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September 1, 2004

FOR IMMEDIATE RELEASE

A Gigantic Tsunami in the Hawaiian Islands 120,000 Years Ago

Honolulu - Arguments rage about onshore marine fossils found in the Hawaiian Islands: were they left by gigantic tsunamis, or are they shoreline deposits exposed by massive uplift of the islands? Controversy continues because the islands involved (Lanai, Molokai, Maui) could be either sinking or rising. Now an international group of scientists led by researchers at the University of Hawaii have sidestepped the problem by looking at Kohala volcano on the island of Hawaii, which is known to be sinking at roughly one inch per decade. Guided by a photograph from the 1930s, the scientists have found an on-shore deposit of smashed-up marine shells, angular chunks of lava rock, lumps of soil, and fragments of coral, all cemented together by what was once coralline sand. The deposit rests on a soil whose upper horizons have been stripped off, leaving behind truncated roots of long-vanished shrubs. All the identifiable species in the deposit are from a back-reef environment quite unlike the present shoreline.

The closest back-reef environment is a drowned coral terrace now at a depth of 1,400 feet. The scientists have dated the on-shore deposit and found that it is 120,000 years old, the same age as the drowned terrace and the same age as the Alika 2 Landslide, the last giant landslide down the western slope of nearby Mauna Loa volcano. They conclude that Alika 2 threw up a gigantic tsunami which surged across the terrace and penetrated almost 4 miles inland, smearing fossil-laden sand up to at least 1,600 feet elevation.

"These giant landslides seem to occur during periods of higher than normal sea level--like we have now," says Gary McMurtry, the lead researcher. "They pose a hazard not just in Hawaii but at all big oceanic volcanoes worldwide."

"Obviously we have to figure out why they occur" says Gerard Fryer, a colleague of McMurtry. "That means more mapping and much more dating. But we are racing galloping development--the best exposure on Kohala has already been damaged by unauthorized bulldozing".

Research Article Citation:

Geology, Volume 32, Number 9 September 2004

Megatsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa.

Gary M. McMurtry, Gerard J. Fryer, David R. Tappin, Ian P. Wilkinson, Mark Williams, Jan Fietzke, Dieter Garbe-Schoenberg and Philip Watts, pages 741-744.

For more information, contact

Gerard Fryer, Associate Professor, Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, gerard@hawaii.edu (808) 956-7875

Gary McMurtry, Associate Professor, Department of Oceanography, School of Ocean and Earth Science and Technology, garym@soest.hawaii.edu (808) 956-6858

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822 United States

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Click on the image to open a larger, complete image in a separate window.

Setting of 120,000-year-old tsunami deposits on the slopes of Kohala Volcano on the island of Hawaii. The red line marks the shoreline at the time of deposition. The deposits were left four miles inland from that shoreline at an elevation of what was then 1,600 feet.

credit: Gerard Fryer, SOEST/University of Hawaii

Above: Photograph by Harold Stearns, about 1936. The outcrop is atypical for a sinking shoreline.

Below: The same site in 2002. The capping white layer was left by a massive tsunami. credit: Gerard Fryer, SOEST/University of Hawaii

Perspective view of the Big Island of Hawaii, looking northeast. The giant Alika landslide descended the western slope of the volcano Mauna Loa (ML). The northern lobe of the landslide, Alika 2, was about 120 cubic miles in volume (the 1980 Mt. St. Helens landslide was less than one cubic mile). Sediments lying on top of the Alika 2 debris are 120,000 years old.

credit: Gerard Fryer, SOEST/University of Hawaii

On the Kohala coast of the Big Island, David Tappin of the British Geological Survey points to the contact between the tsunami deposit above, and older rock below. The tsunami deposit is made up of broken and smashed marine fossils mixed in with chunks of basalt lava. Coral fragments in the deposit are 120,000 years old.

credit: Gerard Fryer, SOEST/University of Hawaii

 


For more information, contact Tara Hicks.

 

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