The Earth's crust is quite thin compared to the diameter of our planet. The crust is made up of sections called plates, ranging from very small to the size of an entire continent. Both the Earth's surface and its interior are constantly moving.
Beneath the rigid plates of the earth's crust, lies the mantle -- mobile rock that slowly moves in circles, like a pot of thick soup that is heated to boiling. The heated soup rises to the surface, spreads and begins to cool, then sinks back to the bottom of the pot, where it reheats and rises again. This cycle is repeated over and over; this is called convective flow. This process occurs much more slowly inside the Earth.
Because of the heat and pressure that builds up beneath the surface, the crust is constantly being stressed, which breaks it up. This is known as plate tectonics. When two plates move away from each other, deep cracks are opened through the crust, allowing magma to rise to the surface and then cool, making new crust. Crust is destroyed when one plate dives under another. Plates can also slide horizontally past each other.
One of the most spectacular visible consequences of plate tectonics is the Himalayan mountain range, the highest continental mountains in the world. It was formed when the continent of India collided with Asia 50 million years ago, pushing up the peaks over millions of years until they reached their present heights.