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!