A photographic collage depicting the diversity and evolution of life on Earth through the last 600 million years.
In some ways, it is more like a book, and the rocks are its pages.
Some of the pages are torn or missing, and the pages are not numbered, but geology gives us the tools to help us read this book.
Scientists who study the past try to put events in their proper order.
Earth is constantly changing—nothing on its surface is truly permanent.
Rocks that are now on top of a mountain may once have been at the bottom of the sea.
Thus, to understand the world we live on, we must add the dimension of time. When we talk about recorded history, time is measured in years, centuries, and tens of centuries.
When we talk about Earth history, time is measured in millions and billions of years. We keep track of time with a marvelous invention, the calendar, which is based on the movements of Earth in space.
One spin of Earth on its axis is a day, and one trip around the Sun is a year.
The modern calendar is a great achievement, developed over many thousands of years as theory and technology improved.
People who study Earth’s history also use a type of calendar, called the geologic time scale.
It looks very different from the familiar calendar.