The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 429

The Solid Earth

Although you might think of earth as solid rock, it is in fact
neither entirely solid nor entirely rock. Th e thin crust seems
solid, but it fl oats and shifts on a semiliquid layer of molten rock
just below the crust. Below that lies a deep, rocky mantle sur-
rounding a core of liquid metal. Much of what you see on Earth’s
surface is determined by conditions and processes in its interior.

Earth’s Interior

Th e theory of the origin of planets from the solar nebula predicts
that Earth should have melted and diff erentiated into a dense
metallic core and a dense mantle with a low-density silicate crust.
But did it? Where’s the evidence? Earth’s average density can be
calculated easily from its known mass and density. Clearly, the
silicate rocks on Earth’s surface have lower density than material
inside the planet. But, what more can be determined about
Earth’s interior?
High temperature and tremendous pressure in Earth’s inte-
rior make any direct exploration impossible. Even the deepest oil
wells extend only a few kilometers down and don’t reach through
the crust. It is impossible to drill far enough to sample Earth’s
core. Yet Earth scientists have studied the interior and found
clear evidence that Earth did diff erentiate (How Do We
Know? 20-2).
Th is exploration of Earth’s interior is possible because earth-
quakes produce vibrations called seismic waves that travel
through the crust and interior and eventually register on sensitive
detectors called seismographs all over the world (■ Figure 20-3).
Two kinds of seismic waves are important to this discussion. Th e
pressure (P) waves are much like sound waves in that they travel
as a sequence of compressions and decompressions. As a P wave
passes, particles of matter vibrate back and forth parallel to the
direction of wave travel (■ Figure 20-4a). In contrast, the shear
(S) waves move as displacements of particles perpendicular to

20-3

the waves’ direction of travel (Figure 20-4b). Th at means that S

waves distort the material but do not compress it. Normal sound

waves are pressure waves, whereas the water waves you can surf

on, and vibrations you see in a bowl of jelly, are shear waves.

Because P waves are compression waves, they can move through

a liquid. S waves can move along the surface of a

liquid, but not through it. A glass of water can’t

shimmy like jelly because a liquid does not have the

rigidity required to transmit S waves.

Th e P and S waves caused by an earthquake do

not travel in straight lines or at constant speed within

Earth. Th e waves may refl ect off boundaries between

layers of diff erent density, or they may be refracted as

they pass through a boundary. In addition, the

gradual increase in temperature and density toward

Earth’s center means the speed of sound increases as

■ Figure 20-4

(a) P, or pressure, waves, like sound waves in air, travel as a region of

compression. (b) S, or shear, waves, like vibrations in a bowl of jelly, travel

as displacements perpendicular to the direction of travel. S waves tend to

travel more slowly than P waves and cannot travel through liquids.

0

Earthquake

occurs in

Mexico

510

Time (min)

15 20 25

First

P waves

arrive

First

S waves

arrive

■ Figure 20-3

A seismograph in northern Canada made this record of seismic waves from an
earthquake in Mexico. The fi rst vibrations, P waves, arrived 11 minutes after
the quake, but the slower S waves took 20 minutes to make the journey.

a

b