Hokule`a Tours Pacific Ocean Hotspots

by Christina Heliker, Geologist at the U.S. Geological Survey's Hawaiian Volcano Obvservatory (This article appear in the Hawaii Tribune-Herald on Sept. 5, 1999.)

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.