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Particle motion

Wave propagation

FIGURE 18.1 This drilling ship samples sediment and rock
from the deep ocean floor. It can only sample materials well within A
the upper crust of Earth, however, barely scratching the surface of
Earth’s interior.
Particle motion
material that is predominantly iron. The environment of the
center of Earth today is extreme, with estimates of pressures up
to 3.5 million atmospheres (3.5 million times the pressure of
the atmosphere at the surface). Recent estimates of the tempera- Wave propagation
tures at Earth’s core are about the same as the temperature of the
surface of the Sun, about 6,000°C (11,000°F).
The melting and flowing of iron to Earth’s center were the
beginnings of differentiation, the separation of materials that
gave Earth its present-day stratified or layered interior. The
different crystallization temperatures of the basic minerals, as B
illustrated in Bowen’s reaction series, further differentiated the
materials in Earth’s interior. FIGURE 18.2 (A) A P-wave is illustrated by a sudden push on a
stretched spring. The pushed-together section (compression) moves
in the direction of the wave movement, left to right in the example.
18.2 EARTH’S INTERNAL STRUCTURE (B) An S-wave is illustrated by a sudden shake of a stretched rope.
The looped section (sideways) moves perpendicular to the direction
The theoretical formation of Earth and the layered structure of of wave movement, again left to right in the illustration.
its interior are supported by indirect evidence from measure-
ments of vibrations in Earth, Earth’s magnetic field, gravity, and
heat flow. First, we will consider how vibrations tell us about liquids do not have the cohesion necessary to transmit a
Earth’s interior. shear, or side-to-side, motion.
If you have ever felt vibrations in Earth from a passing 3. Up-and-down (crest and trough) and side-to-side waves
train, an explosion, or an earthquake, you know that Earth can that travel across the surface are called surface waves that are
vibrate. In fact, a large disturbance such as a nuclear explosion much like waves on water that move across the solid surface
or really big earthquake can generate waves that pass through of Earth. Surface waves are the slowest and occur where

the entire Earth. A vibration that moves through any part of S- or P-waves reach the surface. There are two important
Earth is called a seismic wave. Geologists use seismic waves to types of surface waves: Love waves and Rayleigh waves.
learn about Earth’s interior. Love waves are horizontal S waves that move from side to
Seismic waves radiate outward from an earthquake, spread- side. This motion knocks buildings off their foundations

ing in all directions through the solid Earth’s interior as do sound and can destroy bridges and overpasses. Rayleigh waves
waves from an explosion. There are basically three kinds of waves: are more like rolling water waves. Rayleigh waves are more
destructive because they produce more up, down, and
1. A longitudinal (compressional) wave called a P-wave
sideways ground movement for a longer time.
(Figure 18.2A). P-waves are the fastest and move through
surface rocks and solid and liquid materials below the Using data from seismic waves, scientists were able to
surface. Th e P stands for primary. determine that the interior of Earth can be broken down into
2. A transverse (shear) wave called an S-wave (Figure 18.2B). three zones (Figure 18.3). The crust is the outer layer of rock
Th e S stands for secondary. S-waves are second fastest aft er that forms a thin shell around Earth. Below the crust is the mantle,
the P-waves. S-waves do not travel through liquids because a much thicker shell than the crust. The mantle separates the

18-3 CHAPTER 18 Plate Tectonics 457