Recycling the continents.

Through the tireless process of plate tectonics, Earth continually renews the ocean crust even as it disposes of ocean rock by drawing it into the planet's mantle. The continents are anomalies, however. According to textbooks, old continental rocks are too buoyant to be recycled inside the planet. They just remain at the surface, growing older.

Two geophysicists now challenge the prevailing theory, suggesting that plate tectonics has recycled significant amounts of continental crust over the last 4 billion years. Samuel A. Bowring of the Massachusetts Institute of Technology and Todd Housh of the University of Texas at Austin discuss their controversial idea in the Sept. 15 Science.

The scientists base their theory on measurements of the ratio between two elements--samarium and neodymium--in rocks. When mantle rock melts to form the buoyant crust, it takes away more neodymium than samarium, leaving less in the mantle. This enables scientists to estimate how much crust was present early on. According to Bowring and Housh, the ratio of neodymium to samarium found in fragments of old rocks suggests that large amounts of continental crust formed early in Earth's history. But much of that crust has since been destroyed and replaced with younger rock. That would explain why so little old continental rock remains. Other researchers say that the continents grew gradually, so the scarcity of old rock simply reflects the smaller size of continents on the early planet.