In the beginning: How Earth got its continents
Dry land was essential for life like us to evolve – but how did Earth evolve dry land? A unique mountain landscape in northern Pakistan holds the clues
Earth 15 April 2015
By Jeff Hecht
(Image: DeAgostini/Getty)
OLIVER JAGOUTZ doesn't have much room for rocks in his narrow tenth-floor office at the Massachusetts Institute of Technology. But the geologist keeps a couple of samples on hand to show visitors how Earth produces something unique in the solar system: continents.
The rocks come from a landscape half a world away, in the remote, hostile mountains of northern Pakistan. But they are a rare record of goings-on deep below Earth's surface. Along with three to four tonnes of other rocks from the region that Jagoutz and his colleagues have gathered over the years, they could hold the key to the enduring mystery of our planet's dry land – and much else besides.
Earth's surface is like no other in our solar system. Sitting atop the partially molten mass of the planet's mantle, like the frothy film on the surface of a simmering pot, are a series of vast slabs of solid rock: the tectonic plates of Earth's crust. That's strange enough, but the crustal plates also contain two rather different ingredients, as Jagoutz's samples show. The first – a heavy, dark rock called gabbro – is typical of the basalts that line the ocean basins. The second, a granite characteristic of Earth's continents, feels light by comparison.
It's a small but crucial difference. Oceanic crust floats lower on top of the mantle and sinks back into it at subduction zones, where two tectonic plates collide. The oldest oceanic crust is just 200 million years old. The less dense continental crust, meanwhile, bobs higher like an iceberg on water. Plate collisions tend to push it …