Lying in Wait? Evolution of Dacite Beneath Santa Maria Volcano, Guatemala

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The origin of silica- and volatile-rich andesitic and dacitic magma is a question fundamental to the understanding of how continental crust evolves, how many of the world's active volcanoes grow, and for fully appreciating the hazards posed by these potentially dangerous volcanoes. The main aim of this project is to improve our understanding of the physical and chemical processes by which this kind of magma originates and evolves. This project will focus on Santa Maria Volcano, Guatemala, a relatively simple volcano whose history mirrors that of Mount St. Helens, including explosive eruption of more than 8 cubic kilometers of dacitic ash and pumice in 1902, followed by decades of lava dome extrusion. By linking the record of historic eruptions to a well-dated record of chemical changes in the basaltic lava flows that erupted to form the 12,400 foot high volcano between 72,000 and 36,000 years before the present, our study will provide an unprecedented long-term evaluation of basaltic-andesitic-dacitic magma evolution over the lifetime of a typical subduction zone volcano. This is a unique opportunity to quantify not only the processes responsible for the cataclysmic 1902 eruption, but the time scales and kinetics of these processes as well.The research project is motivated by recent theoretical and numerical modeling of how magma cools within its surrounding rocks when lodged tens of kilometers deep within the crust. These models predict that, rather than the conventional idea of magma stored briefly at shallow depths below the volcano, basaltic magma formed by melting in the Earth's mantle may become trapped within the lower crust for thousands of years, lose heat slowly and crystallize to form volatile-rich dacite before ascending to the surface. In laboratories at the University of Wisconsin-Madison, the team has made advances on several analytical fronts including: age dating of very young lava flows using the 40Ar/39Ar variant of the potassium-argon decay clock, application of short-lived radioactive isotopes of thorium and uranium to determine how long before eruption a magma begins to crystallize, and secondary ion mass spectrometry to measure the concentration of water and carbon dioxide in tiny inclusions of melt trapped within the crystallizing minerals. Analyses using these approaches to study lava flows and ash deposits from Santa Maria Volcano will enable us to track the rates at which the volcano and underlying magma bodies grew, the depth at which the dacite magma began to lose heat and crystallize, and whether, or not, the arrival of a brand new batch of mantle-derived basaltic magma into the crust triggered the massive eruption that killed several thousand inhabitants in October, 1902. Moreover, in Guatemala and other Central American countries, dozens of similarly large volcanoes have erupted in the last few thousand years, or are active today, putting tens of thousands of nearby people at risk. Understanding better the processes that control long-term growth of these volcanoes, and in particular how volatile-rich dacitic magma forms - perhaps deeply hidden for tens of millenia - and erupts violently with little warning, is an overarching goal. As part of this project, the researchers will catalyze collaboration with Guatemalan volcanologists and civilian authorities and involve a graduate student who will pursue the PhD while acquiring expertise in field methods, laboratory analysis, and scientific communication.