mm focal length) television camera with an 800 ?? 800 pixel virtual-phase, charge-coupled detector. It is designed to return images of Jupiter and its satellites that are characterized by a combination of sensitivity levels, spatial resolution, geometric fiedelity, and spectral range unmatched by imaging data obtained previously. The spectral range extends from approximately 375 to 1100 nm and only in the near ultra-violet region (??? 350 nm) is the spectral coverage reduced from previous missions. The camera is approximately 100 times more sensitive than those used in the Voyager mission, and, because of the nature of the satellite encounters, will produce images with approximately 100 times the ground resolution (i.e., ??? 50 m lp-1) on the Galilean satellites. We describe aspects of the detector including its sensitivity to energetic particle radiation and how the requirements for a large full-well capacity and long-term stability in operating voltages led to the choice of the virtual phase chip. The F/8.5 camera system can reach point sources of V(mag) ??? 11 with S/N ??? 10 and extended sources with surface brightness as low as 20 kR in its highest gain state and longest exposure mode. We describe the performance of the system as determined by ground calibration and the improvements that have been made to the telescope (same basic catadioptric design that was used in Mariner 10 and the Voyager high-resolution cameras) to reduce the scattered light reaching the detector. The images are linearly digitized 8-bits deep and, after flat-fielding, are cosmetically clean. Information 'preserving' and 'non-preserving' on-board data compression capabilities are outlined. A special "summation" mode, designed for use deep in the Jovian radiation belts, near Io, is also described. The detector is 'preflashed' before each exposure to ensure the photometric linearity. The dynamic range is spread over 3 gain states and an exposure range from 4.17 ms to 51.2 s. A low-level of radial, third-order, geometric distortion has been measured in the raw images that is entirely due to the optical design. The distortion is of the pincushion type and amounts to about 1.2 pixels in the corners of the images. It is expected to be very stable. We discuss the measurement objectives of the SSI experiment in the Jupiter system and emphasize their relationships to those of other experiments in the Galileo project. We outline objectives for Jupiter atmospheric science, noting the relationship of SSI data to that to be returned by experiments on the atmospheric entry Probe. We also outline SSI objectives for satellite surfaces, ring structure, and 'darkside' (e.g., aurorae, lightning, etc.) experiments. Proposed cruise measurement objectives that relate to encounters at Venus, Moon, Earth, Gaspra, and, possibly, Ida are also briefly outlined. The article concludes with a description of a 'fully distributed' data analysis system (HIIPS) that SSI team members intend to use at their home institutions. We also list the nature of systematic data products that will become available to the scientific community. Finally, we append a short 'historical' note outlining the responsibilities and roles of institutions and individuals that have been involved in the 14 year development of the SSI experiment so far. ?? 1992 Kluwer Academic Publishers.

Space Science Reviews↗

USGS1995

Climate response among growth increments of fish and trees

Significant correlations were found among the annual growth increments of stream fish, trees, and climate variables in the Ozark region of the United States. The variation in annual growth increments of rock bass ( Ambloplites rupestris ) from the Jacks Fork River was significantly correlated over 22 years with the ring width of four tree species: white oak ( Quercus alba ), post oak ( Quercus stellata ), shortleaf pine ( Pinus echinata ) and eastern red cedar ( Juniperus virginiana ). Rock bass growth and tree growth were both significantly correlated with July rainfall and stream discharge. Variations in annual growth of smallmouth bass ( Micropterus dolomieu ) from four streams were significantly correlated over 29 years (1939–1968) with mean May maximum air temperature but not with tree growth. The magnitude and significance of correlations among growth increments from fish and trees imply that conditions such as topography, stream gradient, organism age, and the distribution of a population relative to its geographic range can influence the climatic response of an organism. The timing and intensity of climatic variables may produce different responses among closely related species.

Arkansas, Missouri↗

USGS1977

Application of Sr and O isotope relations to the petrogenesis of the alkaline rocks of the Red Hill complex, New Hampshire, USA

