Chapter 4

Rocks

4.1 Types of Rocks

Lesson Objectives

• Define rock and describe what rocks are made of.


• Know how rocks are classified and described.


• Explain how each of the three main rock types are formed.


• Describe the rock cycle.

Introduction

Have you ever heard something described as “rock solid?” We usually use the phrase to
describe something that does not and cannot change. It also means something is absolutely
sure and will not fail or go wrong somehow. If we say a plan is rock solid that means the
plan is a sure bet—it will not change and it will not go wrong. When you see pictures like
this rocky mountainside in Costa Rica, it is easy to get the feeling that rocks neither change
nor move, but instead always stay the same (Figure4.1).

The Rock Cycle

The truth is, however, that rocks do change. All rocks on Earth change as a result of natural
processes that take place all the time. These changes usually happen very slowly. They may
even happen below Earth’s surface so that we do not notice the changes. The physical and
chemical properties of rocks are constantly changing in a natural, never-ending cycle called
the rock cycle. The rock cycle describes how each of the main types of rocks is formed, and
explains how rocks change within the cycle. This lesson will discuss the characteristics of