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March 12, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Sonar snares seafloor secrets

Two geologists who used to work at the Hawaiian Volcano Observatory were back in the state last week viewing Hawai`i's volcanoes from a different perspective. Cruising among the islands aboard a 270-ft ship, Jennifer Reynolds and Dave Clague (HVO's former scientist-in-charge) thoroughly enjoyed the smooth water and sunny weather. Sounds like fun, but most of the time they were below decks staring at a computer screen. The ship was contracted by the Monterey Bay Aquarium Research Institute (MBARI), where Jennifer and Dave now work, to survey the ocean floor near Hawai`i.

The purpose of the cruise was to map features on the seafloor that hold particular interest to those of us interested in the evolution of the volcanoes that form our islands. The mapping was done using a multi-beamed side-scan sonar system mounted on the ship's hull. Sonar works by transmitting sound waves that are reflected off the bottom, producing an image that looks like a black and white photograph. The seafloor around Hawai`i has been mapped by side-scan sonar before, but the system on board this ship operates at a higher frequency and produces much better images than those obtained earlier. At 2000 m (6560 ft) water depth, this system can distinguish an object 30 m (100 ft) long, as opposed to 250 m (800 ft) with the older systems.

The areas targeted by the scientists on this cruise included the gigantic landslide deposits off the north shore of Moloka`i, the northeast shore of Kohala, and offshore of Haleakala Volcano on Maui. The huge, jumbled blocks that make up these deposits were dumped by catastrophic landslides that carried away parts of growing volcanoes. These events occur only once every few hundred thousand years, so there's no need to lose any sleep worrying about them, but for a geologist this jumbled seascape tells a fascinating story that was only dimly suspected until the seafloor was mapped.

The ship surveyed two submarine rift zones off the Big Island: the Hilo Ridge, subject of a recent Volcano Watch column, and the Puna Ridge, the submarine extension of the east rift zone of Kilauea. Papa`u Seamount, between Kilauea's south coast and Loihi, was also surveyed.

Continuing around the island, the ship paused 50 km (30 miles) offshore of Kawaihae to map Mahukona, the island's first volcano, which now lies submerged under more than 1200 m (4000 ft) of water. Only the western side of this extinct volcano is still exposed; the eastern slopes and summit are buried beneath Kohala and Hualalai Volcanoes. Mahukona was never a huge volcano, but it once rose roughly 800 ft above sealevel. That was about 450,000 years ago; since then, the island has been steadily subsiding.

The history of this subsidence is another tale that can only be read from maps of the seafloor. A series of drowned coral reefs shows up as terraces stepping down into the deeps off the Kohala-North Kona coast. Coral samples dredged from these reefs have been dated by isotopic methods, and tell us that the deepest and oldest reef was alive when Mahukona was still active, about 465,000 years ago. From the depths and ages of these reefs, we know that the island has subsided an average of 2.6 mm (0.1 in) per year since then.

This sinking is due to the weight of the volcanoes, which causes the outer layer of the earth to sag, pushing into the semi-molten mantle below. Even as the island grows higher as more lava erupts on the surface, any given point on the island is slowly sinking relative to sealevel.

The maps compiled from the surveys will be used to plan and guide the next step of seafloor research--sending remotely operated vehicles to the bottom to take samples and pictures. Not only geologists, but also biologists and chemists interested in submarine life will participate in these studies. The new seafloor maps from this cruise will be posted on MBARI's website.
in the next two weeks

Eruption and Earthquake Update, 12 March

During the past week, there was constant effusion of lava from the vent within Pu`u `O`o. Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. There were breakouts from the tube system that fed minor surface flows. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

A resident of Kona Palisades subdivision felt an earthquake at 3:55 p.m. on Saturday, March 7. The location of the magnitude 3.0 earthquake was 4 km (2.4 mi) northeast of Kealakekua at a depth of 25 km (15 mi).

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Updated : 26 March 1998