Earliest evidence of plate tectonics.

Earliest evidence for plate tectonics 

Earth scientists are still trying to piece together the plate-tectonics puzzle. What exactly pushes the vast fragments of the Earth's outer shell around the surface, and when did the crustal plates first break up and start sliding around, warping, tearing and burying each other as they collide?

The Earth's surface provides solid evidence that the crustal plates have been moving for 600 million years. Less direct evidence supports plate tectonics back to 1.9 billion years ago. Now, four researchers say they have found the earliest geologic evidence yet for plate tectonics--in a formation in India where two plates apparently crashed together 2.5 billion years ago, when the Earth was less than half its present age.

These data challenge previous models in which some scientists envisioned the Earth's crust as lacking plate tectonics until fairly recent geological time. "Our findings show that plate tectonic processes in the very early Earth were much like those of the last 600 million years," says Eirik J. Krogstad of the State University of New York, Stony Brook.

Krogstad and co-worker Gilbert N. Hanson, along with S. Balakrishnan and V. Rajamani of Jawaharlal Nehru University and D.K. Mukhopadhyay of Roorkee University, both in India, describe the new findings in the March 10 SCIENCE. They gathered their evidence for early tectonics from a narrow, north-to-south-trending strip called the Kolar schist belt, where rocks of various ages and origins lie juxtaposed. Krogstad says two pieces of continental crust crunched together from the east and west, squeezing up a band of seafloor between them. This former seafloor, or oceanic crust, makes up the belt, and differs in composition and density from the surrounding two continental crusts.

The continental crust on the west side contains much older rocks than the east side. By measuring how much uranium has decayed to lead and comparing other radioactive-isotope ratios, the researchers estimate the oldest material on the west side t about 3.2 billion years old. Krogstad calls the estimated 2.5-billion-year-old rocks on the east side "juvenile." He thinks the age difference indicates these two continents formed in different places and times before colliding.

According to Krogstad, when the two plates moved together they consumed most of the seafloor separating them, leaving just the narrow belt. Lying side by side in the belt are slivers of fairly ordinary ocean crust and of ancient oceanic crust. Moreover, slivers on the east side of the belt of ocean crust contain more light rare-earth elements than those on the west. Because the levels of these elements are thought to vary from place to place within the mantle, the researchers reason that these oceanic crustal slivers with different ingredients had different sources in the mantle. They conclude that the oceanic crust here formed in different times and places, and crunched together as solid plates.

The researchers were able to estimate the time of the cataclysmic event because the collision introduced into the rock new radioactive material that started decaying at that time -- approximately 2.5 billion years ago. In addition to radioactive elements, the tectonic crunch allowed gold to filter up, making the Kolar belt more than just gold mine of knowledge. According to Krogstad, it's also the world's richest gold mine. His team plans to spend more time exploring the area, hoping to turn up more evidence of early tectonics. And perhaps he says, they'll stumble across more gold.