SEARCH · USGS Science

Results for “American Currents”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21Linked to original sources

USGS2019

Glacier retreat in Glacier National Park, Montana

Currently, the volume of land ice on Earth is decreasing, driving consequential changes to global sea level and local stream habitat. Glacier retreat in Glacier National Park, Montana, U.S.A., is one example of land ice loss and glacier change. The U.S. Geological Survey Benchmark Glacier Project conducts glaciological research and collects field measurements across select North American glaciers, including Sperry Glacier located in Glacier National Park. These records provide direct evidence of glacier change. The amount and timing of Earth’s land ice that will be lost in the future depends on the future trajectory of greenhouse gas emissions. Ongoing glaciological research will enhance physical understanding and forecasting capabilities; however, the continued retreat and ice mass loss of small glaciers in Glacier National Park is certain.

Montana↗

USGS2018

American Recent Eulipotyphla: Nesophontids, Solenodons, Moles, and Shrews in the New World

The mammalian taxonomic order Eulipotyphla is comprised of the living taxonomic families Erinaceidae (gymnures, hedgehogs, and moonrats), Solenodontidae (solenodonts), Soricidae (shrews), and Talpidae (desmans and moles). Morphological and molecular studies continue to alter our view of relationships within and among these families, and this research has added considerably to our understanding of the diversity, distributions, and relationships of many of the New World species that belong to them. Currently, there are more than 450 recognized living species worldwide, making the Eulipotyphla the third most speciose order of mammals. New World (North and South America and associated islands) eulipotyphlans currently include 110 recognized species of shrews, seven species of moles, and both living species of solenodons. In this work, I attempt to summarize the taxonomic results of recent studies and provide a guide to the most appropriate current applications of taxonomic names in this region.

Book↗

USGS2011

IASPEI workshop: Seismic modeling of laterally varying structures

During the past 10 years, significant progress has been made in the methods of collection and analysis of seismic reflection and refraction data. This progress has led to the development of new models for the structure and composition of the earth's crust, based on sophisticated analysis of numerous profiles in many areas of geologic importance. The third triannual meeting of the IASPEI (International Association of Seismology and Physics of the Earth's Interior) Commission of Controlled Source Seismology was convened in Park City, Utah, on August 11–17, 1980, to bring together seismologists and geologists to explore and assess the progress of controlled source techniques (controlled sources include explosions, air guns, and Vibroseis‐type sources), and to evaluate its significance in terms of current models of the seismic velocity structure and composition of the crust and upper mantle. Particular attention was paid to the progress and problems in the modeling of two‐ and three‐dimensional structures.

Eos, Transactions, American Geophysical Union↗

USGS2012

Rotenone persistence model for montane streams

The efficient and effective use of rotenone is hindered by its unknown persistence in streams. Environmental conditions degrade rotenone, but current label instructions suggest fortifying the chemical along a stream based on linear distance or travel time rather than environmental conditions. Our objective was to develop models that use measurements of environmental conditions to predict rotenone persistence in streams. Detailed measurements of ultraviolet radiation, water temperature, dissolved oxygen, total dissolved solids (TDS), conductivity, pH, oxidation–reduction potential (ORP), substrate composition, amount of organic matter, channel slope, and travel time were made along stream segments located between rotenone treatment stations and cages containing bioassay fish in six streams. The amount of fine organic matter, biofilm, sand, gravel, cobble, rubble, small boulders, slope, pH, TDS, ORP, light reaching the stream, energy dissipated, discharge, and cumulative travel time were each significantly correlated with fish death. By using logistic regression, measurements of environmental conditions were paired with the responses of bioassay fish to develop a model that predicted the persistence of rotenone toxicity in streams. This model was validated with data from two additional stream treatment reaches. Rotenone persistence was predicted by a model that used travel time, rubble, and ORP. When this model predicts a probability of less than 0.95, those who apply rotenone can expect incomplete eradication and should plan on fortifying rotenone concentrations. The significance of travel time has been previously identified and is currently used to predict rotenone persistence. However, rubble substrate, which may be associated with the degradation of rotenone by adsorption and volatilization in turbulent environments, was not previously considered.

