16 February 2010
In light of the recent dome collapse at Soufriere Hills, I thought I’d expound a little on the subject, which is a major part of my research. Lava domes, if they last long enough, tend to go through cycles of growth and collapse. These can be relatively short, like the domes at Soufriere Hills or Mt. St. Helens (remember, a few years is short even on a volcanic timescale), or long, like at Casita volcano in Nicaragua or my own study area of Santiaguito. The collapses vary in volume: a smallish collapse might comprise a few million cubic meters of material (the 1929 collapse at Santiaguito was about 3 million cubic meters), but the collapse of an entire dome might be in the 100s of millions of cubic meters.
There are a number of reasons why domes collapse. One is gravity; domes can grow on steep slopes or overspill summit craters, which means that parts of them can become very unstable and simply collapse because of their own weight. Another trigger might be an earthquake, which can shake loose dome rock; yet another trigger might be an eruption, which could loosen or even blow up significant portions of a dome. (This could be what happened at Soufriere Hills; there have been some pretty spectacular Vulcanian eruptions going on at the same time as dome-building there.)
My research, however, focuses on how water can cause dome collapses over short and long timescales. In the short term, intense precipitation events (large storms or hurricanes) have been known to cause domes to collapse (especially at Soufriere Hills). This likely has something to do with water saturating the domes to the point where it can penetrate deep into hot dome rock, and either a) sealing in magmatic gases or b) sealing itself in and vaporizing in cracks and fractures. Either way means that pressure builds up in the dome and water can lubricate structural breaks, which reduce the stability of the dome. In the long term, a dome with an active and well-supplied hydrothermal system can form lots of clays, which are very weak and can also trap water. (Some clays even swell when they absorb water, which pushes dome rock around and destabilizes it that way.) This seems to be what happened when some old domes at Casita Volcano in Nicaragua collapsed and formed a devastating lahar, or mudflow of volcanic material.
What I intend to focus on at Santiaguito is why the domes there haven’t experienced any major collapses in the 80 years they’ve been growing, and what part of the complex might be most likely to fail if a collapse did occur. This is going to involve looking at water-dome interaction in both the short and long term; I’m mixing in hydrology and clay mineralogy with my volcanology, and on this trip I hope to collect clay samples from the inactive domes in the complex. (I am not going anywhere near Caliente, the erupting dome, if I can help it; those videos of people standing on the rim during eruptions are just insane. I’ve met people who were at Galeras when it erupted in 1993, and I’ve heard enough about what can happen that I have no intention of putting myself in that kind of danger.)
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