The birth and death of a mountain range

Mountain ranges rise where a sea once met a shore. They start to form when a sea bottom along the edge of a continent begins to fill up with mud and sand. As the mud and sand grow thicker, the sea bottom begins to sink. The mud, sand, and rock drop slowly down into the earth's mantle - the hot rock beneath the crust. They are crushed, squeezed, and melted together by heat and pressure. Hot rock from the mantle is mixed ini with them. All this takes many millions of years.

Mountains are orn when this mixture of rock is pushed up again by earthquakes and other forces in the earth. As the huge pile of rock rises, it pushes the edge of the land into wrinkles and folds. After many more millions of years, the upper part of the long, lumpy mass of rock has risen high above the land.

As soon as mountains are born, they begin to wear down. Rain falls on them. Each drop, like a little bomb, breaks off tiny bits of rock. Wind blows off tiny grains of rock and carries them away. Streams and rivers run down the slopes, cutting great grooves. Sometimes, huge masses of ice and snow move down mountainsides, grinding the rock to powder.

For millions of years, all these forces wear tons and tons and tons of rock from mountains. Slowly, the proud, peaked mountaintops are worn down and smoothed out. After many millions of years, nothing is left of a mountain range but a row of small, smooth hills. After more years, even those are worn away.