As you start this unit, you may be wondering why you need to learn about rocks. Rocks are the history book of the Earth, but that book is written like a mystery. Each layer of rock is a page, telling its own story about the rich history of the planet we call home, but that story is hidden in clues that we have to decipher. To solve those mysteries, we need to know what the clues mean, giving us the knowledge we need to let us read the rocks.

Be sure to check the Rocks Resources page for materials list, suggested reading, where to get specimens, etc.


What is a Rock?

The word "rock" means very different things to different people. As you will see, geologists have their own definition that includes things we don't normally think of as rocks.

Igneous Rocks

All igneous rocks start as magma, but differences in chemistry and cooling rates can produce many different kinds of igneous rocks.

Sedimentary Rocks

Sedimentary rocks are deposited rocks, made up of pieces or chemicals that were weathered or eroded from other rocks. Sedimentary rocks make up less than 4% of the total rocks in the Earth, but they make up more than 75% of the rocks exposed on the surface.

Metamorphic Rocks

Metamorphic rocks also form from other rocks, but instead of being eroded, metamorphic rocks have been changed by heat and/or pressure.

The Rock Cycle

Reading the Rocks

How can you read a rock? It is just like reading a book, but instead of written words, you look for clues about how each rock layer and structure formed.

Rocks Resources