SAN FRANCISCO--A 500-meter-tall tsunami scoured Hawaii's Big Island long ago, geologists say. The new claim, described here 8 December at a meeting of the American Geophysical Union, is the best evidence yet that Hawaii's unstable volcanic slopes threaten to inundate the state--but not often enough to alarm researchers.
Geologists have long debated whether the Hawaiian Islands have been periodically hit by "megatsunamis," triggered by massive landslides plunging into the ocean. Indeed, several such slides have cascaded from the western flanks of the Big Island--including the 350-cubic-kilometer Alika 2 slide about 120,000 years ago. Some researchers think that a megatsunami from that slide deposited coral and seashells high on the slopes of the island of Lanai more than 100 kilometers away. Others reject that interpretation, claiming that the deposits mark the level of shorelines carved into Lanai before tectonic processes lifted the island.
The new research may have washed away that criticism by focusing on the Big Island itself. Geologist Gary McMurtry of the University of Hawaii, Manoa, and his colleagues found corals and other marine fossils well above the ancient coastline of Kohala, the island's oldest volcano. Radioisotope dating of the fossils gave ages of more than 100,000 years. Back then, that part of Kohala--which has sunk steadily for the last half-million years--was at least 400 meters above sea level and 5 kilometers inland, McMurtry says. Only an extraordinary wave could have swept the fossils there, and McMurtry thinks the timing pinpoints the Alika 2 slide as the trigger.
Computer simulations show that the tsunami would have swamped the other islands as well. "These waves were truly catastrophic," McMurtry says. Still, he notes, they would be about as rare as megatsunamis from asteroids hitting the ocean--grave threats, but extremely unlikely in our lifetimes.
Although the evidence for a giant tsunami on Lanai is still in dispute, the case on the Big Island is now "ironclad," says oceanographer David Clague of the Monterey Bay Aquarium Research Institute in Moss Landing, California. Even so, some hardened critics at the meeting refused to yield. "The sedimentology is wrong for a tsunami deposit," says oceanographer Keith Crook of the University of Hawaii. "The painstaking geologic observations have not yet been made."