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September 2, 1999

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


Hokule`a tours hotspots

The Hokule`a set sail from Hilo in June on an eight-month round-trip voyage to Rapa Nui (Easter Island). The crew of this double-hulled sailing canoe is using traditional Polynesian star-compass navigation to find the island, which is about 2,200 km (1,400 mi) from the nearest inhabited land mass.

From a geologist's point of view, the Hokule`a is taking an extended fieldtrip over some of the most fantastic terrain on Earth. The canoe is sailing over a sea floor festooned with chains of enormous volcanoes, landing at the summits of a few of those that are large enough to rise above the waves.

The Hokule`a spent several weeks island-hopping in the Marquesas Islands, the canoe's first landfall. Both the Marquesas and the Society Islands, where the Hokule`a will stop at Tahiti on the way home, are island chains born of a hotspot that feeds magma to the surface. As in Hawaii, the volcanoes of these islands become progressively older to the northwest. That's because, as the Pacific Plate drifts northwestward, each volcano, in turn, is rafted away from the hotspot and dies.

The hotspot that fueled the Marquesas has since closed up shop, but the one beneath the Society Islands is still in business. Mehetia, the easternmost island in the Society group, experienced a seismic swarm in 1981 that was probably generated by an undersea eruption on the southeast flank of the island.

As the Hokule`a sailed through the Marquesas, the crew was struck by the fabulous rock spires that dominate the skyline of Ua Pou. Castle-like pinnacles are typical of eroded volcanic islands in the tropics. Deep river valleys converge in the interior of the island, separated by knife-edged ridges. Some ridges become so narrow that great natural windows open within the valley walls.

On August 24, Hokule`a anchored off the precipitous cliffs of Pitcairn Island. The crew paid a visit to the small community composed mostly of descendants of the HMS Bounty's mutineers and their Tahitian wives. As a measure of how isolated this island is, the crew noted that the Pitcairners ran out of chocolate three months ago.

Pitcairn is an extinct, but young, volcano. Since the 1970s, geologists had suspected that this volcano was at the active end of a hotspot chain that included the Gambier and Duke of Gloucester Islands to the northwest. In 1989 sonar maps of the sea floor confirmed this theory with the discovery of two active submarine volcanoes about 100 km (60 mi) southeast of Pitcairn. One of these rises within 60 m (200 ft) of sea level-close enough to roil the water during an eruption. So beware, Hokule`a!

From Pitcairn the canoe sailed to Mangareva, where the crew is currently preparing for the long crossing to Rapu Nui. Before they sight land again, the Hokule`a will pass over one of the Earth's great boundaries, the East Pacific Rise, a long sea floor ridge where upwelling magma from the mantle is creating new oceanic crust.

West of the East Pacific Rise, the Pacific Plate creeps northwestward with its burden of volcanic islands until it sinks under the lighter continental crust at the western rim of the Pacific Basin. On the opposite side of the East Pacific Rise is the Nazca Plate, which drifts eastward till it dives beneath South America. Rapa Nui rides on the Nazca Plate, holding a one-way ticket to Chile.

Just as the ancient culture that built the monolithic statues of Rapa Nui has provoked endless speculation, so has the origin of the island itself. Early on, archeologists favored the notion that Rapa Nui was the remains of a sunken continent, but geologists soon took the fun out of that idea. The island consists of three relatively young volcanoes that rise from the sea floor. Recent sea floor mapping indicates that Rapa Nui is at the western end of a chain of submarine volcanoes that probably formed over a hotspot.

And what about the famous statues, or moai, of Rapa Nui? If you're a rockhound, you'll want to know that they were carved from yellowish-gray basaltic tuff--a rock that started out as volcanic ash and was subsequently cemented together by mineral-bearing water. The moai originally sported red topknots quarried from welded spatter. This is a familiar commodity to anyone who has visited Pu`u `O`o or other vents on the east rift zone of Kilauea.

Maybe if we're lucky, the Hokule`a will bring back some samples. In the meantime, we can follow the progress of the canoe on the Polynesian Voyaging Society's website at http://leahi.kcc.hawaii.edu/org/pvs.

Eruption Update

The eruption of Kilauea Volcano continued unabated during the past week. Lava from Pu`u `O`o flows through a network of tubes to enter the ocean at the Kamokuna site in Hawaii Volcanoes National Park. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

No earthquakes were reported felt during the week ending on September 2.

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Updated: 4 Oct 1999