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At least 595 records · Page 33Linked to original sources
USGS2017

Emplacement of Holocene silicic lava flows and domes at Newberry, South Sister, and Medicine Lake volcanoes, California and Oregon

This field guide for the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Scientific Assembly 2017 focuses on Holocene glassy silicic lava flows and domes on three volcanoes in the Cascade Range in Oregon and California: Newberry, South Sister, and Medicine Lake volcanoes. Although obsidian-rich lava flows have been of interest to geologists, archaeologists, pumice miners, and rock hounds for more than a century, many of their emplacement characteristics had not been scientifically observed until two very recent eruptions in Chile. Even with the new observations, several eruptive processes discussed in this field trip guide can only be inferred from their final products. This makes for lively debates at outcrops, just as there have been in the literature for the past 30 years. Of the three volcanoes discussed in this field guide, one (South Sister) lies along the main axis defined by major peaks of the Cascade Range, whereas the other two lie in extensional tectonic settings east of the axis. These two tectonic environments influence volcano morphology and the magmatic and volcanic processes that form silicic lava flows and domes. The geomorphic and textural features of glass-rich extrusions provide many clues about their emplacement and the magma bodies that fed them. The scope of this field guide does not include a full geologic history or comprehensive explanation of hazards associated with a particular volcano or volcanic field. The geochemistry, petrology, tectonics, and eruption history of Newberry, South Sister, and Medicine Lake volcanic centers have been extensively studied and are discussed on other field excursions. Instead, we seek to explore the structural, textural, and geochemical evolution of well-preserved individual lava flows—the goal is to understand the geologic processes, rather than the development, of a specific volcano.

California, Oregon↗
USGS2017

Geologic field-trip guide to Mount Shasta Volcano, northern California

The southern part of the Cascades Arc formed in two distinct, extended periods of activity: “High Cascades” volcanoes erupted during about the past 6 million years and were built on a wider platform of Tertiary volcanoes and shallow plutons as old as about 30 Ma, generally called the “Western Cascades.” For the most part, the Shasta segment (for example, Hildreth, 2007; segment 4 of Guffanti and Weaver, 1988) of the arc forms a distinct, fairly narrow axis of short-lived small- to moderate-sized High Cascades volcanoes that erupted lavas, mainly of basaltic-andesite or low-silica-andesite compositions. Western Cascades rocks crop out only sparsely in the Shasta segment; almost all of the following descriptions are of High Cascades features except for a few unusual localities where older, Western Cascades rocks are exposed to view along the route of the field trip. The High Cascades arc axis in this segment of the arc is mainly a relatively narrow band of either monogenetic or short-lived shield volcanoes. The belt generally averages about 15 km wide and traverses the length of the Shasta segment, roughly 100 km between about the Klamath River drainage on the north, near the Oregon-California border, and the McCloud River drainage on the south (fig. 1). Superposed across this axis are two major long-lived stratovolcanoes and the large rear-arc Medicine Lake volcano. One of the stratovolcanoes, the Rainbow Mountain volcano of about 1.5–0.8 Ma, straddles the arc near the midpoint of the Shasta segment. The other, Mount Shasta itself, which ranges from about 700 ka to 0 ka, lies distinctly west of the High Cascades axis. It is notable that Mount Shasta and Medicine Lake volcanoes, although volcanologically and petrologically quite different, span about the same range of ages and bracket the High Cascades axis on the west and east, respectively. The field trip begins near the southern end of the Shasta segment, where the Lassen Volcanic Center field trip leaves off, in a field of high-alumina olivine tholeiite lavas (HAOTs, referred to elsewhere in this guide as low-potassium olivine tholeiites, LKOTs). It proceeds around the southern, western, and northern flanks of Mount Shasta and onto a part of the arc axis. The stops feature elements of the Mount Shasta area in an approximately chronological order, from oldest to youngest.

