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.
One of the best ways to appreciate, and plan for, the hazards associated with active volcanoes is through the development of comprehensive models for how they evolve over tens of thousands of years. This project produced new geological, geochemical and isotopic data from all of the lava flows and volcanic ash deposits that comprise Volcan Santa Maria in Guatemala. This volcano produced one of the deadliest eruptions of the 20th century and may be typical of what other volcanoes in Central America are capable of in the future. Our results were used to model the behavior of magmas that erupted during the past 100,000 years and which lead up to the catastrophic eruption in 1902 A.D. Our data indicate that the 13,000 foot tall symmetrical Mount Fuji-like cone of Santa Maria volcano grew as basaltic lava flows erupted between about 100,000 and 25,000 years ago. We show that as a large volume of basaltic magma ascends from the mantle into the crust below Santa Maria volcano during the early stages of volcano growth this magma reacts strongly with surrounding rocks as it ascends toward the surface. However as the flux of basaltic magma into the lower crust wanes, the pool of magma that remains in the deep crust cools and crystallizes slowly over tens of thousands of years to become a water-rich dacite magma that is capable of erupting in a highly explosive manner. Our findings show that at Volcan Santa Maria, a hiatus in volcanic activity of about 25,000 years preceded the deadly eruption of such a water-rich dacite magma in 1902. Several other volcanoes in Central America resemble the cone of Santa Maria as it stood in repose before the 1902 eruption, thus our findings may help better understand what these volcanoes are capable of in the future. The Principal Investigator was the mentor of two female students who completed their MS degrees at the University of Wisconsin-Madison while participating in this project and contributing to these findings. We also worked closely with Guatemalan scientists and have shared our findings with them.
