22.1  Earth's Structure (continued)

The Science of Geology

  Geology is the study of planet Earth, including its composition and structure. Scientists who study Earth and the processes that have shaped Earth over time are called geologists. Geologists such as the one shown in Figure 2 divide the forces that change Earth's surface into two groups: constructive forces and destructive forces. Constructive forces shape the surface by building up mountains and other land areas. Destructive forces slowly wear away mountains and, eventually, every other feature on Earth's surface.

Figure 1  These workers are drilling a hole that will be more than two kilometers deep. Later they will lower instruments into the hole to record data at that depth.

The modern science of geology began in the late 1700s. James Hutton, a Scottish physician and farmer, made observations of rocks that he could explain only if Earth were far older than most people had imagined. He observed that some rocks were made of particles that came from older rocks. Hutton realized that Earth's surface had changed gradually over time.

Figure 2  This geologist is examining the rocks that make up the side of a canyon.

From his observations, Hutton developed the principle of uniformitarianism. Uniformitarianism is the idea that the geologic processes that operate today also operated in the past. Thus, ancient rocks can be understood by observing present-day geologic processes. According to the principle of uniformitarianism, dramatic features such as mountains and canyons are the result of geologic processes that work very slowly over long periods of time.