The Red Hill ring complex in central New Hampshire is composed of apparently cogenetic syenites, nepheline-sodalite syenite, and granite. The ages and petrogenetic relations among five of the six recognized units have been investigated by rubidiumstrontium and oxygen isotope analysis of whole rocks and separated minerals. Whole-rock samples from three syenite units are consistent with a single Rb-Sr isochron which gives an age of 198??3 m.y. and an initial (87Sr/86Sr)o ratio of 0.70330??0.00016 (??2 sigma; ??=1.42?? 10-11y-1). However, Sr isotope data for two other units, nepheline syenite and granite, are not consistent with this isochron but rather indicate higher initial ratios which range from 0.7033 to about 0.707. Whole-rock O isotope analyses give ??18O values which range from+6.2 to+9.3??? Sr and O isotope analyses on mineral separates indicate that observed whole-rock variations in (87Sr/86Sr)o are primary and are not due to any secondary process. The fact that the isotope systematics correlate with rock type, suggests that crustal interaction is likely to have played a significant role in the development of this over-and undersaturated association. Such process(es), while still not fully delineated, could be of fundamental importance to the genesis of associations of critically undersaturated and oversaturated intrusives. The data support the idea that interaction between magmas and crustal materials strongly influenced the compositional relations of similar complexes elsewhere including those of the White Mountain magma series. ?? 1977 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1973

Tests of pesticidal synergism with young pheasants and Japanese quail

Thirteen pairs of chemicals involving 18 pesticides and two polychlorinated biphenyl preparations were each fed for 5 days to Japanese quail or ring-necked pheasant chicks 7 to 16 days of age. Malathion + EPN, and malathion + trichlorofon were moderately synergistic in tests with both species, whereas joint toxicities of the other chemicals tended to be additive. Comparisons with other studies of joint action of pesticides against mammals and insects suggest that the two species of birds tested are less susceptible to synergism than are mammals or insects. The results also suggest that the likelihood of a factor of synergism greater than three in birds is not great.

Bulletin of Environmental Contamination and Toxico↗

USGS1967

Base surge in recent volcanic eruptions

A base surge, first identified at the Bikini thermonuclear undersea explosion, is a ring-shaped basal cloud that sweeps outward as a density flow from the base of a vertical explosion column. Base surges are also common in shallow underground test explosions and are formed by expanding gases which first vent vertically and then with continued expansion rush over the crater lip (represented by a large solitary wave in an underwater explosion), tear ejecta from it, and feed a gas-charged density flow, which is the surge cloud. This horizontally moving cloud commonly has an initial velocity of more than 50 meters per second and can carry clastic material many kilometers. Base surges are a common feature of many recent shallow, submarine and phreatic volcanic eruptions. They transport ash, mud, lapilli, and blocks with great velocity and commonly sandblast and knock down trees and houses, coat the blast side with mud, and deposit ejecta at distances beyond the limits of throw-out trajectories. Close to the eruption center, the base surge can erode radial channels and deposit material with dune-type bedding.

Bulletin Volcanologique↗

USGS1997

Long-term growth trends of baldcypress (Taxodium distichum (L.) rich.) at Caddo Lake, Texas

Caddo Lake, situated on the border of northeast Texas and northwest Louisiana, USA is a medium-sized lake dominated by stands of baldcypress (Taxodiwn distichum). A study of tree growth was initiated at Caddo Lake to address concerns about the health of the baldcypress ecosystem. The lake has been subjected to several dramatic water-level changes over the past 200 years, including water-level stabilization following dam construction in 1914. To assess the long-term growth trends of baldcypress trees and determine if a recent growth decline is occurring at Caddo Lake, increment cores were taken from 52 trees. The cores were crossdated and rings between the years 1900 and 1992 measured to the nearest 0.01 mm. Most cores were characterized by high variation in year-to-year growth. Although increasing growth rates were observed at most locations, trees from two backwater areas of the lake had recent growth rates lower than their long-term average. Growth amounts at these two sites were, however, within the historic range of variation. No recruitment was observed. From these data, we can conclude that the historic, extreme changes in hydrologic regime and the current stabilized water levels have not resulted in an overall decline in baldcypress growth at Caddo Lake.