California↗
USGS2022

Field-trip guide to continental arc to rift volcanism of the southern Rocky Mountains—Southern Rocky Mountain, Taos Plateau, and Jemez Mountains volcanic fields of southern Colorado and northern New Mexico

The southern Rocky Mountains of northern New Mexico and southern Colorado preserve the Oligocene to Pleistocene record of North American continental arc to rift volcanism. The 35–23 million year old (Ma) southern Rocky Mountain volcanic field (SRMVF), spectacularly preserved in the San Juan Mountains of southern Colorado, records the evolution of large andesitic stratovolcanoes to complex caldera clusters, from which at least 22 major ignimbrite sheets (each 150–5,000 cubic kilometers) were erupted. Outflow deposits of the SRMVF preserved along the broadly uplifted northwest flank of the northern Rio Grande rift basin (the San Luis Valley) provide critical structural and temporal constraints on the inception of crustal extension. Coincident with waning stages of SRMVF caldera-forming volcanism (~25.4 Ma), extensional tectonism was accompanied by a transition from bimodal early Miocene to intermediate-composition late Miocene and dominantly basaltic Pliocene rift volcanism of the Taos Plateau in the southern San Luis Basin. Concomitant rift volcanism in the Española Basin and bordering Jemez Mountains of northern New Mexico records a similar Miocene eruptive history dominated by intermediate-composition volcanism that transitioned locally to Pliocene rift-related basaltic volcanism of the Cerros del Rio volcanic field and culminated in eruptions of the iconic rhyolitic Pleistocene Bandelier Tuff and formation of the Valles Caldera along the northwestern rift-basin margin. This 6-day, 7-night field trip will focus, in broadly equal proportions, on rift-related extensional volcanism of the Jemez Mountains and Taos Plateau regions during the first half of the trip, and on caldera-forming volcanism of the southern Rocky Mountain volcanic field during the second half of the trip. The 35-million-year volcanic history of the region highlighted by new geologic mapping, high-resolution geochronology, petrologic, geochemical, and geophysical data facilitates discussion of (1) the magmatic response to the tectonic transition from subducted-slab arc to continental-rift volcanism; (2) the nature and temporal evolution of rift magmas; (3) fault controls on the spatial evolution of rift magmatism; (4) the diversity of continental-arc ignimbrite volcanism and associated lavas; (5) ignimbrite caldera structure and associated intrusions in three-dimension; (6) the role of recycled crystal mush and magmatic cumulates during growth of Cordilleran batholiths; and (7) high-precision geochronologic contributions to interpretation of relations between regional tectonic and volcanic processes. Most stops will be along roads, but there will be moderate hikes on trails of less than 1-hour duration covering 1–2 kilometers (0.6–1.2 miles) with modest elevation gain of <150 meters (<492 feet). The route will progress in reverse stratigraphic order, starting in the Jemez Mountains of New Mexico and proceed northward to San Luis Basin and San Luis Hills before turning west to the southeast and central San Juan Mountains. Our last full day takes us to the little-visited and only recently mapped, Bonanza caldera of the northeastern San Juan Mountains and on the final day, we leave the San Luis Valley to briefly explore the Tertiary subvolcanic plutons of the Collegiate Range along the west side of the Arkansas Valley rift valley, en route to Denver. The authors of all daily contributions acknowledge the helpful reviews by Amy Gilmer and Joe Colgan and thank Christine Chan and Jeremy Havens for assistance with figures, tables, and guidebook text.

