13 February 2011
What geological concept or idea did you hear about that you had no notion of before (and likely surprised you in some way)?
I can’t think of any particular moments where something like this hit me all at once, but one concept that I’ve encountered as a grad student strikes me as something that I never really thought about much as an undergrad (or as a kid who liked volcanoes, for that matter). It’s the idea that an eruption style at a single volcano – not just in a region – can change dramatically in a relatively short period of time.
When I first started getting into the science of how volcanoes work, I sort of slotted them into categories in my head: This is an effusive eruption, so it forms shields like Hawaii; this is an explosive eruption, it happens at stratovolcanoes; this is a strombolian eruption, it happens at cinder cones, etc. etc. It was really easy to get comfortable with those definitions. But volcanoes, like any other natural system, can’t be categorized like that; they happen on a spectrum of activity, and the categories I thought I knew were really just end-members. The idea that the same volcano could have lava flows, lava domes and explosive eruptions wasn’t really even on my radar until I started digging into the primary literature for projects in advanced classes. When I got to grad school, it became abundantly clear that there was no slotting volcanic activity into neat little categories; my professors at UB study volcanoes that show all sorts of combinations of eruption styles and activity types.
There’s a lot of literature discussing what happens during these eruptions, but we’re still working out the why and how – in fact, it’s the subject of one of the dissertation of one of my fellow UB grads. There are a number of proposed explanations for these, involving everything from compositional changes in the magma, to volatile contents and degassing processes, to magma chamber replenishment by undifferentiated, more primitive melts, to structural changes in the volcano or regional seismic activity. (I’ve gathered a sampling of papers which discuss these hypotheses under the “Additional Reading” section, as well as a couple of links for people who don’t have access to journals.)
Some of my favorite examples of changeable volcanic systems are ones that I hear about all the time in classes and in the office. Llaima volcano in Chile, a basaltic to andesitic stratocone, is the field area that my officemate is working in; it shifts between a passively-degassing, relatively quiet conduit lava lake and effusive lava flows to Strombolian/violent Strombolian eruptions (explosive, closed-system degassing). Stromboli itself (Italy) switches between lava flows and explosive eruptions. On the more silicic side of things, Santa Maria in Guatemala and Mount St. Helens in the Cascades are excellent examples of changing eruption styles in felsic (dacite) volcanism; both of these stratovolcanoes experienced huge Plinian eruptions that were followed years later by lava dome extrusion.
My gradual enlightenment to the spectrum of volcanic eruption styles – and the connections between them – is a way that I’ve come to think about about most geological phenomena. While end-member descriptions are useful when you’re first learning about a concept, it’s important to remember that natural systems rarely fit into neat categories, and they definitely don’t stay there.
How Volcanoes Work: Physiochemical Controls on Eruption Style (San Diego State University Geosciences)
Parfitt & Wilson, 2008, Fundamentals of Physical Volcanology – Ch. 10, Eruption Styles, Scales and Frequencies (Google Books)