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Rock slide under the waves.

The deep sea is not as quiet as it may seem. Oceanographers surveying the volcanic mountain range running down the center of the Atlantic basin have discovered the remains of a massive underwater avalanche that surpasses in size any landslide in recorded history.

The bathymetric survey reveals a large scar on the side of the mid-Atlantic ridge at 26.5 [degrees] N latitude, about 3,000 meters below the surface of the ocean. The mountainside, apparently gave way and slid downhill at tremendous speed, running up and over a smaller ridge farther down the slope in a matter of minutes, suggests Brian E. Tucholke, who reports the finding in the February Geology. This is the first evidence of a rock slide ever found in the rift valley of the mid-ocean ridge.

The avalanche carried about 19 cubic kilometers of rock, Tucholke estimates. By comparison, less than 3 cubic kilometers of rock moved in the landslide that sparked the 1980 eruption of Mount St. Helens.

Tucholke says the mid-Atlantic slide must have occurred sometime in the last 450,000 years. If it happened quickly, the avalanche could have spawned a wave 700 meters high. But by the time the wave reached the North American or African coastline, it would have dwindled to less than a meter in height.

In the past few years, oceanographers have found underwater rock slides far larger even than the example in the mid-Atlantic. An ancient slide discovered along the flank of a Hawaiian volcano measures approximately 5,000 cubic kilometers in size.
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Title Annotation:underwater avalanche in the Atlantic basin
Publication:Science News
Date:Mar 14, 1992
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