Transactions of the American Fisheries Society↗

USGS2007

Climate change: Potential impacts and interactions in wetlands of the United States

Wetlands exist in a transition zone between aquatic and terrestrial environments which can be altered by subtle changes in hydrology. Twentieth century climate records show that the United States is generally experiencing a trend towards a wetter, warmer climate; some climate models suggest that his trend will continue and possibly intensify over the next 100 years. Wetlands that are most likely to be affected by these and other potential changes (e.g., sea-level rise) associated with atmospheric carbon enrichment include permafrost wetlands, coastal and estuarine wetlands, peatlands, alpine wetlands, and prairie pothote wetlands. Potential impacts range from changes in community structure to changes in ecological function, and from extirpation to enhancement. Wetlands (particularly boreal peatlands) play an important role in the global carbon cycle, generally sequestering carbon in the form of biomass, methane, dissolved organic material and organic sediment. Wetlands that are drained or partially dried can become a net source of methane and carbon dioxide to the atmosphere, serving as a positive biotic feedback to global warming. Policy options for minimizing the adverse impacts of climate change on wetland ecosystems include the reduction of current anthropogenic stresses, allowing for inland migration of coastal wetlands as sea-level rises, active management to preserve wetland hydrology, and a wide range of other management and restoration options.

Journal of the American Water Resources Associatio↗

USGS1961

Local evidence of Pleistocene to recent orogeny in the Argentine Andes

Deformed continental sedimentary rocks are exposed in the province of Salta, northwestern Argentina, in one of many intermontane basins of the Puna, a high desert region of subparallel north-trending block-fault ranges. These rocks, formerly thought to be Tertiary but recently dated by fossil diatoms as Pleistocene or younger, comprise several thousand feet of elastics and evaporites interpreted as having accumulated in a structural basin under geologic and climatic conditions much like those of today. They are overlain unconformably by sedimentary rocks and sediments of three distinct depositional periods. The stratigraphic section is as follows: Fan gravels and playa deposits ( Recent ) - Disconformity - Flat-lying lacustrine sandstones and siltstones, minor salines - Angular unconformity - Gently folded conglomerates and sandstones - Angular unconformity - Folded and faulted conglomerate, sandstone, shale, evaporites, and tuffs The basin rocks are folded along north-trending axes and are cut by northeast- to southeast-trending normal faults and by a north-trending reverse fault; the next younger conglomerates and sandstones are gently folded; the two youngest units are undisturbed. The three unconformities, the faults and folds in the older beds, and post-lake-bed faulting of an erosion surface on an adjacent block all indicate intermittent late Pleistocene to Recent local deformation. Neither regional tension nor regional compression can explain both the Pleistocene to Recent movements on the regional block faults and the contemporaneous compressional structures within the basin. The mechanism of horst-wedging, suggested by a current explanation of the analogous ranges of the Great Basin, is proposed as a solution to the dilemma; the horst blocks, forced directly upward along Miocene normal faults, acted as wedges and compressed the sediments accumulating in the graben, creating the pattern of faults and folds now observed. If such structures are ever discovered in the North American Great Basin, as seems reasonable, they should offer new insight into the understanding of basin-and-range structure.

