Giant rocks

Mountains are like great, gray giants. The are huge chunks of rock that have been pushed up out of the earth. Some mountains stand all alone, but most are connected together. Long chains of connected mountains are called mountain ranges.

Mountains are not all alike. Many mountains are bare and rocky. Others have green forests and fields of grass growing on their sides. And most high mountains have snow on their tops all year around. This is because the air on a mountain top is usually cold enough to keep water frozen.

Scientists believe that most of the earth's mountains are many millions of years old. The newest are wrinkled, and have sharp, pointed tops. Older mountains are smooth, with rounded tops. These "old" mountains have been worn down by wind and rain during many millions of years.

The moving continents

It may seem to you that the earth's crust is a single piece of rock. You may think this crust covers the outside of the earth just as an apple's skin covers an apple. But the earth's crust is actually made up of a number of enormous separate pieces, called plates. These plates fit together, like the pieces of a puzzle. And they are all moving, very slowly, in different directions.

Scientists are not sure what makes these huge, curved sheets of rock move. It is thought that the plates slide on the hot, soft rock beneath them. They move only about one inch (2.5 centimeters) or less, a year.

Some of the plates make up part of the ocean floor. Other plates hold the huge, high masses of rock that rise above the sea and form the continents. Thus, as the plates move, they carry the continents and ocean floor with them !

The plates have probably been moving for billions of years. This means that earth's seas and continents have probably had many different shapes ! Scientists think that more than two hundred million years ago the earth's dry land was all bunched together, forming one gigantic continent. Slowly, as the plates moved, the one huge continent broke into two continents. And, as the plates continued to move, both of these continents broke up to form the seven continents we know today.

As the plates keep moving, the continents and seas will keep changing shape. Scientists think that in about fifty million years South America and Africa will be farther apart than they are now. This will make the Atlantic Ocean wider. And Australia may move up and push against Southeast Asia, becoming part of it.