The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 431

which in turn raises the melting point so high that the material
cannot melt (■ Figure 20-6). Th at is why there is an inner core of
solid iron and nickel. Estimates suggest the inner core’s radius is
about 22 percent that of Earth.
Th e paths of seismic waves in the mantle, the layer of dense
rock that lies between the molten core and the crust, show that
it is not molten, but it is not precisely solid either. Mantle mate-
rial behaves like a plastic, a material with the properties of a solid
but capable of fl owing under pressure. Th e asphalt used in paving
roads is a common example of a plastic. It shatters if struck with
a sledgehammer, but it bends under the steady weight of a heavy
truck. Just below Earth’s crust, where the pressure is less than at
greater depths, the mantle is most plastic.
Earth’s rocky crust is made up of low-density rocks and fl oats
on the denser mantle. Th e crust is thickest under the continents,
up to 60 km thick, and thinnest under the oceans, where it is
only about 10 km thick. Unlike the mantle, the crust is brittle
and can break when it is stressed.
Although Earth’s core is only 2000 miles from you, it is
completely inaccessible. Earth’s seismic activity reveals some of
Earth’s innermost secrets. But there is another source of evidence
about Earth’s interior—its magnetic fi eld.

The Magnetic Field

Apparently, Earth’s magnetic fi eld is a direct result of its rapid
rotation and its molten metallic core. Internal heat forces the
liquid core to circulate with convection while Earth’s rotation
turns it about an axis. Th e core is a highly conductive iron–nickel

alloy, an even better electrical conduc-

tor than copper, the material com-

monly used for electrical wiring. Th e

rotation of this convecting, conducting

liquid generates Earth’s magnetic fi eld

in a process called the dynamo eff ect

(■ Figure 20-7). Th at is the same pro-

cess that generates the solar magnetic

fi eld in the convective layers of the sun

(see Chapter 8), and you will see it

again when you explore other planets.

Earth’s magnetic fi eld protects it

from the solar wind. Blowing outward

from the sun at about 400 km/s, the

solar wind consists of ionized gases car-

rying a small part of the sun’s magnetic fi eld. When the solar

wind encounters Earth’s magnetic fi eld, it is defl ected like water

fl owing around a boulder in a stream. Th e surface where the

solar wind is fi rst defl ected is called the bow shock, and the cav-

ity dominated by Earth’s magnetic fi eld is called the magneto-

sphere (■ Figure 20-8a). High-energy particles from the solar

wind leak into the magnetosphere and become trapped within

Earth’s magnetic fi eld to produce the Van Allen belts of radia-

tion. You will see in later chapters that all planets that have

magnetic fi elds have bow shocks, magnetospheres, and radiation

belts.

■ Figure 20-6

Theoretical models combined with observations of the velocity of seismic
waves reveal the temperature inside Earth (blue line). The melting point of
the material (red line) is determined by its composition and by the pressure.
In the mantle and in the inner core, the melting point is higher than the
existing temperature, and the material is not molten.

Center of Earth

T(

°C)

5000

4000

Melting point

Temperature

Solid mantle Liquid

core

Solid

core

3000

2000

1000

0

Depth (km)

1000 2000 3000 4000 5000 6000

Surface of Earth

■ Figure 20-7

The dynamo effect couples convection in the liquid core with Earth’s rota-

tion to produce electric currents that are believed to be responsible for

Earth’s magnetic fi eld.