14: Rocks and Geological Time
- Page ID
- 55126
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- 14.1: The Three Rock Types
- Rocks are classified into three major groups—igneous, sedimentary, and metamorphic—based on how they form. This section examines the processes that produce each rock type and the characteristics geologists use to identify and classify them. It also introduces sedimentary structures and fossils, which preserve important evidence about the environments and conditions that existed when rocks formed.
- 14.2: The Rock Cycle
- The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.
- 14.3: Reading Earth's History
- Rocks preserve a record of events that have occurred throughout Earth’s long history. This section introduces the geological time scale and examines how geologists determine the sequence and ages of past events. Relative dating methods establish whether rocks and events are older or younger than one another, while isotopic dating provides numerical ages. Together, these methods allow geologists to reconstruct Earth’s history across billions of years.
Chapter 14: Rocks and Geological Time — Introduction
In the previous chapter, we examined minerals—the basic building blocks of rocks. We now take the next step by looking at the rocks themselves and the processes that continually create, alter, and recycle Earth materials. Rocks are more than collections of minerals; their composition, texture, and structure preserve evidence about the conditions under which they formed.
We begin with the three major rock types: igneous, sedimentary, and metamorphic. Each forms through different geological processes, but these categories are not permanent. Through melting, cooling, weathering, erosion, burial, heat, and pressure, one type of rock can eventually become another. The rock cycle connects these processes and shows us that Earth's surface and interior are parts of a continuously changing system.
Rocks also provide one of our most important records of the past. Their relationships, structures, fossils, and radioactive isotopes allow geologists to determine the sequence of geological events and estimate when those events occurred. We will therefore finish the chapter by learning how to read Earth's history from rocks, moving from relative dating to numerical ages and the geological time scale. The enormous spans of time involved can be difficult to visualize, but they are essential for understanding the dynamic Earth we will explore in the final chapter.