Falling Flanks

Wednesday, October 01, 1997
Compared with mountains, which take millions of years to form, volcanoes rise in a geologic instant, in just thousands or even hundreds of years. This quick growth can make volcanoes structurally unstable. When Mount St. Helens erupted in Washington in 1980, entire slopes of the volcano collapsed. Most geologists have assumed that such catastrophic failures would be triggered only by an eruption. But researchers at the Open University in England now have evidence that an eruption need not occur. Even dormant volcanoes, the researchers say, can collapse. And since dormant volcanoes are thought to be relatively safe and are therefore rarely monitored by geologists, this puts many areas all over the world at risk.

Volcanologist Benjamin van Wyk de Vries has been studying Mombacho, a volcano in Nicaragua whose collapse within the last thousand years gave it a jagged profile. His studies have shown that two processes make volcanoes unstable. One involves the unique chemistry of volcanic rocks. The cracks within solidified lava inside a volcano are typically filled with hot, acidic gases that react with the surrounding rock and change its chemical structure. It breaks them down from a nice solid rock that you can bang with your hammer without breaking, to something that you can crumble with your fingers, says Van Wyk de Vries. Volcanic slopes can thus collapse under their own weight.

Volcanoes themselves also put tremendous pressure on the ground below them. If a volcano happens to form in an area where the ground consists of soft sediments, the sediments will ooze out from beneath the volcano. If such soft, shifty sediments entirely surround the volcano, it can spread out in all directions and will actually tend to become more stable because it will have a larger base. But in most cases, says Van Wyk de Vries, the sediments can more easily push out in just one direction. This creates uneven stresses on the volcano, which can lead to fractures and ultimately a massive collapse and avalanche.

Just about any volcano, active or not, is capable of collapsing, says Van Wyk de Vries. Many are near populated areas, like Seattle and Mexico City. He is now working on ways to determine which volcanoes are stable and which are at risk of collapse. You look at the structure, and if it’s bulging outward at a constant pace, that’s okay, he says. What we’ve got to watch for is when that movement starts increasing; then you know you’re running into problems.
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