Andes Mountains↗

USGS1987

Oblique map of the northern Sierra Nevada, California, showing location of gold-bearing areas

More than a third of the gold produced by the United States was mined in California. The bulk of this gold was recovered from the western slope of the northern half of the Sierra Nevada between the Merced River in the south and the Feather River to the north, a distance of about 170 mi. Gold was first discovered, in this region, on the American River at Coloma in 1848, triggering the famed California gold rush. Mining was continuously active, somewhere in the area, from the discovery of gold until World War II when mining was legally prohibited. Dramatic increases in gold prices in the past decade coupled with recent advances in extractive techniques have revitalized prospecting, and major deposits are currently being explored and developed. Gold has been found in a variety of geologic environments in the region. In addition to production from the complex vein systems of the historically famous Mother Lode and associated East Gold Belt and West Gold Belt, large amounts of gold have also been recovered from the Grass Valley-Nevada City and other isolated lode districts and from Tertiary river channels and Quaternary alluvium. This oblique map illustrates the relation of the different gold-bearing environments to each other and to the general terrain of the northern Sierra Nevada. The map was derived from the 1970 U.S. Geological Survey 1:500,000-scale topographic map of California and the 1976 U.S. Geological Survey 1:500,000-scale topographic map of Nevada, using an isometrograph, a mechanical instrument that produces an oblique framework by tracing individual contours. Form lines sketched over this framework graphically portray the physiographic configuration of the region. Relief on the oblique map has a 3:1 vertical exaggeration and appears as if viewed from a 30° angle above the horizon. Locations of lode gold prospects and mines shown on the map were obtained from the U.S. Geological Survey's Mineral Resource Data System (MRDS), a computerized mineral-resource information file, and plotted in their respective locations (D.F. Huber, written commun., 1986). Some locations from two northern counties, missing from the MRDS retrival, were added. The twenty lode mines believed to be the most productive are cited in table 1. A total of nearly 4,000 sites, including both prospects and mines, were initially plotted, but about a third of those were obscured by topography on the oblique map. Locations of Tertiary river channels and gold-dredging fields were taken from published general references modified by examining specific sources and by cursory field examination. Seven of the major dredge fields are identified in table 2.

California↗

USGS2015

Aurora painting pays tribute to Civil War's end

This year marks the sesquicentennial anniversary of the end of the American Civil War, a conflict that Abraham Lincoln called a “mighty scourge.” It was one of the most poignant periods in U.S. history, laying bare political, economic, social, and moral divergence between Northern and Southern states. The cause of the divergence that led to war was slavery [e.g., McPherson , 1988, chap. 3]—an institution that, by the 19th century, had been effectively abolished in the North but remained firmly entrenched in the South. War erupted in 1861 after a confederacy of Southern states declared secession from the Union of the United States. When the war finally ended in 1865, the Union had prevailed, and afterward, slavery was abolished throughout the United States. This outcome was obtained at the cost of 750,000 American lives and substantial destruction, especially in the South [e.g., Gugliotta , 2012]. In 1865, the same year the war ended, the American landscape artist Frederic Edwin Church unveiled Aurora Borealis (pictured above), a dramatic and mysterious painting that can be interpreted in terms of 19th century romanticism, scientific philosophy, and Arctic missions of exploration. Aurora Borealis can also be viewed as a restrained tribute to the end of the Civil War—a moving example of how science and current events served as the muses of late romantic artists [e.g., Carr , 1994, p. 277; Avery , 2011; Harvey , 2012].

Eos, Earth and Space Science News↗

USGS2005

Is disease an important mortality factor for Pacific herring?

Marine pelagic forage fishes, including sardines, anchovies, and herring, undergo large oscillations in population abundance. Although over-fishing can have a dramatic impact in reducing population size, this anthropogenic perturbation cannot fully account for all population declines because natural oscillations in marine pelagic fish biomasses occurred prior to the onset of commercial fishing. Among the herring metapopulation in Puget Sound / Straight of Georgia, the mean estimated annual mortality, exclusive of commercial fishing, increased from 20% in the late 1970’s and early 1980’s to 64-87% during 1996-1999. This natural mortality affects primarily the older age cohorts and resulted in recent decreased median ages of adult herring from age 4-6 cohorts to age 2-3 cohorts that die prior to iteroparous spawning. Surveys of wild herring from the region indicate that the protozoan parasite Ichthyophonus sp., is currently ubiquitous among Pacific herring populations in Washington and British Columbia, and prevalence of infection increase directly with herring age, from 12% among juveniles to 58% among the underrepresented age 6+ cohorts. Ichthyophonus can be highly pathogenic to immunologically naïve Pacific herring, causing 80% mortality 2 mo. after exposure in the laboratory. Current laboratory-based studies are underway to determine whether natural Ichthyophonus infections are terminal for the host, and whether these infections account for the decreasing median age of wild Pacific herring populations in recent years.

