Mountains look permanent, but they are constantly being built up and worn down. Most great ranges form where the plates of Earth's crust collide, folding and thrusting rock upward over millions of years. The Himalayas, for example, are still rising a few millimeters each year as India pushes northward into Asia. At the same time, water, ice, and wind attack every exposed surface. Rain seeps into cracks and freezes, splitting boulders apart, while glaciers grind down valleys and rivers carry the debris toward the sea. Eventually the forces of uplift slow, and erosion gains the upper hand.
Click to start typing