Our Earth Is Always Changing  
Top: The Grand Canyon started to form about six million years ago. Bottom: Earthquakes change the landscape suddenly, but are caused by pressure built up over a long time. The Earth formed over 4.5 billion years ago, and it has been changing ever since.

Sometimes these changes happen very fast. An earthquake can split the ground in a few seconds. Lava from a volcanic eruption can spread over the side of a volcano in minutes. A heavy rainstorm can flood a neighborhood in a day. These changes are easy to see.

But most changes happen so slowly we don't notice them at all. The continents slowly creep across the surface of the Earth at an average speed of eight centimeters a year. Over hundreds of millions of years, mountains form, and then slowly erode away.

How do Earth scientists know about these changes? They do a lot of detective work, and they look for clues all over the Earth!