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OVERVIEW

In chapter 17, you learned about rocks and minerals of Earth’s surface. What is below the rocks and minerals
that you see on the surface? What is deep inside 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 18.1). The
deepest mine has penetrated to depths of about 3 km (about 2 mi), and the deepest oil wells may penetrate down
to about 8 km (about 5 mi). But Earth has a radius of about 6,370 km (about 3,960 mi). How far have the mines
and wells penetrated into Earth? By way of analogy, consider the radius of 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
sampled materials directly beneath the surface. What is known about Earth’s interior was learned indirectly, from
measurements of earthquake waves, how heat moves through rocks, and Earth’s magnetic field.
Indirect evidence suggests that 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 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 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 Earth’s interior and the theory of plate
tectonics. A bit of indirect information about 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 Earth.

bardment was substantial as well as lengthy, continuing for
18.1 HISTORY OF EARTH’S INTERIOR
several hundred million years. Calculations of the heating result-
Many of the properties and characteristics of Earth, including ing from this tremendous bombardment indicate that sufficient
the structure of its interior, can be explained from current the- heat was liberated to melt the entire surface of Earth to a layer
ories of how it formed and evolved. Theories and ideas about of glowing, molten lava. Thus, the early Earth had a surface of
how Earth and the rest of the solar system formed were dis- molten lava that eventually cooled and crystallized to solid igne-
cussed in detail in chapter 15. Here is the theoretical summary ous rocks as the bombardment gradually subsided, then stopped.
of how Earth’s interior was formed, discussed as if it were a fact. Then Earth began to undergo a second melting, this time
Keep in mind, however, that the following is all conjecture, even from the inside. The interior slowly accumulated heat from the
if it is conjecture based on facts. radioactive decay of uranium, thorium, and other radioactive
In brief, Earth is considered to have formed about 4.6 bil- isotopes (see chapter 13). Heat conducts slowly through great
lion years ago in a rotating disk of particles and grains that had thicknesses of rock and rock materials. After about 100 million
condensed around a central protosun. The condensed rock, years or so of accumulating heat, parts of the interior became
iron, and mineral grains were pulled together by gravity, grow- hot enough to melt to pockets of magma. Iron and other metals
ing eventually to a planet-sized mass. Not all the bits and pieces were pulled from the magma toward the center of Earth, leav-
of matter in the original solar nebula were incorporated into the ing less dense rocks toward the surface. The melting probably
newly formed planets. They were soon being pulled by gravity did not occur all at one time throughout the interior but rather
to the newly born planets and their satellites. All sizes of these in local pockets of magma. Each magma pocket became mol-
leftover bits and pieces of matter thus began bombarding the ten, cooled to a solid, and perhaps repeated the cycle numerous
planets and their moons. Evidently, the bombardment was so times. With each cyclic melting, the heavier abundant elements
intense that the heat generated by impact after impact increased were pulled by gravity toward the center of Earth, and addi-
the surface temperature to the melting point. Evidence visible tional heat was generated by the release of gravitational energy.
on the Moon and other planets today indicates that the bom- Today, Earth’s interior still contains an outer core of molten

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