Colorado, New Mexico↗
USGS2024

Special topic—Eruption plumes and clouds

Introduction Explosive eruptions create plumes of volcanic ash and gas that can rise more than 30,000 feet (9.1 kilometers [km]) above sea level within minutes of eruption onset. The resulting clouds disperse under prevailing winds and may cause hazardous conditions hundreds to thousands of kilometers from the volcano, including in international airspace. Rapid detection and characterization of explosive activity is vital to mitigate the wide-ranging effects of volcanic ash. Ashfall thicknesses as small as a millimeter or so on the ground can affect infrastructure, agriculture, and air quality, requiring extensive clean-up procedures (Schuster, 1981; Warrick and others, 1981, U.S. Geological Survey, 2022). Volcanic clouds also pose substantial threats to aircraft. Since 1953, 88 encounters between airplanes and ash clouds have been documented worldwide (International Civil Aviation Organization, 2015, appendix F), resulting in aircraft damage and, in 9 cases, engine failure (Guffanti and others, 2010). In 1982, two large passenger planes suffered complete engine failure owing to eruptions in Indonesia (Global Volcanism Program, 1982) and a similar incident occurred over Alaska in 1989 (Casadevall, 1994). In all three cases, they were able to restart some engine capability and land safely once they emerged from the ash clouds, although with substantial damage (Guffanti and others, 2010). The clear threat to aviation has led to establishment of nine Volcanic Ash Advisory Centers (VAAC) around the world to monitor and rapidly disseminate information about volcanic eruptions to the aviation community. U.S. Geological Survey (USGS) volcano observatories issue the Volcano Observatory Notice for Aviation that informs of preeruptive unrest or eruptive activity. When ash-producing eruptions do occur, volcano observatories work closely with their regional VAAC to ensure consistency and accuracy in eruption onset time, cloud altitude, ash production, and duration as reported in Volcanic Ash Advisories. Explosive volcanism in the United States and Commonwealth of the Northern Mariana Islands prompts 50–100 such advisories in any given year (table J1). This collaborative effort is greatly aided by USGS detection and monitoring of eruption clouds to ensure a timely and coordinated response. To support these efforts to provide guidance on ash transport and fallout, the USGS developed the Ash3d volcanic ash dispersion model ( https://vsc-ash.wr.usgs.gov/ash3d-gui ) (Schwaiger and others, 2012). Automated simulations are run daily by the USGS for volcanoes that are in elevated states of unrest, and in response mode when eruptions occur. During eruptions, the model output is provided to local National Weather Service Weather Forecast Offices to guide them in the issuance of their information products (such as special weather statements, ashfall advisories, or ashfall warnings), as well as to State and local governments and the public. Characterization of the eruption source is needed to estimate the parameters used to initialize the Ash3d model, and by the Anchorage and Washington VAACs to initialize other dispersion models that inform forecasts for the airborne volcanic cloud. The source parameters that can be provided by observation during an eruption include eruption start time, eruption cloud height over time, and eruption duration. Other, nonobservable source parameters, such as mass eruption rate and grain-size distribution, are based on empirical correlations and study of historical deposits. The goal is to provide a time series of cloud heights, mass eruption rates, and particle-size distributions that accurately reflects current conditions. When feasible, the USGS also provides guidance on the nature of ongoing eruptions and forecasts future activity using petrologic monitoring of collected tephra samples. The aims of providing accurate observable parameters are achieved through analysis of (1) near-real-time meteorological satellite data, (2) ground-based cameras (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–G, 11 p., https://doi.org/10.3133/sir20245062g.">chapter G , this volume; Orr and others, 2024), (3) weather radar, (4) volcanic lightning detection, and (5) ground-based ash sensors and sampling. Explosive eruptions can be detected by a variety of geophysical monitoring, including infrasound (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–C, 11 p., https://doi.org/10.3133/sir20245062c.">chapter C , this volume; Lyons and others, 2024) and seismicity (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–B, 9 p., https://doi.org/10.3133/sir20245062b.">chapter B , this volume; Thelen and others, 2024). However, those methods cannot quantify the altitude, ash content, and dispersal dynamics of resulting volcanic clouds. Ideally, all available sources of monitoring data are synthesized to develop a coherent understanding of eruptive activity. The guidance summarized here provides a framework for characterizing volcanic clouds in the atmosphere and tracking the evolution of explosive eruption dynamics.

