Education

Ph. D. University of Oregon (1999)
B.S. Humboldt State University (1993)

Publications

Current field areas

I’m currently working on many different volcanoes, some of which are currently active or have been in the recent past. See my pictures and learn more about them at the Global Volcanism Program website. More to come!

Research Interests

My research involves interpreting the conditions of flow, degassing, and eruption of magma, particularly at silicic volcanoes. In the field, I analyze patterns of deformation in lava flows (e.g., folds, crystal and bubble alignments) to better understand lava flow emplacement mechanisms. In the laboratory, I conduct microscopic analyses of textures and structures in volcanic materials in order to decipher changes in the way magma flows in the conduit. I also perform electron beam and infrared spectroscopic measurements to quantify chemical patterns in glasses and minerals. My field and analytical work is complemented by experiments designed to simulate conditions of magma storage and ascent, degassing, and crystallization, including work carried out in our petrology laboratory. Here are a few projects and places I am currently working on:

1.) Microlite crystallization in glassy rhyolite. This project is in collaboration with Jim Gardner (U. Texas) and involves experimental reproduction of groundmass textures and mineral compositions in rhyolitic obsidians collected from the Inyo Volcanic Chain, CA. The goal here is to determine the conditions of magma storage and ascent based on hydrothermal decompression and phase equilibrium experiments. Surprisingly little is known about the rates and paths that magma travels through the shallow crust before it erupts. Our goal is to determine ascent velocities, residence times, and degassing patterns in the Inyo magma leading up to and during its eruption. Some recent products of these experiments are shown below and highlight the textures of alkali feldspar microlites, which are sensitive to PH₂O and therefore, the decompression history.

2.) Degassing behavior of silicic melt inferred from H₂O concentration patterns in rhyolite domes. The rate and extent of degassing (ie., the loss of volatile species from the silicate melt upon decompression or heating) is a dominant control on eruption behavior. I am working with Michael Manga (UC Berkeley) and Michael C. Martin (LBNL) on quantifying the rates of vesiculation based on measurement of H₂O concentrations in effusive obsidians from Big Glass Mountain, CA. Concentration gradients such as the ones shown below can be compared with models for H₂O diffusion in rhyolite melt to infer vesiculation timescales. This information, in turn, is helpful for evaluating the hazardous explosive activity evident in many lava domes and flows. The figure below shows two wafers, cut from banded obsidian, and their respective H₂O profiles. The profiles were measured using a synchrotron-source FTIR instrument at the Lawrence Berkeley National Laboratory Advanced Light Source.

3.) Magma degassing and crystallization in the Bronze-age eruption of Thera volcano, Greece. This project, conducted in conjunction with Ulrich Kueppers and Don Dingwell of the University of Munich, concerns degassing and crystallization of the rhyo-dacitic magma that fed the great Minoan eruption. We are conducting detailed FTIR (Fourier Transform Infrared Spectroscopic) measurements on glassy pyroclasts and pumices collected from multiple phases of the eruption in order to track changes in volatile content with time. Below is a photo from Kameni Island showing products of recent (mid-20th century) vulcanian and effusive activity. Note the large breadcrusted dacite bomb that Ulli stands next to.

4.) Field and experimental study of bimodal volcanism at Cotopaxi Volcano, Ecuador. Cotopaxi volcano, located about 60 km south of Quito, Ecuador, is one of the largest and highest (5897 m) active volcanoes in the world. It has experienced 14 significant eruptions since 1532, in addition to many smaller eruptions. Its activity is characterized by both explosive and effusive eruptions and it has produced tephra falls, lava flows, and pyroclastic flows. Cotopaxi eruptions pose a serious threat to the neighboring city of Quito (~2 million people) because many of its eruptions generate deadly debris flows that can travel several tens of kilometers from the volcano down valleys and into villages and cities (Miller et al., 1978).

In addition to being very hazardous, Cotopaxi is a remarkable volcano in that it has produced magmas that are strongly bimodal in their chemical composition. In other words, this volcano has produced two chemically distinct magma types over its half-million year history: rhyolite (~75 wt% SiO₂) and andesite (~60 wt% SiO₂). During the last 10,000 years the volcano has switched back and forth between these two magma types with very little mixing between the two components. Such rapid changes from andesitic to rhyolitic magma emission suggest: 1) instability in a chemically-layered source magma chamber and/or 2) abrupt switches in a magma plumbing system that taps two separate chambers.

My experimental and field work on Cotopaxi is a collaborative effort with Drs. Peter Hall and Patty Mothes of the Instituto Geofisico in Quito; our collective goal is to determine the depths and temperatures of rhyolite and andesite magmas residing under the volcano. This data might help elucidate whether the magmas constitute singular or separate chambers, which in turn, would help scientists interpret future signs of unrest, such as earthquakes. So far, the experiments are establishing the stability fields of natural mineral assemblages that grew in the andesites and rhyolites prior to their eruption. Minerals such as biotite, feldspar, and hornblende are well preserved in the Cotopaxi rocks, however, they are sensitive to pressure changes, which cause them to react if perturbed outside of their “comfort zones“. Another group of experiments is underway to exploit the disequilibrium response minerals have in the natural magmas that undergo decompression and degassing. The ultimate goal of these kinetic experiments is to estimate the timescales of magma ascent prior to eruptions.

Cotopaxi’s south face with Yan Lavallee examining an early feeder dike in the foreground.