The Earth's crust is broken into many pieces. These pieces are called plates . There are twelve main plates on the Earth's surface. The red lines on this map of the world represent the largest plate boundaries. A plate boundary occurs where two plates come together. There are three kinds of plate boundaries:

Convergent boundary -where two plates collide to form mountains or a subduction zone.
Divergent boundary -where two plates are moving in opposite directions as in a mid-ocean ridge.
Transform boundary -where two plates are sliding past each other as in the San Andreas fault of California.

The Earth's plates are in constant, but very, very slow motion. They move at only 1/2 to 4 inches (1.3 to 10 centimeters) per year!! This does not seem like much, but over millions of years it adds up to great distances of movement.

The Continental Drift Theory states that the continents have moved and are still moving today. In 1912 Alfred Wegener introduced this theory, but he did not fully understand what caused the plates to move. A theory is an explanation of a scientific process that has been successfully tested by many different methods.

The motion of the Earth's plates help scientists to understand why earthquakes, volcanoes, and mountain building occur.

You will learn more about why the plates are moving in the next lesson, "How Plates Move".

Scientists believe these plates have been moving for millions of years. In fact, 250 millions years ago the Earth's seven continents were all grouped together into a supercontinent called Pangea.

Click on the "Next" button, and be transported back 250 million years!!