In previously published papers (Iyer, 1975; Iyer and Stewart, 1977; Iyer, 1979), evidence was presented for the presence of a large body of anomalous material, with compressional velocities considerably lower than normal from the surrounding rock, in the crust and upper mantle under Yellowstone National Park, Wyoming. In this paper, we give a detailed analysis of P-residual data from the 26-element telemetered seismic array and three groups of portable stations operated by the U.S. Geological Survey in the Yellowstone region. Using a variety of analytical techniques, we delineate the shape of the low-velocity body, determine the velocity structure inside the body, and interpret the nature of the material that constitutes the body. We will also outline some tectonic implications of our model.

The spectacular surface geothermal phenomena for which Yellowstone is famous result from deep-seated volcanism. The volcanic evolution of Yellowstone is discussed by Christiansen and Blank (1972) and summarized by Eaton and others (1975). Briefly, the geothermal manifestations in Yellowstone are results of three cycles of Quaternary volcanism, each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows. The oldest cycle ended about 1.9 m.y. ago, forming a caldera from Island Park to, perhaps, the central part of the present Yellowstone caldera. The second cycle climaxed about 1.2 m.y. ago and was confined to the Island Park caldera. The third cycle began soon after the second and is responsible for the 75-by 45-km Yellowstone caldera as it is outlined today. This caldera is the result of a catastrophic eruption approximately 600,000 yr ago. The most recent episode of volcanism probably began about 150,000 yr ago in the western half of the caldera near Old Faithful geyser, and the youngest flows in Yellowstone (about 70,000 yr old) are probably associated with this episode.

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