Scientific Investigations Report↗
USGS2025

Paleomagnetic correlation of surface and subsurface basalt flows in the central and southwestern part of the Idaho National Laboratory, Idaho

The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, used paleomagnetic data from 22 coreholes to construct 3 fence diagrams of subsurface basalt flows in the southern part of the Idaho National Laboratory. These diagrams provide comprehensive descriptions of the horizontal and vertical distribution of basalt flows and sediment layers beneath the surface, aiding geological studies and contributing valuable data to numerical models of groundwater flow and contaminant transport. The correlations established though these diagrams include spatial correlations between basalt flows found in multiple coreholes. Correlations were identified by matching average paleomagnetic inclinations and confirming or denying these correlations using petrology, geochemistry and radiometric ages. The fence diagrams aid in identifying potential locations of subsurface vents, volcanic vents that have been buried by more recent volcanic activity, associated to subsurface basalt flows. By tracing the subsurface flows and analyzing where the greatest thickness occurs, the locations of buried vents can be inferred. Some subsurface flows exhibit correlations across several coreholes and may indicate yet unidentified surface or buried vents, thereby enhancing our understanding of the volcanic history and subsurface geology of the region.

Idaho↗
USGS1952

Stratigraphic sections of the Phosphoria formation in Idaho, 1947-48, Part I

The Permian Phosphoria formation of the western states contains one of the world's largest reserves of phosphate. Although previous investigations (see especially Mansfield, 1927), including reconnaissance geologic mapping and sampling, established the location of most of the important deposits and their quality of scattered localities, they were not sufficiently detailed to permit a comparison of the merits of individual deposits or an appraisal of the reserves of phosphate rock that might be available under present economic conditions. Because the growing importance of the western phosphate deposits requires a better, more detailed understanding of their distribution and quality, the Geological Survey began in 1947 a comprehensive investigation, including (1) reconnaissance geologic mapping, mostly in Montana, of areas in which the Phosphoria formation could occur but where it had not previously been looked for or found; (2) geologic mapping, mostly in Montana, at a scale no smaller than 1:62,500, of several areas known to contain the Phosphoria formation but not previously mapped except in reconnaissance fashion; (3) geologic and topographic mapping, at a scale of 1:12,000, of some of the richest, thickest, and most accessible deposits; (4) measuring, describing, and sampling all beds of the phosphatic and shaly parts, and in some places the full thickness, of the Phosphoria formation and its stratigraphic equivalents at one or two localities per township over the entire field; (5) chemical and spectrographic analysis of the samples for phosphate, fluorine, minor metals, oil, and rock-forming constituents; and (6) petrologic and geochemical studies of the rocks and minerals of the formation. These studies are designed to define the regional and local geologic structures in which the phosphate bed lie, to provide a basis for the estimation of reserves of the inferred class over the entire region, and to determine the origin of the rocks and the elements contained in them. The data collected are not of the detail required to plan actual mining operations, but it is hoped they will guide industry in the selection of individual deposits worthy of further exploration. Most of the field work contemplated as a part of this investigation is now completed. Although the data will not be compiled or published in final form for some years to come, segments of the data, accompanied by little or no interpretation, will be published as preliminary maps or reports as they are assembled. The present report is the first of a series presenting in abbreviated form the description and analyses of the beds measured and sampled at various localities in southeastern Idaho (pl. 1). Companion reports presenting segments of the data from Montana, Wyoming, and Utah (Swanson and others, 1952, and McKelvey and others, 1952a and 1952b) are being released at the same time as this report, and others are in preparation.

