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Stanford Report, January 8, 2003

The 'fixed' hotspot that formed Hawaii may not be stationary, scientists conclude


Geologists have long assumed that the Hawaiian Islands owe their existence to a hotspot, a collection of stationary plumes of magma rising from the Earth's mantle and penetrating the surface that formed Mauna Loa, Kilauea and other massive volcanoes.

The predominant theory is that the Pacific Plate has been moving across a fixed Hawaiian hotspot for millions of years, creating a trail of volcanoes whose peaks emerge from the ocean as Maui, Oahu and the other islands that make up the 49th state. But now a team of researchers is raising questions about this long-standing paradigm and providing new evidence that fixed hotspots may not be stationary after all.

"Our research suggests that the Hawaiian hotspot actually drifted southward during the Late Cretaceous to Early Tertiary times -- some 45 to 81 million years ago," said David Scholl, consulting professor of geophysics at Stanford University and research scientist emeritus with the U.S. Geological Survey in Menlo Park, Calif.

Scholl is one of several researchers who discussed new insights into the origins of the Hawaiian Islands and the adjacent Emperor Seamounts during the annual meeting of the American Geophysical Union in San Francisco in December. Robert A. Duncan of Oregon State University and John A. Tarduno of the University of Rochester chaired the panel.

Textbook example

The Hawaiian Islands are considered a textbook example of the fixed hotspot phenomenon. The islands are part of a long chain of volcanoes collectively known as the Hawaiian-Emperor Seamounts that stretch some 3,600 miles along the floor of the Pacific -- from the Big Island of Hawaii to Alaska's Aleutian Trench.

The segment known as the Hawaiian Ridge, which includes the Hawaiian and Midway Islands, forms a neat line of volcanoes that extends some 2,000 miles northwest across the Pacific. At that point, the Hawaiian Ridge meets the Emperor Seamounts -- an older volcanic chain that abruptly changes course, stretching about 1,500 miles almost due north to the Aleutian Trench. According to most researchers, this sharp northward bend records a change in the direction of the Pacific Plate as it passed over the fixed hotspot about 45 million years ago.

To test this hypothesis and determine the true origin of the Emperor Seamounts, Tarduno, Duncan and Scholl embarked on a two-month excursion aboard the research vessel JOIDES Resolution to collect samples of solidified lava flows from four submerged volcanoes that form part of the Seamounts chain. The expedition was conducted under the auspices of the Ocean Drilling Program, an international research effort designed to study the world's seafloors.

Argon and magnetite

When the voyage ended, the researchers conducted geochemical analyses of the lava samples to determine where and when they formed. Age was determined by the radiometric dating of two chemical elements, potassium and argon. When a rock forms, atoms of potassium begin decaying to argon at a constant rate, regardless of changes in the rock's temperature, chemistry or pressure. By measuring the number of potassium-derived argon atoms in the samples, researchers were able to estimate that the submerged volcanoes formed 81 million to 45 million years ago.

Tarduno and his team were able to determine where the volcanoes formed by analyzing a mineral called magnetite in the rock samples. When hot magma from an erupting volcano cools, magnetite residues -- acting like miniature magnetic needles -- align with the Earth's magnetic pole and become locked in place as the rock solidifies. Researchers were able to verify the latitudes at which the Seamounts formed by determining the angles at which the magnetite had frozen.

Using these techniques, Scholl and his colleagues concluded that the Hawaiian hotspot probably drifted southward between 81 million and 45 million years ago -- a finding that may lead researchers to reconstruct previous assumptions about the movement, size and location of the Pacific Plate during that time.

Caroline Uhlik is a science writing intern at Stanford News Service. SR