When mountains deceive.

For geologists, the lush Hawaiian island of Kauai has always stood as a classic example of how a single volcano can build a towering monolith from the seafloor. A team of researchers now seeks to tear down that simple image. The lava flows of Kauai hide the remains of at least two and possibly more ancient volcanoes that combined to form a mountainous melange, report the scientists in the September Geology.

Robin T. Holcomb of the U.S. Geological Survey in Seattle and his colleagues argue that previous scientists were duped by the island's roughly circular shape, with many lava flows emanating from a central plateau. Lava had apparently erupted from the island's center and spread equally around the flanks of the volcano in the last 6 million years, making a cone-shaped island subsequently sculpted by erosion.

To test that hypothesis, the Washington team analyzed the chemistry of contemporaneous lava flows on different sides of Kauai. If all of these rocks had erupted from a single volcano at roughly the same time, they would bear a close chemical resemblance to each other. The recent tests reveal a problem, however.

Rocks from the east and west flanks of the island have significantly different ratios of several isotopes, Holcomb's group found. The scientists suggest that one volcano, which they named Waimea, built up the western portion of the island, then collapsed along its eastern edge in a giant landslide. Later, a separate volcano, Lihue, constructed eastern Kauai. After the younger volcano had stopped growing, its eastern flank also collapsed. Remnants of these massive avalanches litter the seafloor around Kauai.

The researchers say it is important to work out the history of the island's volcanoes in order to evaluate the threat of giant landslides in the future.