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Geology topics

Lisa A. Morgan

Publications and source records attributed to Lisa A. Morgan.

10 recordsLinked to original sources
USGS2012

Volcanogenic massive sulfide occurrence model

Volcanogenic massive sulfide deposits, also known as volcanic-hosted massive sulfide, volcanic-associated massive sulfide, or seafloor massive sulfide deposits, are important sources of copper, zinc, lead, gold, and silver (Cu, Zn, Pb, Au, and Ag). These deposits form at or near the seafloor where circulating hydrothermal fluids driven by magmatic heat are quenched through mixing with bottom waters or porewaters in near-seafloor lithologies. Massive sulfide lenses vary widely in shape and size and may be podlike or sheetlike. They are generally stratiform and may occur as multiple lenses. Volcanogenic massive sulfide deposits range in size from small pods of less than a ton (which are commonly scattered through prospective terrains) to supergiant accumulations like Rio Tinto (Spain), 1.5 billion metric tons; Kholodrina (Russia), 300 million metric tons; Windy Craggy (Canada), 300 million metric tons; Brunswick No. 12 (Canada), 230 million metric tons; and Ducktown (United States), 163 million metric tons. Volcanogenic massive sulfide deposits range in age from 3.55 billion years to zero-age deposits that are actively forming in extensional settings on the seafloor, especially mid-ocean ridges, island arcs, and back-arc spreading basins. The widespread recognition of modern seafloor Volcanogenic massive sulfide deposits and associated hydrothermal vent fluids and vent fauna has been one of the most astonishing discoveries in the last 50 years, and seafloor exploration and scientific studies have contributed much to our understanding of ore-forming processes and the tectonic framework for volcanogenic massive sulfide deposits in the marine environment. Massive ore in volcanogenic massive sulfide deposits consists of greater than 40 percent sulfides, usually pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena; non-sulfide gangue typically consists of quartz, barite, anhydrite, iron oxides, chlorite, sericite, talc, and their metamorphosed equivalents. Ore composition may be Pb-Zn-, Cu-Zn-, or Pb-Cu-Zn-dominated, and some deposits are zoned vertically and laterally. Many deposits have stringer or feeder zones beneath the massive zone that consist of crosscutting veins and veinlets of sulfides in a matrix of pervasively altered host rock and gangue. Alteration zonation in the host rocks surrounding the deposits are usually well-developed and include advanced argillic (kaolinite, alunite), argillic (illite, sericite), sericitic (sericite, quartz), chloritic (chlorite, quartz), and propylitic (carbonate, epidote, chlorite) types. An unusual feature of VMS deposits is the common association of stratiform "exhalative" deposits precipitated from hydrothermal fluids emanating into bottom waters. These deposits may extend well beyond the margins of massive sulfide and are typically composed of silica, iron, and manganese oxides, carbonates, sulfates, sulfides, and tourmaline.

Scientific Investigations Report↗
USGS2009

Is the track of the Yellowstone hotspot driven by a deep mantle plume? - Review of volcanism, faulting, and uplift in light of new data

