3.1: Introduction

This chapter reviews the major concepts of the structure of the Earth and describes the dynamic processes associated the formation of distinct layers within the earth. This is essential information to understand plate tectonics (discussed in Chapter 4). Plate Tectonics Theory explains how the Earth’s surface is broken up into large plates of rock whose slow movements create earthquakes, volcanoes, and mountains, changing the way the Earth looks over geologic time.

While much has been discovered about the character and natural resources of our planet since the time of Christopher Columbus's first voyage, little was know about the internal character of the Earth until the Cold War era following World War II. Although studies of the internal structure of the earth were first reported in the late 19th century using seismic wave data from great earthquakes, it was the data from testing, spying, and verification of underground nuclear explosions that provided a clearer, more detailed picture of the internal structure of our planet. The earth is composed of several zones, including a central core, a mantle, and a crust (Figure 3.4 and 3-5). Oceans (hydrosphere) and atmosphere rest on the surface of the crust. All parts are held together and have their character based on the force of gravity, their chemical composition, and largely how they formed and changed through geologic time. These same factors apply to other planets and moons as well. The solid earth has a central core (both solid & liquid), a mantle (mostly solid though capable of slow flow by heat convection), and the crust (solid).

The appearance of the world as we see it today is a result of the accumulative effects of all geologic processes that have happened in the past. Some of these processes occur rapidly (such as volcanic eruptions, earthquakes, great storms and flood, and occasional asteroid impacts). However, most features we see on the landscape or in a region (or larger features like continents) involve processes that are far grander, operating both near and deep below the surface, and taking place gradually over long periods of time (in spans measured in millions to hundreds-of-millions of years). For instance, the coast lines of northwest Africa and the eastern United States are currently moving apart at a rate of about 2-4 inches a year. However, about 200 million years ago the two continents were joined together before the opening and formation of the Atlantic Ocean basin! Plate tectonics theory helps explain most of the processes and grand landscape features we observe around the world today, both on land and beneath the oceans.

Figure 3.1. Nearly all geologic processes observed on Earth fit in some way into Plate Tectonics Theory.	Figure 3.2. A simplified model of plate tectonics showing types of lithospheric plate boundaries.