I was interested in what was happening underneath Mount St. Helens, what was driving the eruption. Magma that is ejected during an eruption comes from deep underground - one topic of interest in the early 80's was what was happening below the surface in what we call "magma chambers". Magma is a liquid with dissolved gases, and, commonly, containing crystals. One important feature of magma chambers that no one knew much about was how those crystals form. What I did for my dissertation was to try to understand the rates of crystal formation. Since that time, I have been interested in how the presence of crystals (and bubbles) affects the behavior of magma when it comes out of the ground.