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With all eyes on Mount St. Helens' huffing and puffing, scientists are expressing concern about future eruptions of a volcano in the Canary Islands that could send a huge tsunami crashing over the East Coast of the United States.

Unlike Mount St. Helens, scientists are not watching for signs of an eruption at the Cumbre Vieja ("Old Summit") volcano, located on the island of La Palma. There would be no advance warnings such as those issued for Mount St. Helens.

"Consider the current activity at Mount St. Helens," Steven Ward of the University of California at Santa Cruz said in an interview. "That place is covered with instruments so scientists can make very particular predictions about the size and timing of eruptions. Nobody can do that for Cumbre Vieja."

Cumbre Vieja is the most active volcano in the Canary Islands, which belong to Spain and are in the Atlantic Ocean off the coast of Africa.

Computer models show that a future eruption could send dozen waves, each 60 to 75 feet high, crashing onto the Florida at freeway speeds. The waves would spread easily over Florida's flat land, reaching areas perhaps a mile or more inland. Smaller waves would hit coasts far to the north and south of Florida.

Simon Day of the Benfield Greig Hazard Research Center at University College in London was among the first to notice the threat. Scientists at the internationally known center study volcanoes, earthquakes, floods, and other global geophysical events.

About 10 years ago, Mr. Day began studying the area around Cumbre Vieja. He discovered that a near miss occurred 55 years ago, and set the stage for the current concerns.

Like Mount St. Helens and many other volcanoes, Cumbre Vieja is a mountain, which rises to an altitude of 7,500 feet. Volcanic eruptions often involve earthquakes and explosions. Giant landslides can result. During its 1980 eruption, Mount St. Helens literally blew its top off, producing what geologists described as the biggest landslide in U. S. history. Mount St. Helens was 9,677 feet high before the eruption. The explosions cut it down to 8,363 feet.

Mr. Day found that the western half of the Cumbre Vieja is separating itself from the eastern half. It began during an eruption in 1949, when a massive landslide detached much of the western flank of the mountain. About 120 cubic miles of rock and soil slid 13 feet down the mountainside toward the ocean and then stopped.

That amount of debris would fill a cube with enormous dimensions. Each of its six sides would be as long as the distance from Toledo to Columbus. The mass now rests on the mountain, loose and poised to slip into the Atlantic Ocean when shaken by another eruption.

A tsunami would occur if the landslide hits the water and spreads across the Atlantic Ocean.

Cumbre Vieja erupts every 20 to 200 years. The last eruption was in 1971. However, it occurred on the southern side of La Palma, and did not affect the landslide material.

"We don't know, however, if the collapse will be triggered during the next eruption or five or 10 eruptions down the line," Mr. Day emphasized. "In theory, the collapse could occur during an eruption later this year or 10 eruptions and a couple of thousand years in the future."

Mr. Day and Mr. Ward teamed up to develop a computer model that simulated the tsunami resulting from the landslide collapse. The results, published in the respected American Geophysical Union journal, triggered a continuing scientific debate on whether "mega-tsunamis" are possible. They would be unusually large, fast-moving tsunamis that travel great distances.

A tsunami, often mistakenly called a "tidal wave," has nothing to do with tides. A tsunami is a series of catastrophic ocean waves generated by disturbances - earthquakes, volcanic eruptions, landslides, or asteroids striking the Earth - that displace huge amounts of water.

In the open ocean, tsunamis are very broad waves that may be almost invisible as they travel at speeds that can exceed 400 miles per hour. They can appear like gentle ocean swells only 3 feet high and pass beneath ships unnoticed.

When they approach shallow water along coastlines, however, tsunamis slow down and change from long shallow waves to short high ones - almost like a rug crumpled against a wall.

With little warning, a series of tsunami waves burst onto shore at freeway speeds with enough energy to flatten everything in their path.

The Ward-Day computer model foresees a mega-tsunami, starting with an initial bulge or splash of water more than a half mile high as the landslide material hits the ocean. That would subside and the tsunami waves would spread out across the ocean at jetliner speeds.

After a hour, waves 150 to 300 feet high would hit the North African coast. The United Kingdom would get waves 20 to 30 feet high within two to five hours. The tsunami would arrive at its final destination - the Caribbean, Florida, and the rest of the U. S. East Coast within nine hours.

Other scientists have reported evidence that giant landslides in the past generated mega-tsunamis. One University of Hawaii group, for instance, showed that marine fossils were swept far inland on the Hawaiian Islands 120,000 years ago when a landslide tumbled down the sides of the Mauna Loa volcano.

The Tsunami Society, a group of scientists who specialize in tsunamis, takes issue with the mega-tsunami idea.

"Scare mongering" is how a panel of experts organized by the society described the idea. "While Cumbre Vieja on Las Palma is expected to erupt again, it will not send a large part of the island into the ocean, though small landslides may occur," the panel said.

William McGuire, director of the Benfield Greig Hazard Research Center, said Cumbre Vieja's threat is real. He chaired a British panel that in August urged world leaders to pay more attention to the risk of Cumbre Vieja.

"The U. S. government must be aware of the threat from Cumbre Vieja," Mr. McGuire said at a news briefing. "They certainly should be worried, as should the island states of the Caribbean."

Scientists do not know whether the mega-mass of material in the landslide is stable and safe between eruptions, or actually is slowly slipping toward the Atlantic Ocean, Mr. Ward said. There has been very little monitoring of the volcano - essential to issuing warnings about an impending eruption.

Mr. McGuire was on the team of British and Canadian scientists who took the last major measurements of the landslide's stability at Cumbre Vieja. Done in the late 1990s, they suggested that the landslide continues to slip very slowly toward the ocean.

Contact Mike Woods at: mwoods@nationalpress.com

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