The Wave That Paved Hawaii 100,000 Years Ago

Abstract

iiA _1q About 100,000 years ago, a giant wave surged up the southern slope of the Hawaiian island of Lanai, blanketing the island's flanks with coral fragments and limestone gravels that the churning water had ripped from the seafloor. As the wave receded it stripped off the rich soil. And on its second and final sweep it dropped pieces of island volcanic rocks from its clutches. This was the picture painted last week at the American Geophysical Union meeting in San Francisco in a presentation given by James G. Moore, a volcanologist, and his brother George W Moore, a marine geologist, both of the U.S. Geological Survey (USGS) in Menlo Park, Calif. The prevailing view before the Moores began their work was that the gravels found on Lanai were deposited at several different times during the Pleistocene epoch, when the sea level rose, creating a series of ancient shorelines above the present coast. Since the melting and growth of ice sheets alone could not account for the implied large and rapid sea level changes, other ideas, including the uplift of the island due to the intrusion of magma, were proposed and debated over the past 50 years. However, based on the ages of offshore coral reefs and other measurements, the Moores decided that the southeastern Hawaiian islands are actually sinking so fast that any past shorelines would now lie below the present coast, and hence should not be seen on dry land. Indeed, when the researchers went to Lanai to have a closer look at the marine deposits, they discovered, instead of a series of ancient shorelines, a single gravel layer that went from the coastline up to an altitude of 326 meters. Above 326 meters, the researchers found 3 meters of soil in which pineapples were growing, while at lower altitudes there was no soil at all. Skeletons of corals and other reef organisms were found in the gravel as disordered fragments rather than as remnants of old reef systems. The researchers also discovered that the gravel layer became thinner and the u rocks and coral pieces finer with increasc ing altitude. A notable exception was the Kaluakapo Crater. that trapped larger fragments than those found in the surrounding gravel. Moreover, uranium dating of the coral by Barney J. Szabo at the USGS in Denver, Colo., revealed that samples from different parts of the island are all about 100,000 years old. All of this geologic evidence, added to the finding of similar gravels spread at lower altitudes on Oahu, Molokai, Maui and Hawaii, led the researchers to conclude that a massivre wave surged through the Pacific Basin sometime just before 100,000 years ago. Because of the wave's large size, the Moores have ruled out an underwater earthquake as the cause, since the highest tsunami, recorded in 1946 on Hawaii, reached only 17 meters above the shoreline. A meteorite fall in the nearby sea or a subsea volcanic eruption could have triggered the wave, but the scientists think a more likely explanation is an underwater landslide over an area of about 25 square kilometers south of Lanai on the Hawaiian Ridge. Subsea landslides had previously been observed to produce large waves; in 1958 an underwater slide in an Alaskan fjord generated the tallest recorded wave, running up 524 meters on land. And the Hawaiian Ridge, which is among the steepest and highest landforms on earth, is believed to have suffered a number of major subsea landslides in the past. The Lanai wave might have been created when a massive chunk of land rapidly fell, sending water surging into the void and up the island slope. According to the researchers, whose work also appears in the Dec. 14 SCIENCE, if such a wave did occur, it is probably not unique. "Now we have a classic place to go," says James Moore, "to look and see what deposits like this actually look like, so they can be recognized in other places as well." --S. W?isburd