CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 433
sections of Earth’s crust are in rapid motion relative to the
4.6 billion-year age of the planet.
Most of the geological features you know—mountain ranges,
the Grand Canyon, and even the familiar outline of the
continents—are recent products of Earth’s active surface.
Earth’s surface is constantly renewed. Th e oldest rocks on
Earth, small crystals of the mineral zircon from western
Australia, are 4.4 billion years old. Most of the crust is much
younger than that. Most of the mountains and valleys you
see around you are no more than a few tens of millions of
years old.
Th e average speed of plate movement is slow, but sudden
movements do occur. Plate margins can stick, accumulate stress,
and then release it suddenly. For example, that’s what happened
in 2004 along a major subduction zone in the Indian Ocean. Th e
total motion was as much as 15 meters, and the resulting earth-
quake caused devastating tsunamis (tidal waves). Every day,
minor earthquakes occur on moving faults, and the stress that
builds in those faults that are sticking will eventually be released
in major earthquakes.
Earth’s active crust explains why Earth contains so few
impact craters. Th e moon is richly cratered, but Earth’s surface
has only about 150 impact craters. Plate tectonics and erosion
have destroyed all but the most recent craters on Earth.
You can see that Earth’s geology is dominated by two dra-
matic forces. Heat rising from the interior drives plate tectonics.
Just below the thin crust of solid rock lies a churning molten
layer that rips the crust to fragments and pushes the pieces about
like rafts of wood on a pond. Th e second force modifying the
crust is water. It falls as rain and snow and tears down mountains,
erodes river valleys, and washes any raised ground into the sea.
Tectonics builds up mountains and continents, and then erosion
rips them down.
3
Earth’s Atmosphere
You can’t tell the story of Earth without mentioning its
atmosphere. Not only is it necessary for life, but it is also inti-
mately related to the crust. It aff ects the surface through erosion
by wind and water, and in turn the chemistry of Earth’s surface
aff ects the composition of the atmosphere.Origin of the Atmosphere
Until a few decades ago, planetary scientists thought the early
Earth might have attracted small amounts of gases such as hydro-
gen, helium, and methane from the solar nebula to form a
primeval atmosphere. According to that old hypothesis, slow
decay of radioactive elements eventually heated Earth’s interior;
melted it; caused it to diff erentiate into core, mantle, and crust;
and triggered widespread volcanism.
When a volcano erupts, 50 to 80 percent of the gas released
is water vapor. Th e rest is mostly carbon dioxide, nitrogen, and
smaller amounts of sulfur gases such as hydrogen sulfi de—the
rotten-egg gas that you smell if you visit geothermal pools and
geysers such as those at Yellowstone National Park. Th ese gases
could have diluted the primeval atmosphere and eventually pro-
duced a secondary atmosphere rich in carbon dioxide, nitrogen,
and water vapor.
In contrast, a modern understanding of planet building
shows that Earth formed so rapidly that it was substantially
heated by the impacts of infalling material, as well as by radioac-
tive decay. If Earth’s surface was molten as it formed, then out-
gassing would have been continuous, and the early atmosphere
would have been rich in carbon dioxide, nitrogen, and water
vapor from the beginning. In other words, planetary scientists
now think Earth went straight to the volcanic “secondary atmo-
sphere” and never had a hydrogen-rich primeval atmosphere. You
will fi nd in Chapter 26 that the lack of hydrogen in Earth’s origi-
nal atmosphere has important implications for how life began on
our planet.
Astronomers also have suspected that some of the abundant
water on Earth arrived late in the formation process as a bom-
bardment of volatile-rich planetesimals. Th ese icy bodies, the
theory goes, were scattered by the growing mass of the outer
planets and by the outward migration of Saturn, Uranus, and
Neptune. Th e inner solar system, including Earth, would then
have been bombarded by a storm of comets, some of which
could have supplied some or all of Earth’s water. Th at hypothesis
once faced a serious objection. Spectroscopic studies of a few
comets revealed that the ratio of deuterium to hydrogen in com-
ets does not match the ratio in the water on Earth. Some astron-
omers thought this meant that the water now on Earth could not
have arrived in cometlike planetesimals. However, studies of
Comet LINEAR, which broke up in 1999 as it passed near the
sun, show that the water in that comet had a ratio of isotopes20-4
SCIENTIFIC ARGUMENT
What evidence indicates that Earth has a liquid metal core?
A good scientific argument focuses on evidence. In this case,
the evidence is indirect because you can never visit Earth’s core.
Seismic waves from distant earthquakes pass through Earth, but a
certain kind of wave, the S type, does not pass through the core.
Because the S waves cannot move through a liquid, scientists con-
clude that Earth’s core is partly liquid. Earth’s magnetic fi eld is
further evidence of a liquid metallic core. The theory for the gen-
eration of magnetic fi elds, the dynamo effect, requires a moving,
conducting liquid (for a planet) or gas (for a star) in the interior. If
Earth’s core were not partly a liquid metal, it would not be able to
generate a magnetic fi eld.
Two different kinds of evidence tell you that our planet has a
liquid core. Can you build a new argument? What evidence can
you cite to support the theory of plate tectonics?