Texas↗

USGS2005

Factors affecting the toxicity of methylmercury injected into eggs

We developed a standardized protocol for comparing the sensitivities of the embryos of different bird species to methylmercury when methylmercury was injected into their eggs. During the course of developing this protocol, we investigated the effects of various factors on the toxicity of the injected methylmercury. Most of our experiments were done with chicken ( Gallus domesticus ), mallard ( Anas platyrhynchos ), and ring-necked pheasant ( Phasianus colchicus ) eggs, all of which were purchased in large numbers from game farms. A smaller amount of work was done with double-crested cormorant ( Phalacrocorax auritus ) eggs collected from the wild. Several solvents were tested, and corn oil at a rate of 1 μl/g egg contents was selected for the final standardized protocol because it had minimal toxicity to embryos and because methylmercury dissolved in corn oil yielded a dose–response curve in a range of egg concentrations that was similar to the range that causes reproductive impairment when the mother deposits methylmercury into her own eggs. The embryonic stage at which eggs were injected with corn oil altered mercury toxicity; at early stages, the corn oil itself was toxic. Therefore, in the final protocol we standardized the time of injection to occur when each species reached the morphologic equivalent of a 3-day-old chicken embryo. Although solvents can be injected directly into the albumen of an egg, high embryo mortality can occur in the solvent controls because of the formation of air bubbles in the albumen. Our final protocol used corn oil injections into the air cell, which are easier and safer than albumen injections. Most of the methylmercury, when dissolved in corn oil, injected into the air cell passes through the inner shell membrane and into the egg albumen. Most commercial incubators incubate eggs in trays with the air cell end of the egg pointing upward, but we discovered that mercury-induced mortality was too great when eggs were held in this orientation. In addition, some species of bird eggs require incubation on their sides with the eggs being rolled 180° for them to develop normally. Therefore, we adopted a procedure of incubating the eggs of all species on their sides and rolling them 180° every hour. Little has been published about the conditions of temperature, humidity, and the movements to which eggs of wild birds need to be subjected for them to hatch optimally under artificial incubation. Not unexpectedly, hatching success in an artificial incubator is generally less than what natural incubation by the parents can achieve. However, the survival of control embryos of most wild bird species was good (generally ≥ 80%) up to within 1 or 2 days of hatching when we incubated the eggs at 37.5°C (or 37.6°C for gallinaceous species) at a relative humidity that resulted in an approximate 15% to 16% loss in egg weight by the end of incubation and by incubating the eggs on their sides and rolling them 180°/h. To improve statistical comparisons, we used survival through 90% of incubation as our measurement to compare survival of controls with survival of eggs injected with graded concentrations of mercury.

Archives of Environmental Contamination and Toxico↗

USGS2012

Lake trout otolith chronologies as multidecadal indicators of high-latitude freshwater ecosystems

High-latitude ecosystems are among the most vulnerable to long-term climate change, yet continuous, multidecadal indicators by which to gauge effects on biology are scarce, especially in freshwater environments. To address this issue, dendrochronology (tree-ring analysis) techniques were applied to growth-increment widths in otoliths from lake trout ( Salvelinus namaycush ) from the Chandler Lake system, Alaska (68.23°N, 152.70°W). All otoliths were collected in 1987 and exhibited highly synchronous patterns in growth-increment width. Increments were dated, the widths were measured, and age-related growth declines were removed using standard dendrochronology techniques. The detrended time series were averaged to generate an annually resolved chronology, which continuously spanned 1964–1984. The chronology positively and linearly correlated with August air temperature over the 22-year interval (p < 0.01), indicating that warmer summers were beneficial for growth, perhaps by increasing fish metabolic rate or lake productivity. Given the broad distribution of lake trout within North America, this study suggests that otolith chronologies could be used to examine responses between freshwater ecosystems and environmental variability across a range of temporal and spatial scales.

Alaska↗

USGS2002

Birth of the modern Chesapeake Bay estuary between 7.4 and 8.2 ka and implications for global sea-level rise

Two major pulses of sea-level rise are thought to have taken place since the last glacial maximum — meltwater pulses (mwp) 1A (12 cal ka) and 1B (9.5 cal ka). Between mwp 1B and about 6 cal ka, many of the complex coastal ecosystems which ring the world’s oceans began to form. Here we report data for rhenium, carbon isotopes, total organic carbon, and fossil oysters from Chesapeake Bay which span the transition from fresh to brackish water conditions in the bay in the mid-Holocene. These data constrain sea-level change and resulting environmental change in the bay. They indicate that the transition was rapid, and that it was produced by (1) a third pulse of rapid eustatic sea-level rise, or (2) a geometry of the prehistoric Chesapeake Bay basin which predisposed it to a nonlinear response to a steadily rising sea level. Similar nonlinear changes in vulnerable coastal environments are likely to take place in the future due to polar warming, regardless of the timing or rate of sea-level rise.

