Volcano Watch -- 19 March 1998 [Thu]

[Ryan Kawailani Ozawa]

Volcano Watch -- 19 March 1998 [Thu]
Hawaiian Volcano Observatory/U.S. Geological Survey
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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 radiocarbon
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 at www.mbari.org.

Eruption and Earthquake Update

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).

From:

Hawaiian Volcano Observatory
U.S. Geological Survey
P.O. Box 51
Hawai`i National Park, HI 96718
Phone: (808) 967-7328
Facsimile: (808) 967-8890
Web: http://www.soest.hawaii.edu/hvo/

Republished by the Hawai`i NewsList with the permission of HVO/USGS.
Comments and questions are welcome. Please write Don Swanson at
don...@liko.wr.usgs.gov.