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Heavy Rains Make Volcanoes Erupt

The most dangerous type of volcanic eruption can be set off by heavy rain, meaning that watching the weather could save lives. This type of eruption is a "dome collapse," which has caused more than 70% of the volcano-related deaths over the past century. "They tend to be big, violent eruptions," says volcanologist John Murray.

The build-up leading to an eruption can be seen months or years ahead, as magma pushing from below produces a visible dome on the side of the mountain. Small eruptions of lava add to the bulge, making it more unstable until it eventually collapses, releasing the pressure in a massive explosion. But it?s hard to tell exactly when this will happen.

Researchers studying the Soufriere Hills volcano on the Caribbean island of Montserrat linked the last three eruptions to the weather. The volcano became active on July 18, 1995 after being dormant for 300 years. Before all three eruptions, heavy rain destabilized a dome that was ready to collapse, says Adrian Matthews at the University of East Anglia in the U.K. The eruption on July 29, 2001 occurred immediately after tropical storm Barry hit the island. "That was the first heavy rainfall event in seven months," he says, and within minutes, the dome collapsed.

There?s also a link between the wettest times of the year and eruptions of Mount Etna and Mount Saint Helens, but the connection is not as clear-cut. Matthews says the next step will be to figure out exactly how rainfall leads to an eruption. Large amounts of water may seep into cracks in the dome and be vaporized by the molten rock. The increase in pressure as the water turns to steam may trigger a blast.

Plymouth, the capital of Montserrat, is only a few miles from the Soufriere Hills volcano. After several evacuations, the town was finally abandoned in 1996, and two-thirds of the island is now off-limits to inhabitants. If eruptions could be predicted, some of these people could move back into the vicinity of the volcano.

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