Volcano’s Past Illuminates Future Dangers

The Hawaiian Islands are home to the largest documented shoreline collapse in history—an ancient seaward landslide that sent rocks from the island of Oahu to sites more than 100 miles offshore.

The avalanche of debris from the northeast shore of Oahu probably occurred between 1.5 million and 3 million years ago, and it undoubtedly created one of the largest tsunamis in Earth’s history—a wave large enough to have inundated every coastline of the northern Pacific Ocean.

Today, geologists are studying whether seismic and tectonic forces are creating the potential for a similar disaster on the southeast shore of Hawaii’s Big Island near Kilauea volcano. The world’s most active volcano, Kilauea is continually growing larger. At the same time, its seaward flank is moving toward the Pacific, currently at the rate of about 10 centimeters per year. The entire mountain itself creeps slowly across the seafloor as magma from deep within Earth’s mantle intrudes into the core of the volcano, but some of the movement comes from landslides as layers of lava and sediment atop the mountain are pulled down by the force of gravity.

It is the latter condition that causes concern. “From previous studies, we know that Kilauea is the site of an active landslide, the Hilina slump, which has moved in historic times,” says Julia Morgan, assistant professor of earth science at Rice. “We now recognize that Kilauea also experienced a catastrophic landslide in the past, possibly within 25,000 to 50,000 years, which is quite recent in geologic terms.”

The 10-by-15-mile Hilina slump is partially detached from the seaward flank of Kilauea and is thought to be a candidate for catastrophic collapse. But studies by Morgan and her colleagues, Gregory Moore of the University of Hawaii and David Clague of Monterey Bay Aquarium Research Institute (MBARI), bring some hope. After a comprehensive analysis of two offshore seismic and seafloor mapping surveys conducted in 1998 by the Lamont-Doherty Earth Observatory and MBARI, they have concluded that the debris left over from the last catastrophic landslide on Kilauea is forming a buffer that stabilizes the Hilina slump.

The most recent collapse on Kilauea involved a detached piece of the mountain similar in size to the Hilina slump and located immediately to its northeast. When this section of the volcano slid away, it settled beneath the ocean at the base of Kilauea. As the entire volcano grew and slid oceanward, this debris piled up, much like snow piles up in front of a snowplow. The result is a broad, benchlike, submarine structure that sits at the foot of the mountain, about 15 to 20 miles off the coast. The downslope edge of the Hilina slump now impinges on the outer bench.

“Based on what we’ve seen, we believe that the outer bench is still growing, and we expect that the buttressing effect it exerts on the Hilina slump will increase accordingly,” Morgan says. “This interaction reduces the likelihood of catastrophic detachment of the Hilina slump under present conditions.”

However, because the outer bench contains a good deal of loose sediment and debris, it is also subject to catastrophic failure. For instance, the bench is riddled with small-scale faults and fractures. A massive volcanic eruption or a large earthquake like the 7.2-magnitude temblor that hit Hawaii in 1975 could shake the outer bench to pieces. Morgan says there is geologic evidence that something similar occurred on nearby Mauna Loa about 100,000 years ago.

The research was funded by the National Science Foundation, with additional support from Landmark Graphics Corp., and the results were presented at the fall meeting of the American Geophysical Union in San Francisco.

—Jade Boyd