Sunday, October 26, 2008

Rifts, Geography and Time

Over its 4.6 billion-year history, the surface of our planet has been steadily changing; the size, shape and position of our oceans and continents are in a constant state of flux. A map of the Earth today bears little resemblance to the surface features 200 million years ago and will be just as useless in the distant future.

The motion of the tectonic plates that form the crust of our planet is governed by the opening and closing of oceans; in the former case, a land mass rifts apart while, in the latter, continents are forced to collide and fuse. During the Permian Period, 250 million years ago, Earth's land masses had coalesced into the mega-continent of Pangea. Fifty million years later, the Tethys Sea began to open, rifting Pangea from east to west and separating Laurasia (the northern continents) from Gondwana (the southern continents). About 150 million years ago, the Atlantic opened, rifting these super-continents from north to south and, over the next 100 million years, arms of the Atlantic, Indian and Pacific oceans cut apart Laurasia and Gondwana, producing the map that we see today.

But rifting continues. The Red Sea-East African Rift complex began to develop 40 million years ago and will eventually open a seaway through Africa. Rifting on the east and west sides of Greenland occurred 30 million years ago, producing northern arms of the Atlantic and breaking up a land bridge that had connected Europe and North America. The Gulf of Aden and Rio Grande Rifts began to open 10 million years ago, the Gulf of California opened 5 million years ago and the Lake Baikal Rift began to develop 2.5 million years ago; the latter will eventually connect with the Arctic Ocean, cutting Russia in half. In like manner, further rifting across the Great Basin of North America will merge with the Gulf of California Rift, flooding the Intermountain West with an arm of the Pacific.