Chapter 20 Overview

In the previous chapter you learned about rocks and minerals of the earth's surface. What is below the rocks and minerals that you see on the surface? What is deep inside the earth? What you can see--the rocks, minerals, and soil on the surface--is a thin veneer. Nothing has ever been directly observed below this veneer, however (Figure 20.1). The deepest mine has penetrated to depths of about 3 km (about 2 miles), and the deepest oil wells may penetrate down to about 8 km (about 5 miles). But the earth has a radius of about 6,370 km (about 3,960 miles). How far have the mines and wells penetrated into the earth? By way of analogy, consider the radius of the earth to be the length of a football field, from one goal line to the other. The deep mine represents progress of 4.3 cm (1.7 in) from one goal line. The deep oil well represents progress of about 11.5 cm (about 4.5 in). It should be obvious that human efforts have only scratched the surface, sampling materials directly beneath the surface. What is known about the earth's interior was learned indirectly, from measurements of earthquake waves, how heat moves through rocks, and the earth's magnetic field.

Indirect evidence suggests that the earth is divided into three main parts--the crust on the surface, a rocky mantle beneath the crust, and a metallic core. The crust and the uppermost mantle can be classified on a different basis, as a rigid layer made up of the crust and part of the upper mantle, and as a plastic, movable layer below in the upper mantle.

Understanding that the earth has a rigid upper layer on top of a plastic, movable layer is important in understanding the concepts of plate tectonics. Plate tectonics describes how the continents and the seafloor are moving on giant, rigid plates over the plastic layer below. This movement can be measured directly. In some places, the movement of a continent is about as fast as your fingernail grows, but movement does occur.

Understanding that the earth's surface is made up of moving plates is important in understanding a number of earth phenomena. These include earthquakes, volcanoes, why deep sea trenches exist where they do, and why mountains exist where they do. This chapter is the "whole earth" chapter, describing all of the earth's interior and the theory of plate tectonics. A bit of indirect information about the earth's interior, a theory of plate tectonics, and the observation of a number of related earth phenomena all fit together. You can use this concept to explain many things that happen on the surface of the earth.