Chesapeake Bay↗

USGS2020

Geology and eruptive history of Bogoslof volcano

Bogoslof volcano is a shallow submarine/subaerial volcano in the southern Bering Sea about 100 km west of the community of Dutch Harbor, Alaska. The subaerial parts of the volcano consist of two small islands, Bogoslof Island and Fire Island, that together have a total area of about 1.6 km 2 . Bogoslof was first depicted on a Russian map in 1772 and since then has been observed and visited occasionally. The volcano has had at least nine periods of eruptive activity since 1796 and all of its historical eruptions have been similar in style. Historical Bogoslof eruptions involved the effusion of basalt, trachybasalt, basaltic trachyandesite, and trachyandesite lava domes with above sea level relief of 100–200 m. Many of the eruptions are accompanied by the formation of tuff rings and ejection of ballistic particles. Historical observations suggest that eruption clouds are relatively ash-poor. Minor ash fallout has typically occurred within about 100 km of the volcano. Many of the historical eruptions began at vents that were below sea level, and thus, seawater has played an important role in the style of eruptive activity exhibited by the volcano. At times, eruptive activity has been characterized by Surtseyan style eruptions and magma interaction with wet vent-fill deposits. At other times, the eruptive style has been more magmatically driven and has resulted in the formation of pyroclastic flows and small ash clouds. Preliminary studies of the deposits produced during the 2016–2017 eruption indicate vertical sequences of coarse-grained, horizontally bedded pyroclastic flow and fall deposits with numerous blocks, bombs, and lapilli of dense juvenile and accidental lithic material. These deposits were emplaced by near-vent pyroclastic flows, surges, and explosions some of which originated from shallow, highly crystalline cryptodomes.

Bulletin of Volcanology↗

USGS2023

Spatio-temporal clustering of post-caldera eruptions at Yellowstone caldera: Implications for volcanic hazards and pre-eruptive magma reservoir configuration

At least 28 rhyolitic lava flows, domes, and tuffs erupted within Yellowstone caldera following its formation 631 ka. Understanding the timing of intracaldera eruptions is essential for characterizing natural hazards posed by Yellowstone volcano. We present 40 Ar/ 39 Ar eruption ages for the Mallard Lake Member and Central Plateau Member of the Plateau Rhyolite, which comprise products from 22 eruptions spanning ~160 to ~70 ka. These eruptions occurred along two linear vent zones within Yellowstone caldera that appear to be extensions of major extracaldera normal faults. Eruptions took place in five brief clusters with group mean ages of 160.3±1.0 ka (2σ), 149.8±4.0 ka, 111.4±0.9 ka, 104.1±0.8 ka, and 70.8±0.7 ka. All mapped products within each informal group have indistinguishable 40 Ar/ 39 Ar eruption ages implying brief eruption durations. Eruption products of each group are spatially clustered, with only one of the two linear vent zones being active during an eruption group. Using the oldest group (160 ka), we apply paleomagnetic and geochemical analyses to investigate the duration of these eruption groups and their pre-eruptive magma reservoir configuration. Paleomagnetic analyses suggest that the nine rhyolite eruptions ~160 ka spanned 400 years or less. Glass and sanidine compositions of each rhyolite are nearly identical, which is consistent with pre-eruptive storage of a large (~130 km 3 ), interconnected melt body within Yellowstone volcano’s crystal-rich magma reservoir. These characteristics differ from intracaldera eruptions that vented near the caldera’s inner ring fracture system, which tend to be compositionally disparate with less temporal clustering. These insights inform hazard mitigation scenarios.

Idaho, Wyoming↗

USGS2001

New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy)

The &sim; 150 km 3 (DRE) trachytic Campanian Ignimbrite, which is situated north-west of Naples, Italy, is one of the largest eruptions in the Mediterranean region in the last 200 ky. Despite centuries of investigation, the age and eruptive history of the Campanian Ignimbrite is still debated, as is the chronology of other significant volcanic events of the Campanian Plain within the last 200–300 ky. New 40 Ar/ 39 Ar geochronology defines the age of the Campanian Ignimbrite at 39.28 ± 0.11 ka, about 2 ky older than the previous best estimate. Based on the distribution of the Campanian Ignimbrite and associated uppermost proximal lithic and polyclastic breccias, we suggest that the Campanian Ignimbrite magma was emitted from fissures activated along neotectonic Apennine faults rather than from ring fractures defining a Campi Flegrei caldera. Significantly, new volcanological, geochronological, and geochemical data distinguish previously unrecognized ignimbrite deposits in the Campanian Plain, accurately dated between 157 and 205 ka. These ages, coupled with a xenocrystic sanidine component > 315 ka, extend the volcanic history of this region by over 200 ky. Recent work also identifies a pyroclastic deposit, dated at 18.0 ka, outside of the topographic Campi Flegrei basin, expanding the spatial distribution of post-Campanian Ignimbrite deposits. These new discoveries emphasize the importance of continued investigation of the ages, distribution, volumes, and eruption dynamics of volcanic events associated with the Campanian Plain. Such information is critical for accurate assessment of the volcanic hazards associated with potentially large-volume explosive eruptions in close proximity to the densely populated Neapolitan region.

Mineralogy and Petrology↗