Divergent boundaries -- where new crust is generated as the plates pull away from each other.

When plates move apart, cracks appear in the Earth's crust. The hot molten rock escapes through the cracks, and erupts as a volcano in a shower of dust, smoke,and burning liquid. As the plates move further apart, surface rocks sink forming vertical faults. When two vertical faults occur alongside each other, Rift Valleys are formed. Most of the volcanos in the world have never been seen. They lie deep under the oceans where plates are moving apart.

Convergent boundaries -- where crust is destroyed as one plate dives under another.

When plates push towards each other and collide, rocks are squeezed together. Along the western edge of South America, the Nacza plate is being pushed against the South American plate. This is pushing up the land mass and creating the Andes Mountains. The last phase in the tectonic cycle occurs when two land masses collide and the ocean between them disappears. When this happens, layers of the Earth's crust are squeezed into folds. Over millions of years, mountains and valleys are formed. This happened when the Indo-Australian plate moved north and collided with the Eurasian plate. The ancient Tethys Ocean disappeared and the Himalayas was formed.

A bit of history

In 1912 a German scientist Alfred Lother Wegener studying an atlas, realized that the west coast of Africa was a good fit with the east coast of South America. He was aware that sufficient evidence existed to suggest that a huge ice sheet at one time covered most of South Africa, Southern Australia, and India. It was his supposition that all the Continents at one time formed a single Continent, which he called Pangaea (Greek for "all the Earth"). The theory is that around 225 million years ago pangaea lay near the South Pole. Since then, due to what he called "Continental Drift", the Continents have been drifting apart. He accounted for this movement by the explanation (since proven to be correct) that the Continents were giant blocks or plates floating and moving on the liquid mantle below the Earth's crust. The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent features on the Earth's surface. Not surprisingly, his proposal was not well received.