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

数千万年前，北美西部经历了类似圣海伦斯火山的火山活动，但规模要大得多。大约在32,600万年前，几乎同时发生了一系列强度特别大、规模特别大的猛烈喷发，在美国西南部和墨西哥西部的大部分地区，炽热的白炽火山灰覆盖了1000英尺甚至更深的深处。每百万年1000立方英里的喷发速度，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一个地质瞬间喷发出来时，撤离的岩浆房上的地面塌陷，形成了一个地形洼地——一个火山口——直径几十英里。我们的研究是十年前美国国家科学基金会资助的一项研究的延续，该研究开始于内华达州和犹他州的这些巨大的火山口形成火山爆发。这项研究将使用最先进的氩同位素时钟完成火山喷发的年表，并将测量其他元素的同位素比率，如锶（Sr）、钕（Nd）和铅（Pb），这些元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了解在北美西部板块构造的背景下，如此大量的岩浆是如何形成和喷发的地质过程。

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.