By TOM CALLIS
Tribune-Herald staff writer
Imagine this: a wall of water more than 500 feet tall rising out of the ocean and collapsing on Hawaii Island.
It sounds unbelievable, the kind of far-fetched disaster scenario typically dreamt up by Hollywood.
But a scientist says that’s exactly what happened at South Point — not once but twice — between 12,000 and 50,000 years ago.
Gary McMurtry, an oceanography professor at the University of Hawaii at Manoa, said these megatsunamis were caused by giant submarine landslides on the flanks of Mauna Loa.
The falling flanks, acting like huge paddles, pushed the water ashore, depositing coral and large boulders in the process as far as 200 meters inland.
McMurtry said he came across the rocks, some 16 feet in diameter, last April while doing field work with colleagues and quickly recognized their potential for being the product of a megawave, a topic he has been researching for the past decade.
“We said, ‘They must be tsunami deposits, they are so big,’” he said.
“They are all up and down the coast.”
McMurtry said he and other members of his team, including researchers from Portugal, Germany and France, are in the process of dating the rocks and coral deposits to better pinpoint their age.
The waves are believed to have been at least 500 and 740 feet tall, respectively, based on the location of debris deposits.
Determining their ages is a bit tougher, he said, since the age of the slides have not been pinpointed.
The age of the land beneath or near the boulders and past submarine volcanic activity remain the best indicators.
McMurtry said he expects to be able to date the coral shortly. He plans to publish the research afterward.
So far, all signs point to the two Ka Le landslides as the cause, he said.
The waves wouldn’t be the first megatsunamis to hit the islands, though they would be the most recent.
Large landslides have been common throughout Hawaii’s geologic history, scientists say, with massive, destructive waves being the result.
Jim Moore, a U.S. Geological Survey geologist, said there have been 30 to 40 landslides each covering at least 10 kilometers of the ocean floor in the last 10 million years.
One slide on the northeast side of Oahu alone had a volume of 3,000 cubic kilometers, he said.
In comparison, the largest recorded slide occurred during the eruption of Mt. St. Helens in 1980. It displaced 2.9 cubic kilometers of debris.
“These do not occur often,” Moore said. “We are talking about periodically of something like every 100,000 years.”
But the result is obvious, he said — a massive tsunami.
McMurtry said there have been at least five in the last 500,000 years.
“They are all over the place,” he said. “Even on Oahu.”
Several others have been recorded on the Big Island.
They include two in South Kona roughly 250,000 years ago and one on the east side of Kohala about 335,000 years ago.
The tsunamis would have also hit other islands and made landfall on the West Coast of the United States, though they would have been much smaller by the time they reached the mainland, McMurtry said.
Moore said the volcanic rock that makes up the islands makes them inherently unstable and the landslides help contribute to their eventual demise.
“There’s a very delicate balance between gravity stress on a volcano and also the volcanic activity,” he said.
There also may be a connection between climate change and the gigantic landslides, McMurtry said.
He said they tend to occur when the climate is warmer and wetter with increased rainfall possibly helping the land to slip.
While a major landslide hasn’t happened while people have populated the islands, recent history has shown that the land is anything but settled.
Land subsided twice — in 1975 and 1868 — along Kilauea’s rift zones, causing beaches to sink.
The south flank of the active volcano is also moving away from the rest of the island a few centimeters each year, said Jim Kauahikaua, the lead scientist at the Hawaii Volcano Observatory.
That slow, but continuous lurching makes it a potential source for the next large landslide.
“It’s thought that kind of activity may ultimately precede a larger disastrous collapse of the flank but we don’t have any evidence of that,” he said.
The movement is likely caused by magma rising into the rift zone and pushing the land outward, Kauahikaua said.
McMurtry said he hopes that the South Point deposits will help lead to a better understanding of these massive slides, particularly because they are well preserved.
“That’s kind of exciting,” he said.
Email Tom Callis at email@example.com.