Earth Rocks On

Some rocks in Greenland show that Earth's crust was moving even 3.8 billion years ago.

By Emily Sohn, 00:00 AM March 23, 2007

Most of the time, the ground feels solid beneath our feet.

That's comforting. But it's also misleading because there's actually a lot going on underground. Masses of land (called plates) slip, slide, and bump against each other, slowly changing the shape of continents and oceans over millions and billions of years.

Layers of volcanic rock within this 3.8-billion-year-old piece of crust reveal that plate tectonics started early in Earth's history.

Layers of volcanic rock within this 3.8-billion-year-old piece of crust reveal that plate tectonics started early in Earth's history.

Science

Scientists know that Earth formed about 4...

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