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Mauna Loa


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March 25, 1999

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

Scientists explore Mauna Loa's submarine flanks

Earth scientists know a lot about Mauna Loa above sea level but much less about it under water. Studies have naturally focused on the easily accessible island, where one can directly observe and sample rock exposures, gauge the time between eruptions, and trace evolutionary changes in the chemical makeup of the lava flows. Unfortunately, though, such exposures are rarely more than 300 m (1,000 ft) thick. Consequently we can easily study only the most recent activity--that of the past few thousand years. To learn about most of Mauna Loa's history, we must go under water.

The submarine flanks of Mauna Loa have received little attention, because of their difficult access. That is changing, however, as technology advances. Early this year, scientists from the Universities of Hawai`i, Massachusetts, and Tasmania teamed up with colleagues from Woods Hole Oceanographic Institute and the Hawaiian Volcano Observatory to look at Mauna Loa's submarine flanks, using an oceanographic research vessel. Some readers may have noticed a ship cruising back and forth off South Point and south Kona during mid-January. The ship was the modern, fully equipped research vessel, Moana Wave, owned by the University of Hawai`i.

What do the researchers hope to discover? Some want to determine the chemical composition of the plume that carries magma from its deep mantle source up into Mauna Loa. Others are interested in the shallow volcanic plumbing system and the movement of magma within Mauna Loa. Still others are interested in comparing and contrasting the development of the submarine flanks to that of the on-land portion. Are the flanks below sea level comprised of broken rubble that may favor the development of large landslides?

The ship-board scientists dredged the flanks of the volcano in order to collect rock samples for chemical analysis and volcanologic study. Previous investigators discovered giant submarine landslides on Mauna Loa. The slides give us an opportunity to sample the interior of a Hawaiian volcano by, in essence, bringing the mountain piecemeal to Mohammed. From outcrops made by the landslides, we collected samples that will enable us to explore Mauna Loa's history for 200,000 years earlier than we now can.

What did the scientists find? In ten days, 27 dredge hauls were recovered at water depths from 1,600 m (5,250 ft) to over 4,500 m (14,750 ft) on the south and west flanks. In general, rocks collected from the submarine extension of the southwest rift zone are rich in the heavy green mineral olivine. As we moved west and northwest, away from the rift zone, the olivine abundance decreases in the lava flows. Landslide debris was dredged on the south and west flanks of the volcano; this debris includes parts of lava flows that had been erupted on land and carried out to sea by the slides. We also dredged samples that probably erupted from nearby vents far from a rift zone.

Several preliminary interpretations have been reached. Olivine-rich lava off South Point is supplied from deep within Mauna Loa's plumbing system. There the magma is hotter than that above and may contain heavy crystals of olivine that sank to those depths before erupting. The rest of the remaining lava, cooler before eruption and poorer in olivine because of settling, is supplied from higher levels in the magma storage system. The recovery of rocks erupted above sea level but collected on the sea floor far from shoreline verifies that landslides have carried parts of the island into the sea.

From this research, the scientific team will gain greater appreciation for how Hawaiian volcanoes grow and develop. Laboratory work on the recovered samples is underway to disclose Mauna Loa's secrets more fully.

Comment: Three incorrect changes were made by the Trib to last week's Volcano Watch. The distance that imposes the natural quarantine is 2000 miles. The non-native bird is spelled mejiro. The third felt earthquake, also a magnitude-4.0, was felt at 12:52 p.m. by residents of Hilo, Papaikou, and Puna on March 18.

Eruption Update

Eruptive activity from the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Lava is flowing away from the vent through a network of tubes to the ocean near Kamokuna and extending the coastline. The public is reminded that the ocean entry areas are extremely hazardous, with frequent explosive activity and occasional collapse of the new land. The steam clouds are highly acidic and laced with glass particles.

There were no felt earthquakes since March 18.

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Updated: 25 Mar 1999