A Tale of Two Caldera Complexes: Contrasting Evolution of the Indian Peak and Central Nevada Caldera-Forming Magma Systems, Middle Tertiary Great Basin Ash-Flow Province

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9706363 Johnson Tens of millions of years ago western North American experienced volcanism like that of Mt. St. Helens, but of considerably greater magnitude. Virtually simultaneously, about 31-26 million years ago, an especially intense and voluminous series of violent eruptions broadcast blankets of hot, incandescent ash over large parts of the southwestern U.S. and western Mexico to depths of a thousand feet and more locally. Eruption rates, on the order of 1000 cubic miles per million years, were unusually large for this type of volcanism. As these vast volumes of gas-charged magma were erupted in almost a geologic instant, the ground overlying the evacuating magma chamber collapsed to create a topographic depression--a caldera--tens of miles in diameter. Our research is a continuation of an NSF-funded study begun a decade ago on these colossal caldera-forming eruptions in Nevada and Utah. The research will complete the chronology of the eruptions using state-of-the-art argon isotopic clocks and will measure ratios of other isotopes of elements such as strontium (Sr), neodymium (Nd), and lead (Pb) that indicate relative contributions of crustal and mantle material to the erupted magmas. This information will help us constrain the geological processes responsible for the creation and eruption of how such enormous volumes of magma in the context of the plate tectonics of western North America.