Content provided by Becky Oskin, OurAmazingPlanet
It's almost unimaginable: a tsunami more than 1,000 feet (300 meters) high bearing down on the island of Hawaii.
But scientists have new evidence of these monster waves, called
megatsunamis, doing just that. The findings were presented Wednesday (Dec. 5) at the annual meeting of the American Geophysical Union.
Unlike tsunamis from earthquakes, the Hawaiian tsunamis strike when the
island chain's massive volcanoes collapse in humongous landslides. This
happens about every 100,000 years, and is linked to climate change,
said Gary McMurtry, a professor at the University of Hawaii in Honolulu.
Sitting about 30 feet (10 m) away from today's Ka Le (South Point)
seashore are boulders the size of cars. Some 250,000 years ago, a
tsunami tossed the enormous rocks 820 feet (250 m) up the island's
slopes, said Fernando Marques, a professor at the University of Lisbon
in Portugal. (The boulders are closer to the shore now because the main
island of Hawaii is one of the world's largest volcanoes, and its
massive weight sends it sinking into the Earth at a rate of about 1
millimeter a year.)
McMurtry's team found two younger and slightly smaller tsunami deposits
at South Point on the main island of Hawaii, one 50,000 years old and
one 13,000 years old. He suggests the tsunami source is the two Ka Le
submarine landslides, from the flanks of the nearby Mauna Loa volcano.
The waves carried corals and 3-foot (1 m) boulders 500 feet (150 m)
inland.
Deadly, landslide-triggered tsunamis happen at volcanic islands around the world, and are a potential hazard for the Eastern United States.
"We find them everywhere, but we don't know of any historical cases, so
we have to go back in time," said Anthony Hildenbrand, a volcanologist
at the University of Paris-Sud in France, who helped identify the
ancient tsunami deposit.
The falling rock acts like a paddle, giving the water a sudden push. While landslide tsunamis
may have a devastating local effect, they lose their power in the open
ocean and don't destroy distant coastlines like earthquake tsunamis.
The giant landslides seem to happen during periods of rising sea levels,
when the climate is also warmer and wetter, Hildenbrand told
OurAmazingPlanet. Researchers speculate that the change from lower sea
level to higher may destabilize a volcanic island's flanks, and heavier
rains could soak its steep slopes, helping trigger landslides.
There are at least 15 giant landslides that have slid off the Hawaiian
Islands in the past 4 million years, with the most recent happening only
100,000 years ago, according to the U.S. Geological Survey. One block
of rock that slid off Oahu is the size of Manhattan.
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