Landslide likely key Coarse sand and a volcanic layer that appears explosive in origin are puzzling discoveries of a deep-sea drilling program conducted northeast of Oahu for University of Hawaii scientists.
to ocean mystery
UH scientists are trying to find the
source of deep-sea deposits off Oahu
Undersea seismometer 'going strong' scientist says
By Helen Altonn
haltonn@starbulletin.comGregory Moore and Michael Garcia, geology and geophysics professors, are trying to figure out where the explosive volcanic rocks and sand deposits could have come from.
Deep ocean sands usually are found only close to an island or coastline, and these are more than 186 miles from Oahu, Garcia said, explaining it would take a big event to transport them so far.
The source of the explosive volcanic rocks also is a big question, he said.
Some people on the drill ship JOIDES Resolution believed the volcanic layer came from Oahu and was hot, he said.
"Getting something that far, hot, through the ocean is an unusual event," he said. "It supposedly baked the underlying sediment and still had boiling water within it by the time it was put in place."
Some suggested that the source of the volcanic material was closer to where it was found in the ocean, Garcia said.
Others speculated the deposits occurred in a phenomenon similar to Mount St. Helens, when a landslide took off the top of that volcano and exposed the magma chamber, leading to an explosive eruption, he said. The UH scientists have been studying giant landslides around the islands for several years, particularly the catastrophic Nuuanu landslide that removed about 40 percent of the Windward side of Oahu more than a million years ago.
Sarah Sherman, postdoctoral researcher in geology and geophysics who works with Garcia, participated in the drill project aboard the JOIDES Resolution.
She said the sediment was expected to be about 330 feet deep, based on seismic data, but the drill hit rock at about 42 feet. The hole was drilled about 138 feet deep.
Sherman returned with samples of different types of rocks that, she said, "tell us it's more complicated than what we thought." Preliminary data on glass from the 42-foot level indicate it has chemistry like Hawaiian lavas, she said.
"Some of the glasses (in the volcanic rocks) looked like Koolau glass," she said.
That is exciting, she said, because all the samples have low sulfur content, suggesting they were "degassed" and erupted from land, not in the ocean.
Scientists are interested in the Nuuanu landslide because it may have generated enormous tsunamis that reached the West Coast. One question is whether it was one colossal event or a succession of collapses. Learning more about what happened may help predict the next event, Garcia said.
The UH scientists previously used ships and deep-submergence subs from the Japan Marine Science and Technology Center to investigate the Nuuanu landslide.
They said they learned a lot because the Japanese have the ability to map the ocean floor, still poorly known around Hawaii. They even discovered a volcano related to the Hawaiian chain about 46 miles off Waianae, Garcia said.
"It seems to be very young, based on a lack of sediments," Moore said, estimating it is less than 1 million years old. "Size-wise, it is about the same size as Koolau or Waianae volcanoes. It's a big feature."
It rises about 3,300 feet from the sea floor and is about 13,200 feet from the ocean surface, he said.
These discoveries aside, the researchers were frustrated that they could not get beneath the Nuuanu landslide deposits.
They sent a proposal to the Ocean Drilling Program to drill a hole through the deposits, and the project was added to a cruise scheduled by the JOIDES Resolution in this area after Christmas.
Garcia said a layer was found beneath the deposits that the ship's scientists initially called a lava flow but was eventually found to be explosive in origin.
The scientists said they have learned in the last 10 years that eruptions related to the Hawaiian hot spot occur far out on flanks of the islands, as well as on the islands.
A volcanic field, called North Arch Volcanics, was discovered extending from about 62 miles north of Kahuku to about 136 miles in the center, Garcia said.
"It is truly enormous," he said, "with little cones and fissures like those seen on Kilauea. ... We didn't expect such deposits from the North Arch fields because these are lava flows, whereas the deposits they found in the cores are explosive in origin."
If they were explosive eruptions, he said, "that would be a new discovery, and if they (deposits) came from Oahu, that would be a frightening discovery in the sense that such a big event occurred."
Moore said, "The other part of this volcano story is, you wouldn't expect to find an explosive volcano at that water depth (16,500 feet)."
Garcia added: "We're waiting for proof to come in. That part about having it happen on Oahu and getting it that far while it's still hot, that's the part that's troublesome."
Hawaii Institute of Geophysics and Planetology
