Millions of years ago, Mt. Everest was not a mountain at all. It was underneath the ocean! A lot has changed since then. The outer skin of the earth – both land and sea – rides on gigantic “plates”. Over millions of years, these moving plates collide with one another to form spectacular mountains.

How to make a mountain

Here are some of the ways that mountains form:

Two plates can press against each other until the land is lifted and folded over itself.

Folded mountains of the Tibetan Plateau (photo by Alton Byers)

One plate can push on top of another one. As one plate slides downward into the earth, it begins to melt. The melted rock rushes upward along cracks and weak spots, bursting out as fiery volcanoes. You can see a huge “ring of fire” where volcanoes circle the Pacific Ocean.

Plates can stretch until they crack and slide, forming fault-block mountains.

In the ocean, great underwater mountains are formed when plates spread away from one another, and melted rock pushes up through the gap.

Ice sculpture

Glaciers carve out spectacular scenery in mountains. Glaciers pluck rock right out of the mountainside and carry it downward in a river of ice. Glaciers form sharp horns, rounded bowls, and U-shaped valleys.

The famous Matterhorn… and a glacier that is still carving rock nearby. (photos by Elizabeth Byers)

Old mountains

Given enough time, millions of years generally, all mountains crumble. High jagged peaks become low rounded hills. Finally, mountains wear away to become soil on plains, sand on beaches, or sediments in oceans.

What makes mountains crumble? Rain, wind, and ice wear away mountains. Water gets into the cracks in rocks and pushes the cracks wider as it expands with both heat and cold. Water also dissolves some minerals, washing them out of the rocks. Eventually, the rocks crack and split off. Some crash down the mountain as boulders, others move only a tiny bit as clay particles.

Would you like to speed up geologic time to see mountains being made? Try these links:

Break-up of Pangaea
Animated GIF begins with globe showing the ancient super-continent of Pangaea. Continents move apart to their present positions (USGS National Park Service)

Mountain Maker, Earth Shaker: You Try It!
Shockwave interactive animation of plate tectonics (A Science Odyssey, WGBH, PBS)

Birth of the Himalaya
Shockwave animation sequence (PBS)

Making mountains under the ocean
Animated GIF begins with close-up showing mid-ocean ridge topography with magma chamber below. Magma rises and new ocean plate spreads away from ridge (USGS National Park Service)

SOURCE:http://www.mountain.org