Idaho↗
USGS2023

Extrusion tectonism of Indochina reassessed: constraints from 40Ar/39Ar geochronology from the Day Nui Con Voi metamorphic massif, Vietnam

The extrusion tectonic model for the southeastern margin of the Himalayan orogeny links the crustal shear activity along the Red River Shear Zone (RRSZ) to the opening of the South China Sea (SCS). The Day Nui Con Voi (DNCV) metamorphic massif in northern Vietnam strikes NW-SE, is bounded by the RRSZ to the south and continues along the strike where it meets the SCS. The DNCV is thus a critical area to document thermotectonic history in order to advance our understanding of the tectonic evolution of Indochina extrusion and its relationship to the opening of the SCS. Our new 40 Ar/ 39 Ar data combined with microstructural and petrological analyses constrained the timing of the left-lateral shearing of the RRSZ and revealed the thermal evolution of the DNCV metamorphic massif. Three ductile deformation events were observed. D 1 formed NNW-SSE striking upright folds under granulite to upper amphibolite facies conditions. D 2 was a horizontal to sub-horizontal folding event that occurred at amphibolite facies conditions. D 3 was a doming event that formed NW-SE striking up-right folds bounded by left-lateral shearing mylonite belts along the two limbs. The S/C fabrics were defined by muscovite fish, quartz + albite + K-feldspar aggregates, and muscovite folia. The D 3 doming event exhumed the DNCV metamorphic massif from amphibolite facies conditions to the lower greenschist facies conditions. The 40 Ar/ 39 Ar ages obtained from amphibole (∼26 Ma), phlogopite (∼25 Ma), muscovites (∼24-23 Ma), biotite (∼25-23 Ma), and K-feldspars (∼25-22 Ma) from different structural domains of the DNCV metamorphic massif indicated a rapid exhumation ∼26–22 Ma. We interpreted this as the time period for the D 3 event, with the onset of left-lateral shearing occurring around 24 Ma based on ages obtained from syn-kinematic muscovites. This age was much younger than the initiation of sea-floor spreading of the SCS (since 32 Ma) but coincided with the age for the ridge jump event in the SCS. Based on these new data, we proposed that extrusion tectonism cannot be the cause for the initial opening of the SCS. Rather, the extrusion of the Indochina block was temporally correlative with the southward ridge jump event of the already opened SCS.

Day Nui Con Voi massif↗
USGS2023

Editorial: Remote sensing of volcanic gas emissions from the ground, air, and space

When magma rises in volcanic systems, volatile species exsolve from the melt and are outgassed to the atmosphere. The melt composition and temperature, depth at which degassing occurs, extent of gas-water-rock interactions, and volume of ascending magma are all factors that determine the composition and rate of gas emissions at the surface. Interpreted in a petrological framework, gas measurements thus provide information on these fundamental parameters of volcanic systems. Volcanic gases have traditionally been sampled in the field and later analyzed with standard laboratory methods, but remote sensing measurements are playing an increasingly central role in characterizing emissions and the volcanoes from which they originate. The 17 contributions in this Research Topic summarize the state-of-the-art in volcanic gas remote sensing and identify key areas in which the field could further improve our understanding of global volcanism and its impact on Earth’s environment in the next decade.

Frontiers in Earth Science↗
USGS2013

The Chaitén rhyolite lava dome: Eruption sequence, lava dome volumes, rapid effusion rates and source of the rhyolite magma

