The Yellowstone hotspot

Abstract Direct evidence for a plume-plate interaction as the mechanism responsible for the Yellowstone-Snake River Plain (YSRP), 16-Ma volcanic system is the 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 Yellowstone hotspot. Caldera-forming rhyolitic volcanism, active crustal deformation, extremely high heat flow (about 30 times the continental average), and intensive earthquake activity at 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 at 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 reflects long-wavelength tumescence of the hotspot. Yellowstone is also the center of a + 10 m to + 12 m geoid anomaly, the largest in North America, and extends about 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. The 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. The 16-0 Ma 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 YSRP 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 composed 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 granitic upper crust. Direct evidence for a mantle connection for the YSRP system is from anomalously low, P-wave velocities that extend from the crust to depths of ∼ 200 km. These properties and the kinematics of the YSRP are consistent with an analytic model for plume-plate interaction that produces a “bow-wave” or parabolic pattern 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 rhyolitic 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.