Two plates are better than one.

Forty million years ago, India and Australia rode as fellow passengers on board a great tectonic plate steaming slowly northward. But the two travelers have started moving in different directions in recent geologic time, causing the once united Indo-Australian plate to begin splitting apart, concludes a new study by James Van Orman, James R. Cochran, and Jeffrey K. Weissel of the Lamont-Doherty Earth Observatory in Palisades, N.Y., and Florence Jestin of the Ecole Normale Superieure in Paris.

Tectonic plates are large sections of Earths outer shell, or lithosphere, that float on a semisoft layer of rock in the mantle. Geologists at present recognize a dozen major tectonic plates and several smaller ones. The new findings, if confirmed by later studies, would bring the number of major plates to 13.

Researchers have long suspected that the Indo-Australian plate behaved differently from the others. As a rule, tectonic plates deform only along their margins, where they bump and grind into one another. But various scientists over the last 25 years have noted signs that the lithosphere was crumpling south of India - a region far from the recognized boundaries of the Indo-Australian plate.

Van Orman, now at the Massachusetts Institute of Technology, and his colleagues studied the deformation between India and Australia by analyzing measurements of the seafloor collected during oceanographic expeditions in 1991 and 1986. Judging from the faults in the ocean bottom, Van Orman and his coworkers calculated that India and Australia are currently moving in different directions and therefore must sit on separate plates.

The boundary between the putative Indian and Australian plates extends across a zone of deformed lithosphere nearly 900 kilometers across, making it much wider than the borders between other tectonic plates. Cochran explains that the break formed in the last 8 million year - recently by geologic standards - and has yet to develop into a sharp border between the plates.