We use geologic field mapping and sampling, photogrammetric analysis of oblique aerial photographs, and digital elevation models to document the 2008-2009 eruptive sequence at Chaitén Volcano and to estimate volumes and effusion rates for the lava dome. We also present geochemical and petrologic data that contribute to understanding the source of the rhyolite and its unusually rapid effusion rates. The eruption consisted of five major phases: 1. An explosive phase (1-11 May 2008); 2. A transitional phase (11-31 May 2008) in which low-altitude tephra columns and simultaneous lava extrusion took place; 3. An exogenous lava flow phase (June-September 2008); 4. A spine extrusion and endogenous growth phase (October 2008-February 2009); and 5. A mainly endogenous growth phase that began after the collapse of a prominent Peléean spine on 19 February 2009 and continued until the end of the eruption (late 2009 or possibly earliest 2010). The 2008-2009 rhyolite lava dome has a total volume of approximately 0.8 km3. The effusion rate averaged 66 m3s-1 during the first two weeks and averaged 45 m3s-1 for the first four months of the eruption, during which 0.5 km3 of rhyolite lava was erupted. These are among the highest rates measured world-wide for historical eruptions of silicic lava. Chaitén’s 2008-2009 lava is phenocryst-poor obsidian and microcrystalline rhyolite with 75.3±0.3% SiO2. The lava was erupted at relatively high temperature and is remarkably similar in composition and petrography to Chaitén’s pre-historic rhyolite. The rhyolite’s normative composition plots close to that of low pressure (100-200 MPa) minimum melts in the granite system, consistent with estimates of approximately 5 to 10 km source depths based on phase equilibria and geodetic studies. Calcic plagioclase, magnesian orthopyroxene and aluminous amphibole among the sparse phenocrysts suggest derivation of the rhyolite by melt extraction from a more mafic magmatic mush. High temperature and relatively low viscosity enabled rapid magma ascent and high effusion rates during the dome-forming phases of the 2008-2009 eruption.

Chaitén Volcano↗
USGS2025

Zircon as a pathfinder to REE mineralization

Carbonatites and alkaline silicate rocks are major primary sources of the rare earth elements (REE) and other critical metals, such as Nb. Despite the economic significance of these rocks, their formation and the processes of REE enrichment are poorly understood. Here, statistical analysis of a global dataset demonstrates that zircon geochemistry is a powerful recorder of REE metallogenesis and a potential pathfinder for REE deposits. Zircons from REE and Nb fertile intrusions lack Eu anomalies and have elevated Gd/Yb and Th/Yb, indicating they crystallised from magmas that originated from deep, oxidised and enriched mantle sources. Complexes with Nb enrichment have low U/Nb, reflecting an enriched mantle source, whereas high U/Nb in REE-only fertile intrusions suggest a subduction-metasomatised mantle source. Machine learning models demonstrate high accuracy in classifying zircon from barren and fertile deposits. Classification of detrital zircons shows that REE-enriched deposits correlate with supercontinent assembly, whereas Nb fertile complexes are associated with supercontinent breakup. This approach offers a new, mineral to global scale, petrologic and exploration tool that enhances understanding of REE metallogenesis.

Geochemical Perspectives Letters↗
USGS2010

Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry

New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 &plusmn; 7 Ma, 1180&ndash;1155 Ma, and 1061 &plusmn; 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33&ndash;1.29 Ga age in 1161 &plusmn; 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz &plusmn; clinopyroxene indicates that prograde metamorphism at 1170&ndash;1145 Ma and 1060&ndash;1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 &plusmn; 40 &deg;C during both episodes. The chemical composition of 1327 &plusmn; 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180&ndash;1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160 and 1060 Ma correlates temporally with the Shawinigan orogeny and Ottawan phase of the Grenvillian orogeny, respectively, suggesting that the Blue Ridge was part of Rodinia dating back to ca. 1180 Ma.

North Carolina;Virginia↗
USGS1984

SHUTTLE IMAGING RADAR PROVIDES FRAMEWORK FOR SUBSURFACE GEOLOGIC EXPLORATION IN EGYPT AND SUDAN.

Shuttle Imaging Radar provides a pictorial framework to guide exploration for mineral resources (potential placers), groundwater sources, and prehistoric archaeological sites in the Western Desert of Egypt and Sudan. Documented penetration by the SIR-A signal of dry surficial sediment to depths of a meter or more revealed bedrock geologic features and networks of former stream valleys otherwise concealed beneath windblown sand, alluvium, and colluvial deposits. 'Radar units' mapped on SIR-A images according to relative brightness and degree of mottling correspond to subsurface geologic and topographic features identified in more than 50 test pits. Petrologic examination of pit samples confirms that a variety of depositional environments existed in this now hyper-arid region before it was mantled by windblown sand sheets and dunes. Wet sand was discovered in two buried valleys shown on the radar images and located in the field with the aid of co-registered maps and Landsat images, and a satellite navigation device. Buried valleys whose streams once traversed mineralized zones are potential sites of placers (gold, tin).

