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.

Direct evidence for a plume-plate interaction as the mechanism responsible for the Yellowstone-Snake River Plain (YSRP), 16 Ma volcanic system is observation of a linear age-progression of silicic volcanic centers along the Snake River Plain 800 km to the Yellowstone caldera-- the track of the Yellowston hotspot. Caldera-forming rhyolitic volcanism, active crustal deformation, extremely high heat flow (30 times the continental average), and intensive earthquake activity in Yellowstone National Park mark the surface manifestations of the hotspot. Anomalously low, P-wave velocities in the upper-crust of the Yellowstone caldera are interpreted as solidified but still hot granitic rocks, partial melts, hydrothermal fluids and sediments. Unprecedented deformation of the Yellowstone caldera of up to 1 m of uplift from 1923 to 1984, followed by subsidence of as much as ~12 cm from 1985 to 1991, clearly reflects a giant caldera unrest. The regional signature of the Yellowstone hotspot is highlighted by an anomalous, 600 m high, topographic bulge centered on the caldera and that extends across a ~600 km-wide region. We suggest that this feature reflecs long-wavelength tumescence of the hotspot. Yellowstone is also the center of a +20 m geoid anomaly, the largest in North America, and extends ~500 km laterally from the caldera, similar in width to the geoid anomalies of many oceanic hotspots and swells. The 16 Ma trace of the Yellowstone hotspot, the seismically quiescent Snake River Plain, is surrounded by "bow-wave" or parabolic shaped regions of earthquakes and high topography. Whereas systematic topographic decay along the Snake River Plain, totaling 1,300 m, fits a model of lithospheric cooling and subsidence which is consistent with passage of the North American plate across a mantle heat source. We note that the rate of 4.5 cm/yr silicic, volcanic age progression of the YSRP includes a component of southwest motion of the North American plate, modeled at ~2.5 cm/yr, and a component of concomitant crustal extension estimated to be 1 to 2 cm/yr. The USRP also exhibits anomalous crustal structure which we believe is inherited from magmatic and thermal processes associated which the Yellowstone hotspot. This includes a thin, 2-5 km-thick surface layer compses of basalts and rhyolites and an unusually high-velocity, 6.5 km/s, mid-crustal mafic layer that we suggest reflects extinct "Yellowstone" magma systems that have replaced much of the normal granite upper-crust. Direct evidence for a mantle connection for the YSRP system is from anomalously low, P-wave velocities which extend from the crust to depths of ~200km. These properties and the kinematics of teh YSRP are consistent with an analytic model for plume-plate interaction that produces a "bow-wave" or parabolic patter of upper-mantle flow southwesterly from the hotspot, similar to the systematic patterns of regional topography and seismicity. Our unified model for the origin of the YSRP is consistent with the geologic evidence where basaltic magmas ascend from a mantle plume to interact with a silicic-rich continental crust producing partial melts of rhyolite composition and the characteristic caldera-forming volcanism of Yellowstone. Cooling and contraction of the lithosphere follows the passage of the plate over the hotspot with continuing episodic eruptions of mantle-derived basalts along the SRP.

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 17Mawith 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 (N7000 km3)?, and high temperature rhyolitic eruptions (N1000 °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 0.4mm/yr.Drainage divides havemigrated northeastwardwith the hotspot. The Continental Divide and a radial drainage pattern nowcenters 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 activitywith 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 plumemay reflect southeast flow of the upper mantle.

"The Yellowstone supervolcano erupted roughly 640,000 years ago, covering much of North America in a thick coat of ash. Material ejected from the volcano devastated the surrounding area, and particles injected into the atmosphere changed the Earth's climate. Over the past 18 million years the Yellowstone hot spot has powered a series of similar eruptions. In southern Idaho, the 640-kilometer-long Snake River Plain traces the path of the Yellowstone hot spot over this period." -- First paragraph.

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than 200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis. --Abstract.