August 8, 2002

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

Deep-diving submersible unveils secrets of the volcanoes

The research vessel Yokosuka, mother ship of the deep-diving submersible Shinkai 6500, is back in Hawaiian waters again for a series of scientific studies on a wide variety of submarine features. Spurred by the outstanding results of their surveys in 1998 and 1999, investigators on the Japan Marine Science and Technology Center (JAMSTEC) ship planned more than 30 dives during this cruise on sites from the giant Nu`uanu slide north of O`ahu and Moloka`i to Lo`ihi, the southernmost and youngest volcano of Hawai`i.

The first leg of the cruise started on July 14, and the last leg will end on September 2. The research team on the Yokosuka includes members of the U.S. Geological Survey, two of whom, James Moore and Peter Lipman, are former HVO geologists. James Moore, Scientist-in-Charge of HVO from 1962 to 1964, was given the distinction of being the researcher on the 700th dive of the Shinkai 6500, a state-of-the-art submersible that can reach a depth of 6,500 m (21,300 ft).

Several dives were planned on the Hilo Ridge of Mauna Kea and the Puna Ridge of Kilauea, but rough sea conditions forced the researchers to go to alternate sites in the calmer waters on the leeward side of the Big Island. Two dives were made on Hualalai Volcano, one on the south arch volcanic field, and five on the southwestern slope of Mauna Loa, including on the young Alika slide.

When ocean conditions permitted, dives were made on Lo`ihi Volcano. As in the past, when the submersible was diving near Lo`ihi, HVO was asked to closely monitor the seismic activity from the area. We were to warn the ship if a swarm of earthquakes were detected. Fortunately, the Lo`ihi work concluded without a seismic event.

Four dives were made on the southeast rift zone of Haleakala Volcano, and the huge Nu`uanu slide was the locale of several dives. Sea conditions will dictate the choice of future dive locations during this survey.

The submarine research program is important because most, if not all, of a Hawaiian volcano is under the sea. With the submersible, researchers are able to gather samples from known locations and are able to observe and record the detailed geologic structure and morphology of an area.

Researchers have developed realistic hypotheses on Kilauea's growth and magmatic evolution from the distribution and chemical composition of many rock samples collected during dives in 1998 and 1999 on the south flank of Kilauea Volcano. The researchers were greatly aided by information on the rock core of a deep drill hole in Puna.

The chemical composition of a few scattered sand grains of the deepest and probably oldest rock samples from Kilauea's south flank looks a lot like that of the 75,000-year-old alkalic rock types found in the Laupahoehoe Volcanics of Mauna Kea. Radiometric dating of the south flank sandstones set their youngest age at 200,000 years. This age indicates that the origin of the highly alkalic grains in the sandstone was not Mauna Kea, but entirely Kilauea.

Another discovery from dives to the deepest part of Kilauea's south flank was the absence of tholeiitic basalt fragments. Tholeiitic basalt forms the present shield of Kilauea, and its absence in broken pieces on the lowest slope indicates that no major slope failure has occurred along the south flank since the shield started more than 200,000 years ago.

A second conclusion from the above discovery is the possibility that the presently active Hilina fault system on the south flank is young and will continue to grow. This means that large-scale faulting will continue to occur and generate tsunamis. The magnitude-7.2 earthquake in 1975 is an example of a large section of the south flank slipping a short distance and causing a tsunami.

One of the discoveries during the current dives was a manganese-encrusted nodule on the lower slope of Lo`ihi. A rough estimate for the age of the nodule is 100,000 years, based on the rate of nodule formation. This estimated age would make the area where the nodule was found to be at least that old or older.

If you see a 105-m-long (350-ft-long) white ship dead in the water off the Big Island, it's only the research vessel Yokosuka tending the deep-diving submersible Shinkai, which is gathering more information on the early history of our volcanoes.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. The West Highcastle ocean entry of the Mother's Day flow sputtered to a halt on August 5. Four active lobes approach the remnant segments of the old roadway at Highcastle. Throughout the week, multiple entries were observed off the Wilipe`a bench which has an area of 3.8 hectares (9.5 acres). Lava viewing is spectacular, and the National Park Service is allowing visitors to hike out and get up close to the active flows.

The eastern Boundary flow emanating from the "rootless" shields was dark on Friday, but lava may be flowing beneath the crusted surface.

One earthquake was reported felt during the week ending on August 8. A resident of Kailua-Kona felt an earthquake at 8:19 a.m. on August 4. The magnitude-1.8 earthquake was located 32 km (19.2 mi) west of Honokohau harbor at a depth of 12.1 km (7.3 mi).