Conference Paper↗
USGS1993

Magma transport and metasomatism in the mantle: a critical review of current geochemical models

Conflicting geochemical models of metasomatic interactions between mantle peridotite and melt all assume that mantle reactions reflect chromatographic processes. Examination of field, petrological, and compositional data suggests that the hypothesis of chromatographic fractionation based on the supposition of large-scale percolative processes needs review and revision. Well-constrained rock and mineral data from xenoliths indicate that many elements that behave incompatibly in equilibrium crystallization processes are absorbed immediately when melts emerge from conduits into depleted peridotite. After reacting to equilibrium with the peridotite, melt that percolates away from the conduit is largely depleted of incompatible elements. Continued addition of melts extends the zone of equilibrium farther from the conduit. Such a process resembles ion-exchange chromatography for H2O purification, rather than the model of chromatographic species separation. -from Authors

American Mineralogist↗
USGS1996

Hawaii scientific drilling protect: Summary of preliminary results

Petrological, geochemical, geomagnetic, and volcanological characterization of the recovered core from a 1056-m-deep well into the flank of the Mauna Kea volcano in Hilo, Hawaii, and downhole logging and fluid sampling have provided a unique view of the evolution and internal structure of a major oceanic volcano unavailable from surface exposures. Core recovery was ~90%, yielding a time series of fresh, subaerial lavas extending back to ~400 ka. Results of this 1993 project provide a basis for a more ambitious project to core drill a well 4.5 km deep in a nearby location with the goal of recovering an extended, high-density stratigraphic sequence of lavas.

Hawaii↗
USGS1996

Porphyry deposits of the Canadian Cordillera

Porphyry deposits are intrusion-related, large tonnage low grade mineral deposits with metal assemblages that may include all or some of copper, molybdenum, gold and silver. The genesis of these deposits is related to the emplacement of intermediate to felsic, hypabyssal, generally porphyritic intrusions that are commonly formed at convergent plate margins. Porphyry deposits of the Canadian Cordillera occur in association with two distinctive intrusive suites: calc-alkalic and alkalic. In the Canadian Cordillera, these deposits formed during two separate time periods: Late Triassic to Middle Jurassic (early Mesozoic), and Late Cretaceous to Eocene (Mesozoic-Cenozoic). Deposits of the early Mesozoic period occur in at least three different arc terranes (Wrangellia, Stikinia and Quesnellia) with a single deposit occurring in the oceanic assemblage of the Cache Creek terrane. These terranes were located outboard from continental North America during formation of most of their contained early Mesozoic porphyry deposits. Some of the deposits of this early period may have been emplaced during terrane collisions. Metal assemblages in deposits of the calc-alkalic suite include Mo-Cu (Brenda), Cu-Mo (Highland Valley, Gibraltar), Cu-Mo-Au-Ag (Island Copper, Schaft Creek) and Cu-Au (Kemess, Kerr).The alkalic suite deposits are characterized by a Cu-Au assemblage (Copper Mountain, Afton-Ajax, Mt. Milligan, Mount Polley, Galore Creek). Although silver is recovered from calc-alkalic and alkalic porphyry copper mining operations, silver data are seldom included in the published reserve figures. Those available are in the range of 1-2 grams per tonne (g??t-1). Alkalic suite deposits are restricted to the early Mesozoic and display distinctive petrology, alteration and mineralization that suggest a similar tectonic setting for both Quesnellia and Stikinia in Early Jurassic time. The younger deposits, late Mesozoic to Cenozoic in age, formed in an intracontinental setting, after the outboard host arc and related terranes accreted to the western margin of North America. These deposits are interpreted to occur in continental arc settings, and individual deposits are hosted by a variety of older country rocks. These younger deposits also show a spectrum of metal associations: Cu-Mo (Huckleberry, Berg), Cu-Au (-Mo) (Bell, Granisle, Fish Lake, Casino), Mo (Endako, Boss Mountain, Kit-sault, Quartz Hill), Mo-W (Logtung), Au-W (Dublin Gulch) and Au (Ft. Knox). There may be a continuum between Mo, Mo-W, Au-Mo-W and Au deposits. The distribution and timing of these post-accretion deposits likely reflect major crustal structures and subduction geometry. Cordilleran porphyry metallic deposits show the full range of morphological and depth relationships found in porphyry deposits worldwide. In addition, the Cordillera contains numerous alkalic suite deposits, which are rare worldwide: the unusual, possibly syntectonic Gibraltar deposit; and end-member gold-rich granite-hosted deposits, such as Ft. Knox (Alaska).