American Association of Wildlife Veterinarians↗

USGS2002

Transforming the Rockies: Human forces, settlement patterns, and ecosystem effects

The current ecological condition of the Rocky Mountains can be viewed from two somewhat opposing perspectives. The first is that human occupation has had relatively little effect on the Rockies: large natural, if not pristine, areas remain, and the region's open spaces provide wildlife habitat, majestic scenery, and a sense of wildness. Unlike the situation in, say, the Swiss Alps, where even high-elevation meadows have been mown and grazed intensively for as long as 500 years and many large mammals have been extirpated, most elements of Rocky Mountain landscapes and biota are reasonably unaltered. Even the presumption that Native Americans changed regional landscapes with deliberately set fires has been challenged by Baker and Ehle (2001) and others who think that most fires were lightning-caused or accidental ignitions. The second view is that humans have dramatically transformed the Rockies, at least since Euro-American settlement in the mid- to late 1800s. The slaughter of vast buffalo herds, the clearing of timber for railroad ties, and even the removal of whole hillsides in hydraulic placer mining represented substantial transformation. Ranch, resort, and residential development marks the latest incarnation of this transformation. Numerous, complex layers of land use have left landscape legacies, some of which may be unrecognized or underappreciated in modern assessments (Wohl 2001). Here we consider both perspectives because we are impressed with both the many effects of human use of the Rockies and the region's remaining wild landscapes. Ironically, much of the recent population growth and development in the Rockies is driven by the region's wild landscapes, which make the present widespread transformation seem all the more significant. It is, of course, the rapid clip of current human transformation—high population growth rates, pervasive rural residential development, and landscape fragmentation (Baron, Theobald, and Fagre 2000)—that worries ecologists and others concerned with Rocky Mountain ecosystems. So although we offer an overview of historical changes in the Rockies, especially since the 1800s, much of our attention here is on land uses, economies, and settlement patterns since the 1970s and on their future trends.

Book chapter↗

USGS2021

U.S. Geological Survey migratory bird science, 2020–21

Bird conservation as an endeavor engages a broad range of partners and a coordinated effort across State and Federal agencies, nongovernment organizations, universities and, at times, international partnerships. To understand information needs and respond to the many challenges in bird conservation, U.S. Geological Survey (USGS) scientists participate in Flyway committees, on Joint Venture boards and working groups, in professional organizations, and in other conservation partnerships. These activities connect USGS scientists to conservation partners with whom they work to address substantial challenges. More than one hundred USGS scientists are dedicated to the scientific study of migratory birds. This report presents the current (2021) representative breadth of activities of USGS scientists supporting the conservation and management of migratory birds. Ninety USGS scientists contributed to the project descriptions and other information detailing the work of the USGS. The science herein is organized and presented thematically by research strengths and by management topics. The report emphasizes the geographic framework of the North American Flyway councils through which USGS engages regularly with Federal and State government agencies and others who are responsible for managing migratory bird populations.

