Each statement represents a primary learning objective for parts of this unit. After you complete the unit, you should be able to:

• Sketch, label, and explain the rock cycle.

• Describe the two criteria used to classify igneous rocks and explain how the rate of cooling influences the crystal size of minerals.

• List and describe the different categories of sedimentary rocks and discuss the processes that change sediment into sedimentary rock.

• Define metamorphism, explain how metamorphic rocks form, and describe the agents of metamorphism.

• Distinguish between metallic and nonmetallic mineral resources and list at least two examples of each.

Why study rocks? You have already learned that some rocks and minerals have great economic value. In addition, all Earth processes depend in some way on the properties of these basic Earth materials. Events such as volcanic eruptions, mountain building, weathering, erosion, and even earthquakes involve rocks and minerals. Consequently, a basic knowledge of Earth materials is essential to understanding most geologic phenomena.

Every rock contains clues about the environment in which it formed. For example, some rocks are composed entirely of small shell fragments of marine organisms, which tell Earth scientists that the rock likely originated in a shallow marine environment. Other rocks contain clues indicating that they formed from a volcanic eruption or deep in Earth during mountain building. Thus, rocks contain a wealth of information about events that have occurred over Earth’s long history.