Even Without An Eruption, Soft Spots On Volcanoes Can Trigger Deadly Mudflows, UB Scientists Find

SAN FRANCISCO -- Just because a volcano isn't erupting doesn't mean it poses no danger.

In papers being presented today (Tuesday, Dec. 14) at the annual meeting of the American Geophysical Union, University at Buffalo volcanologists show how, in some cases, soft spots on volcanoes that simply collapse from the side may trigger mudflows that potentially can be more devastating than eruptions.

"This is the frightening part about it. These volcanoes don't even have to erupt. They just sit there and a part of it comes roaring off," said Michael F. Sheridan, Ph.D., professor of geology and lead researcher.

"Our findings show us that we have to pay more attention to these older volcanoes that have been sitting around for awhile, sort of stewing in their own juices," said Sheridan.

He explained that as these hydrothermal fluids circulate, they change the structure of the rocks and minerals, softening the volcanoes and making them more vulnerable to collapse.

"That explains why so many volcanoes are usually kind of craggy and steep near the top," he said. "They get that way when soft parts slump or a big slab breaks off, leaving a cliff face."

Sheridan said this new realization about the dangers that volcanic mudflows present to populations is particularly relevant because more towns are being built on volcanic slopes. And that is the case not just in Mexico and other countries known for their volcanoes, but in the U.S. as well.

"Avalanches and mudflows could be a big issue for Mt. Rainier in Washington for example," he said. "There's a lot of pressure in the Seattle and Tacoma areas to start new developments that extend right up into the mountains, but people are building subdivisions in areas that, in the not too distant past, were overcome by mudflows. People are not willing to accept the concept that this is a really dangerous area."

Sheridan said he and his colleagues are applying to volcanoes concepts about hydrothermal alteration that were developed in the 1970s for the purposes of exploration for economically important mineral deposits. Alteration zones and weaknesses in rock structures were targeted then as places to search for these minerals.

Now these weak areas, which UB researchers are pinpointing using satellite data, are turning out to be red flags that could be the source of mudflows so deadly they can travel as far as 80 miles from a source and are capable of wiping out whole towns and villages.

Sometimes, when a piece of a volcano comes loose, an avalanche is triggered.

"Avalanches in themselves can be disastrous," Sheridan noted, "but they don't travel nearly as far as mudflows and they are confined to relatively steep slopes, whereas a mudflow will go for miles. This is where the real danger lies; if you have a really tall volcano and all that energy is driving a mudflow, it could be devastating."

Dangerous mudflows develop when clay is present in the material derived from the volcano's slopes. UB researchers use spectrometers mounted in aircraft or satellites to collect data about the volcano's surface that will help to pinpoint soft spots. This data is interpreted to outline hazard zones for potential mudflows or lahars.

So far, Sheridan and his team have uncovered alarming soft spots on Pico Orizaba, the highest mountain in Mexico, and have mapped areas of potential mudflows at Colima, Mexico's most active volcano.

"That data is going to surprise a lot of people," said Sheridan. "When we started our work on Pico, we were told by geologists in Mexico who had studied the volcano that we would not find any soft spots there, that there simply wasn't any alteration there. In the end, we found alteration everywhere."

While Vera Cruz, the nearest large city with a population of 2 million, is not in danger, Sheridan said that other cities that are closer to the volcano, with populations of around 150,000 or 200,000, are threatened.

The conclusions of this research stem in part from the UB scientists' visit to Nicaragua last fall after torrential rainfall from Hurricane Mitch triggered devastating mudflows that killed about 1,600 people.

"Nicaragua was a real eye-opener," said Sheridan. "There was such a small catchment basin where the rain accumulated, but because the conditions were favorable for this kind of event, a massive, tragic mudflow resulted. These areas are at extremely high risk if people are living nearby," he said.

The research is being funded by the National Aeronautical and Space Administration.