Geoscience Canada↗
USGS1997

Late Pleistocene-Holocene cataclysmic eruptions at Nevado de Toluca and Jocotitlan volcanoes, central Mexico

This field guide describes a five day trip to examine deposits of Late Pleistocene-Holocene cataclysmic eruptions at Nevado de Toluca and Jocotitlan volcanoes in central Mexico. We will discuss the stratigraphy, petrology, and sedimentological characteristics of these deposits which provide insights into the eruptive history, type of volcanic activity, and transport and emplacement mechanisms of pyroclastic materials. These parameters will allow us to discuss the kinds of hazards and the risk that they pose to populations around these volcanoes. The area to be visited is tectonically complex thus we will also discuss the location of the volcanoes with respect to the tectonic environment. The first four days of the field trip will be dedicated to Nevado de Toluca Volcano (19 degrees 09'N; 99 degrees 45'W) located at 23 km. southwest of the City of Toluca, and is the fourth highest peak in the country, reaching an elevation of 4,680 meters above sea level (m.a.s.l.). Nevado de Toluca is an andesitic-dacitic stratovolcano, composed of a central vent excavated upon the remains of older craters destroyed by former events. Bloomfield and Valastro, (1974, 1977) concluded that the last cycle of activity occurred nearly equal 11,600 yr. ago. For this reason Nevado de Toluca has been considered an extinct volcano. Our studies, however, indicate that Nevado de Toluca has had at least two episodes of cone destruction by sector collapse as well as several explosive episodes including plinian eruptions and dome-destruction events. These eruptions occurred during the Pleistocene but a very young eruption characterized by surge and ash flows occurred ca. 3,300 yr. BP. This new knowledge of the volcano's eruptive history makes the evaluation of its present state of activity and the geological hazards necessary. This is important because the area is densely populated and large cities such as Toluca and Mexico are located in its proximity.

Brigham Young University Geology Studies↗
USGS1997

Bimodal magmatism, basaltic volcanic styles, tectonics, and geomorphic processes of the eastern Snake River Plain, Idaho

Geology presented in this field guide covers a wide spectrum of internal and surficial processes of the eastern Snake River Plain, one of the largest components of the combined late Cenozoic igneous provinces of the western United States. Focus is on widespread Quaternary basaltic plains volcanism that produced coalescent shields and complex eruptive centers that yielded compositionally evolved magmas. The guide is constructed in several parts beginning with discussion sections that provide an overview of the geology followed by road directions, with explanations, for specific locations. The geology overview briefly summarizes the collective knowledge gained, and petrologic implications made, over the past few decades. The field guide covers plains volcanism, lava flow emplacement, basaltic shield growth, phreatomagmatic eruptions, and complex and evolved eruptive centers. Locations and explanations are also provided for the hydrogeology, groundwater contamination, and environmental issues such as range fires and cataclysmic floods associated with the region.

Brigham Young University Geology Studies↗