June 7, 1996

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

Research in the Austral Islands

When Polynesian voyagers reached Hawaii, they found volcanic islands much like their homeland in the South Pacific. Geologically speaking, the Marquesas, Tuamotus, Society Islands, Cook Islands, and Austral Islands are about the same age and made of the same kind of volcanic rocks as are the Hawaiian islands. The Hawaiian islands, however, are extremely isolated, while central Polynesia has the highest density of volcanoes of any region on earth.

For the past 25 years, geologists have believed that the island chains of Polynesia were formed by lava from hot spots deep in the Earth's interior, perhaps starting near the Earth's metallic core. The Hawaiian chain is the classic example of this kind of volcanism. Magma from a hot spot eats a path up through the crust and erupts on the surface, forming a volcano. These hot spots are thought to be stationary, but the Earth's crust is not. As the crust moves over the hot spot, a line of volcanoes forms, one after another, in the direction of the crust's movement. But in Polynesia, some of the lines of volcanoes point in other directions, or form groups rather than lines, and in several the ages of the volcanoes do not decrease in order along the chain. So why are these patterns different than in Hawaii? And why are there so many volcanoes in Polynesia in the first place?

For answers to these questions, geologists from the U.S. Geological Survey's Hawaiian Volcano Observatory and from several Mainland universities spent seven weeks at sea around the Austral Islands. Volcanoes in the Austral Islands are younger toward the southeast, as in Hawaii. The newest volcano in the chain, Macdonald seamount, still lies 100 feet below sea level. Many volcanoes stall at this point; the last step in becoming an island is a difficult one, because as soon as new eruptions build the volcano upward, erosion by waves cuts it down again. Macdonald is definitely trying, however. In 1987, an oceanographic research ship reported that Macdonald seamount was erupting, bombarding the ship's hull with hot rocks and gas bubbles. Sailing over an erupting volcano is extremely rare, and is not something that ship's captains are eager to try, because the hot, gas-filled water might suddenly become less dense than the ship -- and down goes the ship.

Fortunately for us, the U.S. Navy declassified Geosat satellite data last year, following the end of the Cold War. These data told us the general shape of the sea floor and the location of the submarine volcanoes. During the cruise, we made detailed maps using a sophisticated echo-sounding system, which draws a strip of seafloor 4-5 miles wide as the ship steams along. We collected volcanic rocks for dating and chemical analysis, using tried-and-true 19th century technology, by dragging an iron bucket along the sea floor and pulling it up to see what we got. Because the sea floor is 2.5 miles deep around the Austral Islands, we needed a very long steel cable to reach the bottom!

The seismic equipment was much more high-tech. Our ship, the R/V Ewing, has the largest seismic exploration system of any academic ship worldwide. We used this system to "map" the layers of rock underneath the sea floor, as far as 4 miles down. This was done by generating sound waves that traveled through the water, into the sea floor, and back out again, returning to the sea surface.

The sound waves are generated by 20 airguns firing simultaneously every 22 seconds, for days on end. People get so used to the rhythm that they wake up in their bunks when the shots stop. These airguns are powered by compressed air; they were invented to replace the dynamite that used to be lit by hand and tossed off the stern of the ship. (Few people were actually injured that way.) Each shot is equivalent to the explosive power of one pound of TNT. The airguns are started up by firing one gun, then two, then four, increasing slowly to warn fish and other marine life to move away.

The sound waves returning from the sea floor are recorded by a "streamer," a 2.5-mile long, oil-filled tube full of electronic cables and sensors that is towed behind the ship. The oil keeps the streamer from sinking and protects the electronics from corrosion by the seawater. The Austral region seismic experiment worked well, after a lot of sweat and blood from the scientists and crew. When we get the data sorted out, we hope to understand more about the volcanoes of Polynesia and the patterns of the island chains.

Eruption Update

There have been two felt earthquakes since late April. On May 30 at 3:45 a.m., an earthquake located 2.8 miles below Kilauea's Middle East Rift Zone had a magnitude of 1.5 and was felt in Kaumana. On June 3 at 1:49 a.m., an earthquake of magnitude 2.6 was located 1.8 miles below Kilauea's caldera, and was felt in Volcano.

The eruption of Kilauea stopped for several days, beginning on May 29. On June 4, lava started flowing through the tubes again and broke out onto the surface, forming channelized flows on Pulama Pali and large sheet flows at the coast. One of the sheet flows spilled into the ocean at Lae'apuki on June 6. The large surface flows have continued for several days, providing spectacular nighttime lava viewing from the end of Chain of Craters Road in Hawaii Volcanoes National Park.