Circular↗

USGS2022

Characterization of the bathymetry, hydrodynamics, water quality, infrastructure, and channel condition of the Old Erie Canal from DeWitt to its junction with the current Erie Canal in Verona, near Rome, New York, 2018–19

The Old Erie Canal has undergone sedimentation and aquatic growth that have restricted flow and diminished the aesthetic quality of the canal during the nearly 200 years since its construction. During 2018–2019, the U.S. Geological Survey (USGS) in cooperation with the Madison County Planning Department and the New York State Canal Corporation conducted a study of the Old Erie Canal between the Town of DeWitt, New York, and its junction with the current Erie Canal of the New York State Canal System near Rome, N.Y. The study comprised bathymetric, velocity, and water-quality surveys and documentation of the canal infrastructure. The USGS established benchmarks and staff gages along the 30.8 miles of the canal study area to reference the water-surface level in the canal to the North American Vertical Datum of 1988 (NAVD 88). Bathymetric survey results indicated that during the time of the survey, the canal depths ranged from 1.26 feet (ft) to 7.33 ft between the Butternut and Durhamville aqueducts (with a mean depth of 3.52 ft). Shallow depths are located throughout the canal, but the section north of the Durhamville aqueduct was the shallowest, with depths ranging from 0.68 ft to 2.44 ft (and a mean depth of 1.36 ft). The reach-averaged water velocity was 0.28 feet per second. The system generally flows west to east from the Butternut aqueduct to the entrance to the Erie Canal. Water-quality data (dissolved oxygen, water temperature, specific conductance, pH, and turbidity) were collected concurrently with the bathymetric survey (spring 2018) to characterize changes in water quality along the length of the canal. Specific-conductance values measured upstream from the hamlet of Kirkville, Manlius, N.Y. may reflect road salts being flushed into the canal through the Butternut and Limestone feeder system (designed to divert water from nearby creeks to supply water for the Old Erie Canal) from recent stormwater runoff. Increases in pH in the downstream direction are possibly caused by increasing amounts of aquatic vegetation. During the time of the survey, turbidity was highest near inflows from the canal feeder system and tributary inputs which were elevated by stormwater runoff that transported sediment into the canal. The canal infrastructure was documented to provide a baseline assessment. The feeder system, designed to bring water into the canal, does not deliver flow when the creeks supplying water to those feeders are at base flow, but does bring water into the system when flows in the feeder creeks are elevated. A recent report provides an example of repair work completed on the Chittenango feeder to improve flow through the feeder into the canal (Welch and Madison County Planning Department, 1996). Two non-regulated tributaries, Meadow Brook and Pools Brook, consistently delivered flow to the canal. Outfalls where canal water discharges into nearby creeks were sealed in the Butternut and Limestone aqueducts. Outfalls in the Chittenango, Cowaselon, and Durhamville aqueducts were found with flashboards installed at an elevation that allows water to be discharged from the canal. These structures are designed to accept additional flashboards to raise the canal water surface with the potential to convey flow farther down the system. The general condition of the channel was open and navigable between Butternut aqueduct and Chittenago aqueduct. On the segment of the canal east of the Chittenango aqueduct, an increasing number of downed trees and tangled wads of vegetation affected flow and made navigation by boat difficult to the Durhamville aqueduct. North of the Durhamville aqueduct, numerous downed trees and an increased density of aquatic vegetation limited navigation by boat and reduced the flow rate.

New York↗

USGS2019

Chemical controls for an integrated pest management program

Chemical controls ranging from natural products to synthesized chemicals are widely used in aquatic pest management activities. Chemicals can be used to lure organisms to traps or can cause direct mortality by altering the physiological function of an organism. Much of what is known about controlling pests with chemicals is from research done on terrestrial pesticides. This paper focuses on how chemicals might be used as aquatic pesticides to control or eradicate aquatic invasive species. Current control tools are described, as are new technologies designed to selectively target the pest to reduce risks to nontarget organisms and the environment.

