Hawaii's Ancient Super Wave

[Guest guest]

Clues to ancient

tsunami found

UH scientists report that a

landslide once sent waves four

miles inland on the Big Isle

http://starbulletin.com/2004/09/02/news/index4.html

By Craig Gima

cgima

 

A giant tsunami caused by the collapse of part of Mauna Loa volcano

120,000 years ago surged more than four miles inland and deposited

coral at the 1,600-foot level of the Big Island, according to

University of Hawaii scientists.

 

Gary McMurtry, an associate professor in the Department of

Oceanography, and Gerard Fryer of the Hawaii Institute of Geophysics

and Planetology studied a deposit in Kohala of coral, smashed-up

marine shells, lava rock and soil on shore cemented together by what

was once coralline sand. The researchers tied the deposit to similar

material at a coral terrace now 1,400 feet below sea level.

 

The scientists believe the coral deposit is evidence of a so-called

megatsunami. Their findings are reported in an article this month's

Geology research journal.

 

The deposit is 120,000 years old, the same age as the underwater coral

terrace and what is known as the Alika 2 Landslide.

 

The scientists theorize that the landslide, which sent 120 cubic miles

of material into the ocean from Mauna Loa, created a megatsunami that

swept miles inland. In contrast, the material from the 1980 Mount St.

Helens eruption and landslide was less than one cubic mile, the

scientists said.

 

"These giant landslides seem to occur during periods of

higher-than-normal sea level -- like we have now," said McMurtry.

"They pose a hazard not just in Hawaii, but at all big oceanic

volcanoes worldwide."

 

However, "the chances of it happening during the lifetime of anyone

presently alive is so small that it isn't something that you should

lose sleep over," Fryer noted.

 

These gigantic landslides occur in Hawaii about every 200,000 years,

Fryer said. The largest giant landslide occurred several hundred

thousand years ago off Oahu when half of the Koolau Volcano fell off,

creating what is now the Windward side of Oahu.

 

While it is not likely to happen any time soon, Fryer said it is

important to study these giant landslides and megatsunamis because of

the potential for destruction.

 

If a similar event were to occur today on the Big Island, a

megatsunami would flow over the isthmus of Maui, wipe out Honolulu and

could reach Wahiawa, Fryer said.

 

The theory of megatsunamis is controversial among geologists, Fryer said.

 

Other scientists believe that while a giant landslide would have

generated a tsunami, they disagree about how large the tsunami was.

 

Some scientists believe the coral deposits were created not by a giant

tsunami, but by the rising and sinking of the islands.

 

Scientists believe that as the Big Island grows, it is sinking, but

the sinking is creating a seesaw effect on the earth's crust that is

raising other islands like Oahu.

 

Fryer said there is a coral deposit at the 1,000-foot elevation level

on Lanai that they would like to compare and date to coral deposits

offshore. If they can date the coral deposit to the Alika 2 Landslide,

they believe they will be able to show that the megatsunami also hit

Lanai.