Sea Floor Holds Story Of Hawaiian Isles' Doom

By WALTER SULLIVAN

Published: December 3, 1991

http://www.nytimes.com/1991/12/03/science/sea-floor-holds-story-of-hawaiian-isles-doom.html

HONOLULU— OCEANOGRAPHIC surveys are throwing dramatic new light on the cataclysmic landslides that lopped off a large part of the Hawaiian island of Oahu and may one day strike the chain's big island, Hawaii.

Oahu's lost half has been missing for some time, probably a million years or more, but only recently have the missing pieces been located. The new oceanographic surveys have mapped in greater detail the massive fields of avalanche debris. Some of the fragments are gigantic blocks measuring 10 miles on a side.

New land observations also suggest that as a large part of the island plunged into the sea, a titanic ocean wave swelled up against the neighboring island of Lanai, reaching a height of 2,300 feet.

The new findings paint a more detailed picture of the process in which the lava that built the islands fell away in cataclysmic avalanches even as the islands themselves were on the rise. The result of this process is Hawaii's remarkably beautiful steep-cliffed landscape.

The islands of the Hawaiian chain are the tops of great mountains that rise as much as 29,000 feet from the sea floor.

Oahu was born four million years ago as two giant volcanoes. The remnants of one form the Waianae Range at the island's west end. Remnants of the other, the Koolau Range, rise gently behind Honolulu and Waikiki. They end in a mighty cliff, the Nuuanu Pali. The cliff, now a major tourist attraction, marks where much of the island fell away.

The Pali is the remaining edge of the giant basin, or caldera, formed by the volcano. At its base is the town of Kaneohe, and beyond that, open ocean. The other half of the caldera, an area the size of Brooklyn, tor away and tumbled into the ocean abyss. In that event, according to measurements taken by a new survey, it appears that 1,800 cubic miles of rock cascaded into the sea. The underwater debris field left by the avalanche reaches more than 140 miles from the island and covers more than 9,000 square miles. Over all, sea-floor debris from avalanches and slumps in the Hawaiian islands covers more than five times the area of the islands themselves.

The Oahu landslide happened a million or more years ago, but the forces of change are still at work in the Hawaiian island chain. Earthquake analysis indicates that Hawaii, the largest and only volcanically active island in the chain, is gradually splitting apart along a rift extending east from Kilauea volcano.

Geologists believe the cause of the splitting is lava that pushes into the rift and spurts onto the surface. Hawaii, at the southeast end of the chain, was formed by huge volcanic eruptions fueled by a "hot spot" in the earth's deep interior from which molten magma penetrates the ocean floor. The older islands in the chain were formed by the same hot spot, but have been carried past it by the motion of the Pacific sea floor, which creeps northwest across the hot spot at the rate of a few centimeters a year.

The island of Hawaii began to form some 800,000 years ago from the eruptions of giant volcanoes, of which Kilauea is the most active. Flows of lava from the volcanoes have heaped atop sediments that built up ages before on the sea bottom. Analysis of an earthquake that occurred in June 1989 indicates that the east side of Kilauea is slipping seaward, lubricated by the buried sediments, six miles below, on which the bottom of the volcano rests.

Studies of the underwater debris fields tell much about the islands' geological past and future. The current surveys are being carried out as part of the effort by the United States to map its entire exclusive economic zone, extending 200 nautical miles off all its shores. Recent findings, analyzed by Dr. James G. Moore, a specialist in Hawaiian volcanism, and five colleagues at the United States Geological Survey, have been published in The Journal of Geophysical Research.

Preliminary results have come from a British sonar system known as Gloria, and surveys now under way by American researchers. Gloria can map a 60-mile-wide swath of sea floor on a single pass. It was towed behind the British research vessel Farnella, which docked a few weeks ago in Honolulu. The survey was jointly sponsored by the Institute of Oceanographic Sciences in Britain and the United States Geological Survey. A 2,300-Foot Wave

The Americans are concentrating on recent landslides from the island of Hawaii, using multibeam sonars that can map the bottom more precisely than the Gloria system. This has produced a perspective diagram of one slide from the west coast of Hawaii and a survey of another that has slipped into the sea from Kilauea.