North American Journal of Fisheries Management↗

USGS2025

Comparing surgery time and short-term incision healing for Largemouth Bass using smooth- and barbed-absorbable sutures

Objective Barbed sutures have become increasingly favored over traditional smooth sutures in human medicine but remain understudied in fisheries biotelemetry applications. Our objectives were to (1) compare surgery time and recovery time for Largemouth Bass Micropterus nigricans when using smooth- versus barbed-absorbable sutures to close the incisions and (2) compare the short-term healing rates of incisions that were closed with the two sutures. Methods We conducted a surgical trial using two incision closure methods: a smooth suture method, which used three interrupted sutures, and a barbed suture method, which used a running style suture. Surgery and surgical recovery times were recorded for each surgery. At 14 d postsurgery, each incision was scored for openness and inflammation. Results Barbed sutures (median surgery time = 133.5 s) resulted in higher surgical efficiency than smooth sutures (median surgery time = 255.0 s). Surgical recovery time did not differ between the two suture types. Mortality did not differ between suture types (overall mortality = 21%) and was similar to previously reported mortality rates. Incision site inflammation and openness were variable but did not differ between suture types. Conclusions Our results indicate that barbed sutures may increase surgical efficiency while maintaining currently observed survival and healing rates for Largemouth Bass.

North American Journal of Fisheries Management↗

USGS1998

Ontogenic and spatial patterns in diet and growth of lake trout in Lake Michigan

Lake trout Salvelinus namaycush in nearshore waters of Lake Michigan grow faster than lake trout residing offshore on Sheboygan Reef, which is in midlake. We examined the stomachs of lake trout, spanning ages 1 through 16, caught in both nearshore and offshore environments of Lake Michigan during 1994 and 1995 to determine whether diet differences may be responsible for the difference in growth rate. A comparison of the diets, coupled with bioenergetics modeling, indicated that juvenile lake trout on Sheboygan Reef experienced slow growth due to low food availability rather than to cold water temperatures. The availability of appropriate‐size prey appeared to regulate lake trout growth. Small prey fish were probably not readily available to small (200‐ to 399‐mm total length) lake trout on Sheboygan Reef, a substantial portion of whose diet consisted of invertebrates; in contrast, nearshore juveniles had a nearly 100% fish diet. Growth rate on the reef remained slow through intermediate lake trout sizes (400–599 mm total length), presumably due to low availability of rainbow smelt Osmerus mordax on the reef. Once lake trout achieved total lengths of approximately 600 mm, they grew slightly faster on Sheboygan Reef than near shore, indicating that large (>170‐mm total length) prey fish were readily available to lake trout in the reef area. On a wet‐weight basis, alewife Alosa pseudoharengus dominated the diet of large (≥600 mm total length) lake trout from both the nearshore and offshore regions of the lake, although bloater Coregonus hoyi composed over 30% of the diet on Sheboygan Reef and in southeastern nearshore Lake Michigan. Size of alewife prey increased with lake trout size. The bloater population currently represents the bulk of the biomass of the adult prey fish community, so our diet analysis suggests that large lake trout are continuing to select alewives.

Illinois, Indiana, Michigan, Wisconsin↗

USGS2022

Special section overview: Effects of ecosystem change on North American percid populations.

Walleye Sander vitreus , Sauger S. canadensis , and Yellow Perch Perca flavescens (referred to as percids herein) are collectively among the most culturally and ecologically important fish species in North America. As ecosystems change in response to environmental drivers, such as climate change, nutrient loading, and invasive species, there is a need to understand how percid populations respond to these changes. To address this need, a symposium was held during the 81st Annual Midwest Fish and Wildlife Conference to bring fishery scientists and managers together to describe and discuss percid population responses to ecosystem change. Prevailing symposium themes included the challenge of identifying mechanisms responsible for population-level changes, developing strategies to adaptively manage for resilient fisheries, and consideration of scale, context, and methods when interpreting variable results. Given the uncertainty of how ecosystem changes affect percid populations, participants emphasized the importance of communicating uncertainties to stakeholders, implementing data-driven management strategies, setting realistic goals, and revising management actions in an adaptive framework. There was universal agreement on both the challenge and necessity of facilitating constructive engagement among stakeholders in cooperative decision making. Symposium participants identified knowledge gaps and discussed future efforts to build on our current understanding of percid populations, including continuation of long-term monitoring, improved standardization of evaluation metrics, implementing adaptive management experiments to identify causal relationships, development of more robust analytical methods, use of historical data sources, and refining techniques to realistically convey management options to stakeholders.