Geophysical imaging of a tilted mantle plume extending at least 500 km beneath the Yellowstone caldera provides compelling support for a plume origin of the entire Yellowstone hotspot track back to its inception at 17 Ma with eruptions of flood basalts and rhyolite. The widespread volcanism, combined with a large volume of buoyant asthenosphere, supports a plume head as an initial phase. Estimates of the diameter of the plume head suggest it completely spanned the upper mantle and was fed from sources beneath the transition zone, We consider a mantle–plume depth to at least 1,000 km to best explain the large scale of features associated with the hotspot track. The Columbia River–Steens flood basalts form a northward-migrating succession consistent with the outward spreading of a plume head beneath the lithosphere. The northern part of the inferred plume head spread (pancaked) upward beneath Mesozoic oceanic crust to produce flood basalts, whereas basalt melt from the southern part intercepted and melted Paleozoic and older crust to produce rhyolite from 17 to 14 Ma. The plume head overlapped the craton margin as defined by strontium isotopes; westward motion of the North American plate has likely “scraped off” the head from the plume tail. Flood basalt chemistries are explained by delamination of the lithosphere where the plume head intersected this cratonic margin. Before reaching the lithosphere, the rising plume head apparently intercepted the east-dipping Juan de Fuca slab and was deflected ~ 250 km to the west; the plume head eventually broke through the slab, leaving an abruptly truncated slab. Westward deflection of the plume head can explain the anomalously rapid hotspot movement of 62 km/m.y. from 17 to 10 Ma, compared to the rate of ~ 25 km/m.y. from 10 to 2 Ma. A plume head-to-tail transition occurred in the 14-to-10-Ma interval in the central Snake River Plain and was characterized by frequent (every 200–300 ka for about 2 m.y. from 12.7 to 10.5 Ma) “large volume (> 7000 km 3 )”, and high temperature rhyolitic eruptions (> 1000 °C) along a ~ 200–km-wide east–west band. The broad transition area required a heat source of comparable area. Differing characteristics of the volcanic fields here may in part be due to variations in crustal composition but also may reflect development in differing parts of an evolving plume where the older fields may reflect the eruption from several volcanic centers located above very large and extensive rhyolitic magma chamber(s) over the detached plume head while the younger fields may signal the arrival of the plume tail intercepting and melting the lithosphere and generating a more focused rhyolitic magma chamber. The three youngest volcanic fields of the hotspot track started with large ignimbrite eruptions at 10.21, 6.62, and 2.05 Ma. They indicate hotspot migration N55° E at ~ 25 km/m.y. compatible in direction and velocity with the North American Plate motion. The Yellowstone Crescent of High Terrain (YCHT) flares outward ahead of the volcanic progression in a pattern similar to a bow-wave, and thus favors a sub-lithospheric driver. Estimates of YCHT-uplift rates are between 0.1 and 0.4 mm/yr. Drainage divides have migrated northeastward with the hotspot. The Continental Divide and a radial drainage pattern now centers on the hotspot. The largest geoid anomaly in the conterminous U.S. is also centered on Yellowstone and, consistent with uplift above a mantle plume. Bands of late Cenozoic faulting extend south and west from Yellowstone. These bands are subdivided into belts based both on recency of offset and range-front height. Fault history within these belts suggests the following pattern: Belt I — starting activity but little accumulated offset; Belt II — peak activity with high total offset and activity younger than 14 ka; Belt III — waning activity with large offset and activity younger than 140 ka; and Belt IV — apparently dead on substantial range fronts (south side of the eastern Snake River Plain only). These belts of fault activity have migrated northeast in tandem with the adjacent hotspot volcanism. On the southern arm of the YCHT, fault activity occurs on the inner, western slope consistent with driving by gravitational potential energy, whereas faulting has not started on the eastern, outer, more compressional slope. Range fronts increase in height and steepness northeastward along the southern-fault band. Both the belts of faulting and the YCHT are asymmetrical across the volcanic hotspot track, flaring out 1.6 times more on the south than the north side. This and the southeast tilt of the Yellowstone plume may reflect southeast flow of the upper mantle.

Wyoming↗
USGS2009

Hydrothermal processes above the Yellowstone magma chamber: Large hydrothermal systems and large hydrothermal explosions

