432 PART 4^ |^ THE SOLAR SYSTEM
Earth’s magnetic fi eld produces the dramatic and beautiful
auroras, glowing rays and curtains of light in the upper atmo-
sphere (Figure 20-8b). Th e solar wind carries charged particles
past Earth’s extended magnetic fi eld, and this generates tremen-
dous electrical currents that fl ow into Earth’s atmosphere near
the north and south magnetic poles (see Chapter 8). Th e currents
ionize gas atoms in Earth’s atmosphere, and when the ionized
atoms capture electrons and recombine, they produce an emis-
sion spectrum as if they were part of a vast “neon” sign (see
Chapter 7).
Although you can be confi dent that Earth’s magnetic fi eld is
generated within its molten core, many mysteries remain. For
example, rocks retain traces of the magnetic fi eld in which they
solidify, and some contain fi elds that point backward. Th at is,
they imply that Earth’s magnetic fi eld was reversed at the time
they solidifi ed. Careful analysis of such rocks indicates that
Earth’s fi eld has reversed itself in a very irregular pattern, on aver-
age every 700,000 years or so during the past 30 million years,
with the north magnetic pole becoming the south magnetic pole
and vice versa. Th e reversals are poorly understood, but they may
be related to changes in the core’s convection.
Convection in Earth’s core is important because it generates
the magnetic fi eld. As you will see in the next section, convection
in the mantle constantly remakes Earth’s surface.
Earth’s Active Crust
Earth’s crust is composed of low-density rock that fl oats on the
mantle. Th e image of a rock fl oating may seem odd, but recall
that the rock of the mantle is very dense. Also, just below the
crust, the mantle rock tends to be highly plastic, so great sections
of low-density crust do indeed fl oat on the semi-liquid mantle
like great lily pads fl oating on a pond.
Th e motion of the crust and the erosive action of water
make Earth’s crust highly active. Read The Active Earth on
pages 434–435 and notice three important points and six new
terms:
Plate tectonics, the motion of crustal plates, produces much
of the geological activity on Earth. Plates spreading apart can
form rift valleys, or, on the ocean fl oor, midocean rises where
molten rock solidifi es to form basalt. A plate sliding into a
subduction zone can trigger volcanism, and the collision of
plates can produce folded mountain ranges. Chains of volca-
noes such as the Hawaiian islands can result when a plate
moves horizontally across a hot spot.
Notice how the continents on Earth’s surface have moved
and changed over periods of hundreds of millions of years.
A hundred million years is only 0.1 billion years, so
1
2
■ Figure 20-8
(a) Earth’s magnetic field dominates space around Earth by deflecting
the solar wind and trapping high-energy particles in radiation belts. The
magnetic fi eld lines enter Earth’s atmosphere around the north and south
magnetic poles. (b) Powerful currents fl ow down along the magnetic fi eld
lines near the poles and excite gas atoms to emit photons, creating auroras.
Colors are produced as different atoms are excited. Note the meteor (shoot-
ing star). (Jimmy Westlake)
Solar
wind
a
Bo
w
sh
oc
k
Radiation belts
b
Rays follow Earth’s
magnetic field.
Dark clouds
silhouetted against the
much higher aurora
Magnetosphere: the
region controlled by the
planet’s magnetic field