New Volcano Threat: Just When You Think It's Safe ...

When a volcano blows and you think you're safe by the sea after the main event subsides, watch your back. That's the message in a new study of an eruption in the Caribbean.

The research confirmed what scientists had expected, and the scenario is a bit frightening.

The Soufriere Hills Volcano, on the island of Montserrat, is an ongoing menace. When its dome collapsed in 2003, it shot out tremendous flows of hot gas and ash. These pyroclastic flows, as they're called, hit the ocean at the mouth of a river. You might think that'd be the end of it.

Image Galleries The Fury of Volcanoes Mount St. Helens in 2004

But the interaction with cool seawater created a fresh pyroclastic explosion and ash surge that spread back onto the island, geologists said this week. The expanding turbulent cloud of rock, steam and ash flowed back onto land at temperatures of 600 degrees and speeds of about 130 miles per hour.

The reverse surge reached 1,050 feet above sea level and flowed nearly two miles inland, devastating an area of nearly three square miles that had not been affected by the main eruption.

Plants were scorched to the ground and some cows were killed, but thanks to evacuations no people were hurt by the secondary flow.

"This interaction between pyroclastic flows and seawater is rarely observed, but presents a very real hazard where populations reside between certain volcanoes and the sea," said the U.S. Geological Survey's Marie Edmonds, who led the research.

Places at particular risk:

• Mount Augustine and many Aleutian volcanoes in Alaska
• Caribbean volcanoes such as Mont Pelee
• Unzen in Japan

Evidence for similar hydrovolcanic explosion's, as they are also known, exists in Earth's natural record of some prehistoric volcanoes.

The research will be detailed in the April edition of journal Geology.

• The Science and History of Volcanoes
• Hawaii's Kilauea Volcano Poses Health Risk
• Super Volcano Will Challenge Civilization, Geologists Warn