Sometimes an entire ocean closes as tectonic plates converge, causing blocks of thick continental crust to collide. A collisional mountain range forms as the crust is compressed, crumpled, and thickened even more. The effect is like a swimmer putting a beach ball under his or her belly—the swimmer will rise up considerably out of the water. The highest mountains on Earth today, the Himalayas, are so high because the full thickness of the Indian subcontinent is shoving beneath Asia.
The Appalachian Mountains formed during a collision of continents 500 to 300 million years ago. In their prime they probably had peaks as high as those in the modern zone of continental collision stretching from the Himalayas in Asia to the Alps in Europe. But over the past 300 million years, the Appalachians have eroded to more modest heights.