Hydrothermal explosions are violent and dramatic events resulting in the rapid ejection of boiling water, steam, mud, and rock fragments from source craters that range from a few meters up to more than 2 km in diameter; associated breccia can be emplaced as much as 3 to 4 km from the largest craters. Hydrothermal explosions occur where shallow interconnected reservoirs of steam- and liquid-saturated fluids with temperatures at or near the boiling curve underlie thermal fields. Sudden reduction in confining pressure causes fluids to flash to steam, resulting in significant expansion, rock fragmentation, and debris ejection. In Yellowstone, hydrothermal explosions are a potentially significant hazard for visitors and facilities and can damage or even destroy thermal features. The breccia deposits and associated craters formed from hydrothermal explosions are mapped as mostly Holocene (the Mary Bay deposit is older) units throughout Yellowstone National Park (YNP) and are spatially related to within the 0.64-Ma Yellowstone caldera and along the active Norris-Mammoth tectonic corridor. In Yellowstone, at least 20 large (>100 m in diameter) hydrothermal explosion craters have been identified; the scale of the individual associated events dwarfs similar features in geothermal areas elsewhere in the world. Large hydrothermal explosions in Yellowstone have occurred over the past 16 ka averaging ~1 every 700 yr; similar events are likely in the future. Our studies of large hydrothermal explosion events indicate: (1) none are directly associated with eruptive volcanic or shallow intrusive events; (2) several historical explosions have been triggered by seismic events; (3) lithic clasts and comingled matrix material that form hydrothermal explosion deposits are extensively altered, indicating that explosions occur in areas subjected to intense hydrothermal processes; (4) many lithic clasts contained in explosion breccia deposits preserve evidence of repeated fracturing and vein-filling; and (5) areal dimensions of many large hydrothermal explosion craters in Yellowstone are similar to those of its active geyser basins and thermal areas. For Yellowstone, our knowledge of hydrothermal craters and ejecta is generally limited to after the Yellowstone Plateau emerged from beneath a late Pleistocene icecap that was roughly a kilometer thick. Large hydrothermal explosions may have occurred earlier as indicated by multiple episodes of cementation and brecciation commonly observed in hydrothermal ejecta clasts. Critical components for large, explosive hydrothermal systems include a water-saturated system at or near boiling temperatures and an interconnected system of well-developed joints and fractures along which hydrothermal fluids flow. Active deformation of the Yellowstone caldera, active faulting and moderate local seismicity, high heat flow, rapid changes in climate, and regional stresses are factors that have strong influences on the type of hydrothermal system developed. Ascending hydrothermal fluids flow along fractures that have developed in response to active caldera deformation and along edges of low-permeability rhyolitic lava flows. Alteration of the area affected, self-sealing leading to development of a caprock for the hydrothermal system, and dissolution of silica-rich rocks are additional factors that may constrain the distribution and development of hydrothermal fields. A partial low-permeability layer that acts as a cap to the hydrothermal system may produce some over-pressurization, thought to be small in most systems. Any abrupt drop in pressure initiates steam flashing and is rapidly transmitted through interconnected fractures that result in a series of multiple large-scale explosions contributing to the excavation of a larger explosion crater. Similarities between the size and dimensions of large hydrothermal explosion craters and thermal fields in Yellowstone may indicate that catastrophic events which result in large hydrothermal explosions are an end phase in geyser basin evolution. The Mary Bay hydrothermal explosion crater complex is the largest such complex in Yellowstone, and possibly in the world, with a diameter of 2.8 km in length and 2.4 km in width. It is nested in Mary Bay in the northern basin of Yellowstone Lake, an area of high heat flow and active deformation within the Yellowstone caldera. A sedimentary sequence exposed in wave-cut cliffs between Storm Point and Mary Bay gives insight into the geologic history of the Mary Bay hydrothermal explosion event. The Mary Bay explosion breccia deposits overlie sand above varved lake sediments and are separated locally into an upper and lower unit. The sand unit contains numerous small normal faults and is coextensive with the Mary Bay breccia in its northern extent. This sand may represent deposits of an earthquake-generated wave. Seismicity associated with the earthquake may have triggered the hydrothermal explosion responsible for development of the Mary Bay crater complex. Large hydrothermal explosions are rare events on a human time scale; however, the potential for additional future events of the sort in Yellowstone National Park is not insignificant. Based on the occurrence of large hydrothermal explosion events over the past 16,000 yr, an explosion large enough to create a 100-m-wide crater might be expected every 200 yr.

Wyoming↗
USGS2009

A New Occurrence Model for National Assessment of Undiscovered Volcanogenic Massive Sulfide Deposits

Volcanogenic massive sulfide (VMS) deposits are very significant current and historical resources of Cu-Pb-Zn-Au-Ag, are active exploration targets in several areas of the United States and potentially have significant environmental effects. This new USGS VMS deposit model provides a comprehensive review of deposit occurrence and ore genesis, and fully integrates recent advances in the understanding of active seafloor VMS-forming environments, and integrates consideration of geoenvironmental consequences of mining VMS deposits. Because VMS deposits exhibit a broad range of geological and geochemical characteristics, a suitable classification system is required to incorporate these variations into the mineral deposit model. We classify VMS deposits based on compositional variations in volcanic and sedimentary host rocks. The advantage of the classification method is that it provides a closer linkage between tectonic setting and lithostratigraphic assemblages, and an increased predictive capability during field-based studies.

