When Earth Swallows a Continent

Earth can do strange things to continents. Like eat them, for example.

Earth can do strange things to continents. Like eat them, for example.

Previously, the planet's hot interior was only known to have an appetite for ocean crust, consuming it in subduction zones around the world. Continental crust was thought to be too buoyant to be able to sink. But as we're seeing with India, that's not always true.

Now a new paper suggests an even stranger possibility: the Alps Mountains are a giant scar leftover from a time in the Jurassic era when Earth suddenly opened and gobbled down a good portion of land in what would today be southern Europe.

Of course 'suddenly' is all relative. It took about 70 million years for the planet to swallow the continent, spanning a good deal of the Jurassic and Cretaceous eras. Still, you won't hear about this in a textbook. We know that continents can be torn in half, drift around the world, and glued back together again into the supercontinents Pangea, Gondwanaland, and Rodinia. But swallowed down, down into the mantle? Tectonic blasphemy.

Scientists think it all started around 165 million years ago when a piece of continent south of where the Alps are today stretched out. That left only a thin veneer of buoyant material on top of a denser material - the continent's roots. As the whole thing cooled, the roots got even weightier, and began dragging the land down with it. A subduction zone opened, swallowing much of the continental crust and thrusting what was left up in shards that gave birth to the famous mountain range.

Continents are our constants. Rocks in the interior of North America are over 4 billion years old, and the oldest minerals on the planet - some 4.4 billion years old - are found in Australia. Subduction, the planet's great tectonic destroyer of crust, used to be confined to the ocean and island chains.

The idea that a hole could open up in a continent and just swallow the land would change all that, but it's hard to prove. Scientists have been considering the idea for a while now, but they don't have much evidence to go on - the Alps may be one case, and China's Tien Shan mountains another, and that's about it for the whole planet. Like the India case, these kind of tectonic puzzles highlight how little we know about the ground beneath our feet.

Image: Wikipedia