Midwest volcanologist and geology professor Susan W. Kieffer holds a Charles R. Walgreen Jr. Chair at Illinois. In an interview with News Bureau Physical Sciences Editor Jim Kloeppel, Kieffer talked about the recent volcanic activity of Mount Merapi in Indonesia.
Is Mount Merapi a typical volcano?
Yes and no! Volcanoes are like kids: They’re all different! But, like kids, groups of them have some traits in common. Merapi is a typical active stratovolcano, with sisters like Mounts Fuji, Vesuvius, Rainier, and St. Helens. The so-called ‘Pacific Rim of Fire’ is the classic place for these volcanoes. The Cascades of the U.S. and the volcanoes of Japan, Indonesia, New Zealand and the west side of South America are all parts of the rim. In these settings one tectonic plate of Earth’s surface is subducted below another, forming a unique setting where gas-charged magmas are produced and rise up to create the stratovolcanoes. Merapi is typical of these – though a bit hyperactive! It holds the dubious distinction of producing more dangerous ash flows than any other volcano in the world.
What makes a Merapi-type eruption so dangerous?
Merapi-type eruptions are more dangerous than, say, Hawaiian eruptions, because the magma in stratovolcanoes is charged with gas. As a result, when such magma finds a conduit to the surface, it can build up a sticky, hot, gas-rich plug known as a volcanic dome. The dome can build up higher and higher, and then suddenly collapse under its own weight. This creates a very dangerous mixture of hot gas, ash and dome fragments that can flow downhill as much as 5-10 miles because of gravity, can wipe out anything on ridgetops because of the gas expansion, and can melt glaciers and mix with the meltwater to form very dangerous, hot and mobile mudflows. Such a mudflow killed 23,000 people when Ruiz in Colombia erupted in 1985.
Is the volcano’s increased activity related to the May 27 earthquake that struck the area and killed more than 5,500 people?
Almost certainly. We know, for example, that earthquakes in Alaska can affect the geysers in Yellowstone, so there’s good reason to suspect that an earthquake only 30 miles from Merapi would affect the eruptions. Is the activity in early June the response? Or will something bigger happen in the near future? We don’t know. The proximity of stratovolcanoes to faults that can generate the large-magnitude earthquakes is very worrisome. Fortunately, in the recent International Decade of Natural Disaster Reduction, Merapi was one of 16 volcanoes chosen for expedited instrumentation and concentrated study. Fortunately, also, was that Mount Rainier was chosen as well.
Does recent swelling of the volcano’s lava dome indicate imminent collapse?
Almost certainly. The dome collapses, reforms, collapses, reforms. This is dangerous enough, but the geologic record shows that Merapi has another trick that we haven’t seen recently. Sometimes these relatively small collapses unload a deeper magma chamber and a much larger eruption, called a Plinian eruption, occurs. This is the type of eruption that Vesuvius is famous for – the eruption in A.D. 79 that Pliny the Younger documented. This is also what happened at Mount St. Helens in 1980 – the dome there was buried in the north flank of the mountain, and when it blew out in the lateral blast, a bigger eruption was triggered – the one that covered part of the U.S. with a nasty layer of ash.
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