North American Journal of Fisheries Management↗

USGS2007

Comparative performance of invasive and native Celastrus species across environmental gradients

The ability to understand and predict the success of invasive plant species in their new ranges is increased when there is a sympatric native congener available for comparison. Celastrus orbiculatus (oriental bittersweet) is a liana introduced into the United States in the mid-1800s from East Asia as an ornamental plant. Its native congener, Celastrus scandens (American bittersweet), ranges from the east coast of the United States as far west as Wyoming. In the Northeastern United States, C. orbiculatus is continuing to expand its range while C. scandens appears to be in serious decline. One hypothesis for this decline is that C. scandens does not have such a wide range of ecological tolerances in the current landscape as C. orbiculatus, which seems to tolerate a greater range of resource conditions. To investigate this hypothesis, we transplanted these two species into ten sites that spanned a full range of light and soil moisture conditions to compare their establishment and performance in terms of aboveground growth (biomass and height) and mortality. After two years, C. orbiculatus showed significantly lower mortality and greater biomass across all resource conditions compared to C. scandens. In addition, C. orbiculatus preferred more mesic soil moisture conditions, while C. scandens performed better in drier soil moisture conditions. Since much of the Northeastern United States is now forested, this preference for mesic soil conditions could make it more successful than C. scandens in the region. This study shows the utility of manipulative experiments, particularly those using congeneric native species as benchmarks, for assessing the causes and predicting the course of invasions. ?? 2007 Springer-Verlag.

Oecologia↗

USGS1998

Origin of the Columbia River basalts: Melting model of a heterogeneous plume head

In order to study the origin of the Grande Ronde basalts (GRs) erupted in the climax stage of the Columbia River basalts (CRBs), we carried out high pressure melting experiments on four of the most primitive rock compositions representing the Yakima group of the CRBs. The voluminous GRs (constituting >80 vol% of CRBs) are totally aphyric basaltic andesites. GRs show very narrow and coherent chemical trends both in major and trace elements as well as isotopes. The silica-rich GRs (SiO 2 = 52–56 wt%) can be produced by direct partial melting of a MORB like source material (CRB72-31) at ∼2 GPa or ∼70 km depth. By 30–50% partial melting of the CRB72-31, the entire compositional range of the GRs can be produced in a narrow temperature interval (1300–1350°C) at ∼2 GPa. The aluminous clinopyroxene that appears in the above melting range is consistent as the major controlling phase of the GR trends. The partial melts are very similar to the GRs except for Al 2 O 3 and FeO which could be due to the mismatch in the source rock composition. Judging from the variation in REE, involvement of garnet in GR magma genesis can be ruled out. Small amounts of plagioclase (10–30 wt%) may be present in the partial melting residue. Judging from REE patterns and Nd isotopes of the GRs, the source rock should be unfractionated in REE. Based on the melting experiments, a heterogeneous plume model is proposed for the initial stage of the Yellowstone hot spot. Large lithologically distinct blobs of old oceanic crust components were included in the plume head. The GR magmas were produced by partial melting of the oceanic crust components at the bottom of the North American lithosphere. Similar melting processes of basalt/peridotite composite source may be operating in other LIPs (large igneous provinces). The GR type genuine oceanic crust derived melts may be seen where the ambient peridotite remains under subsolidus conditions. Volume and temperature of mantle plumes may have been overestimated, because contributions from the recycled oceanic crust is so large and the current mantle melting models concern only peridotite source.

Idaho, Nevada, Oregon↗