Scars from an ancient collision.

Scars from an ancient collision

The Alps, Appalachians, and Himalayas all evolved out of earth-wrenching crashes between two or more of the slow but relentlessly moving tectonic plates that cover the planet's surface. Similar continental smashups have occurred for at least 2 billion years, but much of the evidence from earlier collisions has been wiped out or covered by subsequent events, making the ancient history of the world a dim realm for geologists.

Now, a project in the Bay of Bothnia -- between Finland and Sweden -- has provided some of the clearest information yet about an ancient crash relatively early in the Proterozoic era, which lasted from 2.5 billion to 570 million years ago. To probe the geology beneath the bay, researchers used a technique called seismic reflection profiling, which sends sound waves down into the earth and measures the waves reflected back from structures in the interior. The British, Danish, Finnish, German and Swedish scientists who collaborated on this project report on their work in the Nov. 1 NATURE.

Seismic profiles of the rock structure beneath the bay show a sharp break in the Moho -- the boundary between the Earth's crust and the mantle. On the southwestern side of the break, the Moho dips downward into the mantle, suggesting this region represents an ancient "suture zone" where one plate slid beneath another as the two collided. The rocks in this area date to between 1.9 billion to 1.8 billion years ago.

Crust from the Proterozoic era is much thicker than crust from more recent periods. But the features seen beneath the Bay of Bothnia closely resemble structures in younger collision zones, indicating that the style of plate collisions has not changed much in 1.9 billion years.