Open-File Report↗
USGS2008

Track of the Yellowstone hotspot: Young and ongoing geologic processes from the Snake River Plain to the Yellowstone Plateau and Tetons

This field trip highlights various stages in the evolution of the Snake River Plain–Yellowstone Plateau bimodal volcanic province, and associated faulting and uplift, also known as the track of the Yellowstone hotspot. The 16 Ma Yellowstone hotspot track is one of the few places on Earth where time-transgressive processes on continental crust can be observed in the volcanic and tectonic (faulting and uplift) record at the rate and direction predicted by plate motion. Recent interest in young and possible renewed volcanism at Yellowstone along with new discoveries and synthesis of previous studies, i.e., tomographic, deformation, bathymetric, and seismic surveys, provide a framework of evidence of plate motion over a mantle plume. This 3-day trip is organized to present an overview into volcanism and tectonism in this dynamically active region. Field trip stops will include the young basaltic Craters of the Moon, exposures of 12–4 Ma rhyolites and edges of their associated collapsed calderas on the Snake River Plain, and exposures of faults which show an age progression similar to the volcanic fields. An essential stop is Yellowstone National Park, where the last major caldera-forming event occurred 640,000 years ago and now is host to the world's largest concentration of hydrothermal features (>10,000 hot springs and geysers). This trip presents a quick, intensive overview into volcanism and tectonism in this dynamically active region. Field stops are directly linked to conceptual models related to hotspot passage through this volcano-tectonic province. Features that may reflect a tilted thermal mantle plume suggested in recent tomographic studies will be examined. The drive home will pass through Grand Teton National Park, where the Teton Range is currently rising in response to the passage of the North American plate over the Yellowstone hotspot.

Idaho, Wyoming↗
USGS2007

Integrated Geologic, Geochemical, and Geophysical Studies of Big Bend National Park, Texas

Introduction Big Bend National Park (BBNP), Texas, covers 801,163 acres (3,242 km2) and was established in 1944 through a transfer of land from the State of Texas to the United States. The park is located along a 118-mi (190-km) stretch of the Rio Grande at the United States border with Mexico. The U.S. Geological Survey (USGS) began a 5-year project in 2003 with the objective of studying a number of broad and diverse geologic, geochemical, and geophysical topics in BBNP. This fact sheet describes results of some of the research by USGS scientists working in BBNP.

Fact Sheet↗
USGS2007

Preliminary Assessment of Volcanic and Hydrothermal Hazards in Yellowstone National Park and Vicinity

Possible future violent events in the active hydrothermal, magmatic, and tectonic system of Yellowstone National Park pose potential hazards to park visitors and infrastructure. Most of the national park and vicinity are sparsely populated, but significant numbers of people as well as park resources could nevertheless be at risk from these hazards. Depending on the nature and magnitude of a particular hazardous event and the particular time and season when it might occur, 70,000 to more than 100,000 persons could be affected; the most violent events could affect a broader region or even continent-wide areas. This assessment of such hazards is presented both as a guide for future activities of the Yellowstone Volcano Observatory (YVO) and to aid appropriate response planning by the National Park Service and surrounding agencies and communities. Although the assessment is presented here in some technical detail, this summary is intended to be understandable to non-scientists. The principal conclusions also will be made available in other forms, more accessible to general readers.

