Spinning spectacle under the Pacific.

Mapping of the Pacific floor has revealed that an Ohio-size patch of the ocean bottom is performing the geological equivalent of a pirouette. The region, a so-called microplate, has spun 90 degrees in the geologically short time of 4 million years, reports a group of scientists from the United States, England and Canada.

Like cracked eggshell, the Earth's outer skin in broken into a dozen large pieces called plates that roam around the world, crashing together in some places and moving apart in others. Much smaller blocks, or microplates, sometimes appear at the junction between two or three plates.

In the late 1980s, oceangraphers discovered that the Easter microplate, near Easter Island, has rotated nearly a quarter turn in 3 million to 4 million years. Now, mapping of a microplate father south reveals a similar phenomenon, suggesting that most microplates behave the same way, even if they form for very different reasons, says Roger L. Larson of the University of Rhode Island in Narragansett. He and his colleagues report their results in the April 16 Nature.

The Juan Fernandez microplate sits at 110[degrees]W and 33[degrees]S, the junction between the Antarctic, Pacific and Nazca plates. Seafloor spreading along the border between the plates is adding new ocean crust onto their edges, causing the plates to diverge like three people walking away from each other. Using sonar to map the ocean bottom's topography, the oceanpgraphers discovered ridges along the southeast edge of the microplate that fan out like the spokes of a bicycle wheel. The same spoke pattern on a map of seafloor magnetic measurements, which reveals the history of seafloor spreading in the area. Such agreement between both maps indicates that the spoke-like ridges formed as the microplate rotated in a clockwise direction. The scientist suggest that the various seafloor-spreading ridges in the area exerted a torque on the microplate, causing it to spin.

The turning has slowed in the last million years as the microplate has expanded. Oceanographers cannot tell whether the microplate will grow into a true plate or remain small and bond onto an already established plate. If it keeps growing, it may face an impressive future. Some oceangraphers have posited that the huge Pacific plate started as a tiny microplate some 190 million years ago.