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

9706906最好的几千万年前，北美西部经历了火山活动，如山。圣海伦斯，但规模更大。几乎在同一时间，大约3100万到2600万年前，一系列特别强烈和巨大的猛烈爆发将炽热的灰烬覆盖在美国西南部和墨西哥西部的大部分地区，达到1000英尺的深度，局部地区更深。 喷发率，在每百万年1000立方英里的数量级上，对于这种类型的火山活动来说是非常大的。当这些大量的充满气体的岩浆几乎在地质学上的瞬间喷发时，覆盖在疏散岩浆库上的地面坍塌，形成了一个直径数十英里的地形凹陷--一个破火山口。我们的研究是国家科学基金会资助的一项研究的延续，该研究始于十年前，研究对象是内华达州和犹他州形成巨大火山口的喷发。该研究将使用最先进的氩同位素时钟完成喷发的年代学，并将测量锶（Sr），钕（Nd）和铅（Pb）等元素的其他同位素的比率，这些同位素表明地壳和地幔物质对喷发岩浆的相对贡献。这些信息将帮助我们限制地质过程，负责创造和喷发，如何在北美西部的板块构造的背景下，如此巨大的岩浆量。

9706906 Best 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.