Open-File Report↗
USGS2007

The Yellowstone hotspot, Greater Yellowstone ecosystem, and human geography

Active geologic processes associated with the Yellowstone hotspot are fundamental in shaping the landscapes of the greater Yellowstone ecosystem (GYE), a high volcanic plateau flanked by a crescent of still higher mountainous terrain. The processes associated with the Yellowstone hotspot are volcanism, faulting, and uplift and are observed in the geology at the surface. We attribute the driving forces responsible for the northeastward progression of these processes to a thermal plume rising through the Earth’s mantle into the base of the southwest-moving North American plate. This progression began 16 million years ago (Ma) near the Nevada-Oregon border and arrived at Yellowstone about 2 Ma. Before arrival of the hotspot, an older landscape existed, particularly mountains created during the Laramide orogeny about 70–50 Ma and volcanic terrain formed by Absaroka andesitic volcanism mostly between 50–45 Ma. These landscapes were more muted than the present, hotspot-modified landscape because the Laramide-age mountains had worn down and an erosion surface of low relief had developed on the Absaroka volcanic terrain. The Yellowstone Plateau was built by hotspot volcanism of rhyolitic lavas and caldera-forming rhyolite tuffs (ignimbrites). Streams eroding back into the edges of this plateau have created scenic waterfalls and canyons such as the Grand Canyon of the Yellowstone and Lewis Canyon. Rhyolite is poor in plant nutrients and forms sandy, well-drained soils that support the monotonous, fire-adapted lodgepole pine forests of the Yellowstone Plateau. Non-rhyolitic rocks surround this plateau and sustain more varied vegetation, including spruce, fir, and whitebark pine forests broken by grassy meadows. Heat from the hotspot rises upward and drives Yellowstone’s famed geysers, hot springs, and mudpots. These thermal waters are home to specialized, primitive ecosystems, rich in algae and bacteria. The rock alteration associated with hydrothermal systems creates the bright colors of Yellowstone’s Grand Canyon. Basin-and-range-style faulting has accompanied migration of the hotspot to Yellowstone and formed the linear mountains and valleys that occur north and south of the hotspot track, which is the present-day eastern Snake River Plain. High rates of basin-and-range faulting occurred adjacent to the migrating Yellowstone hotspot, creating distinctive landscapes within the GYE such as the Teton Range/Jackson Hole, with characteristic rugged, forested ranges and adjacent flat-floored grassy valleys. The difference in altitude between the mountains and valleys provides a topographic gradient in which vegetation maturation advances with altitude; animal-migration patterns also follow this trend. The valleys provide natural meadows, agricultural land, town sites, and corridors for roads. Uplift of the GYE by as much as 1 km (3,000 ft) during the last 5 million years has resulted in ongoing erosion of deep, steep-walled valleys. Many prominent ecological characteristics of Yellowstone derive from this hotspot-induced uplift, including the moderate- to high- altitude terrain and associated cool temperatures and deep snowfall. Modern and Pleistocene climate and associated vegetation patterns strongly relate to the topography created by the hotspot and its track along the eastern Snake River Plain. Winter air masses from the moist northern Pacific Ocean traverse the topographic low of the Snake River Plain to where orographic rise onto the Yellowstone Plateau and adjacent mountains produces deep snow. A winter precipitation shadow forms on the lee (eastern) sides of the GYE. During Pleistocene glacial times, this moisture conduit provided by the hotspot-track-produced ice-age glaciers that covered the core of the present GYE. These glaciers sculpted bedrock and produced glacial moraines that are both forested and unforested, sand and gravel of ice-marginal streams and outwash gravels that are commonly covered with sagebrush-grassland, and silty lake sediments that are commonly covered by lush grassland such as Hayden Valley. The effects of the Yellowstone hotspot also profoundly shaped the human history in the GYE. Uplift associated with the hotspot elevates the GYE to form the Continental Divide, and streams drain radially outward like spokes from a hub. Inhabitants of the GYE 12,000–10,000 years ago, as well as more recent inhabitants, followed the seasonal green-up of plants and migrating animals up into the mountain areas. During European immigration, people settled around Yellowstone in the lower parts of the drainages and established roads, irrigation systems, and cultural associations. The core Yellowstone highland is too harsh for agriculture and inhospitable to people in the winter. Beyond this core, urban and rural communities exist in valleys and are separated by upland areas. The partitioning inhibits any physical connection of communities, which in turn complicates pursuit of common interests across the whole GYE. Settlements thus geographically isolated evolved as diverse, independent communities

Idaho, Montana, Utah, Wyoming↗
USGS2005

Steam explosions, earthquakes, and volcanic eruptions -- what's in Yellowstone's future?

Yellowstone, one of the world’s largest active volcanic systems, has produced several giant volcanic eruptions in the past few million years, as well as many smaller eruptions and steam explosions. Although no eruptions of lava or volcanic ash have occurred for many thousands of years, future eruptions are likely. In the next few hundred years, hazards will most probably be limited to ongoing geyser and hot-spring activity, occasional steam explosions, and moderate to large earthquakes. To better understand Yellowstone’s volcano and earthquake hazards and to help protect the public, the U.S. Geological Survey, the University of Utah, and Yellowstone National Park formed the Yellowstone Volcano Observatory, which continuously monitors activity in the region.

Wyoming↗