Julia Rosen writes: It’s no secret that water shapes the world around us. Rivers etch great canyons into the Earth’s surface, while glaciers reorganize the topography of entire mountain ranges. But water’s influence on the landscape runs much deeper than this: Water explains why we have land in the first place.

You might think of land as the bits of crust that just happen to jut up above sea level, but that’s mostly not the case. Earth’s continents rise above the seas in part because they are actually made of different stuff than the seafloor. Oceanic crust consists of dense, black basalt, which rides low in the mantle — like a wet log in a river — and eventually sinks back into Earth’s interior. But continental crust floats like a cork, thanks to one special rock: granite. If we didn’t have granite to lift the continents up, a vast ocean would cover our entire planet, with barely any land to speak of.

Gritty, gray granite and its rocky relatives dominate the continents. It forms the sheer walls of Yosemite Valley and the chiseled faces of Mount Rushmore (and also gleams from many a kitchen counter and shower stall). If you don’t see granite at the surface, you can bet it’s hiding just a few kilometers below your feet, unless you’re cruising over the middle of the ocean in a boat or plane. But what’s special about granite is that it’s relatively buoyant, for a rock—and that to make it, you need water. [Continue reading…]