Geologists have identified two distinct processes whereby the Hawaiian Islands have shed land: slow slumping and catastrophic landslides. The Nuuanu landslide that swept away the northeast side of Oahu is thought to have been of the later type.

So great was its momentum that it crossed the Hawaiian Deep, 15,000 feet below sea level and moved up the slope beyond to 1,000 feet above the deep. The Hawaiian Deep is sea floor depressed by the enormous weight of the volcanoes.

Nearer shore is a giant amphitheater, now submerged, but presumably above water when the slide occurred. The island, as it aged and moved away from the "hot spot," has sunk 1,000 feet.

That the avalanche was sudden and swift has been inferred from the immense blocks of debris showing in the sonar. According to Dr. Barry Raleigh, dean of the School of Ocean and Earth Science Technology at the University of Hawaii, the resulting wave swept to an elevation of 2,300 feet on nearby Lanai island, swamping 95 percent of its surface and depositing coral high on its slopes.

He and Dr. Moore hope that, by dating this coral and by drilling down to coral beneath the debris slide, it should be possible to learn when the avalanche occurred.

Traces of great avalanches have been found on both sides of Oahu as well as alongside all the other islands. Avalanches are at present prone to occur along the steep sides of Loihi, the submerged volcano southeast of Hawaii that should eventually form a new island. Critical but Irregular Events

Debate about the possibility of large Hawaiian landslides dates at least from 1890, when James Dwight Dana, a leading 19th century geologist, proposed that such immense cliffs as the Nuuanu Pali of Oahu, the Napali Coast of Kauai and northern Molokai were produced by fracturing and "downdrop" of the seaward side beneath the sea.

But Dr. Moore and his colleagues said that only now has the extent of such processes been shown.

Discovery of evidence for such dramatic events further discredits doctrinaire uniformitarianism -- the belief that all phenomena that have shaped the earth are currently active. Other examples of critical but irregular events include asteroid impacts such as the one that probably created the moon, or the periodic shedding of Mount Rainier's ice into Puget Sound.

As each island of the Hawaiian chain is pared down by landslides and sinks deeper into the sea, new islands should arise to the southeast, as is the case now with Loihi. That this process has been going on for tens of millions of years is evident from the islands and atolls that extend far to the northwest. Because motion of the Pacific floor changed direction some 30 million years ago, the chain then bends north, becoming the Emperor Seamounts that extend almost to the northwest corner of the ocean.

Photo: The Hawaiian island of Oahu was born four million years ago as two giant volcanoes. The remnants of one is this cliff, the Nuuanu Pali, now a major tourist attraction. The rest of the caldera of the volcano, an area the size of Brooklyn, has torn away and tumbled into the ocean abyss. (Photo Resource Hawaii/Marc Schechter) (pg. C10) Diagram: "Oahu Shaped By Giant Slide" About a million years ago, long before Polynesians arrived, the northern side of Oahu's giant Koolau volcano slid to sea, spreading debris as much as 140 miles offshore. A great cliff, the Nuuano Pali, is all that remains of the volcano. The south-facing slope in back of the cliff is the site of Honolulu suburbs. Giant Blocks of Evidence Mapping by sonar has shown blocks of debris 10 miles on a side. The total volume is estimated at 1,800 cubic miles, covering 9,000 square miles of ocean floor. Another great landslide slid southwest into the sea from the Waianae Volcano on Oahu's west side. Enormous Ocean Wave So swift and massive was the avalanche as it plunged into the sea that it caused a wave that deposited coral 2,300 feet above sea level on the nearby island of Lanai. Lava eruption helped split the island. Land lost in avalanche. Fragments and debris from avalanche (Source: James G. Moore) (pg. C1) Map of Hawaii highlighting submarine landslide areas (Source:Journal of Geophysical Research) (pg. C1)