CHAPTER 1 An Introduction to Geology
22
that iron and nickel began to melt. Melting
produced liquid blobs of heavy metal that
sank toward the center of the planet. This
process occurred rapidly on the scale of
geologic time and produced Earth’ s dense
iron-rich core.
The early period of heating resulted in
another process of chemical differentiation,
whereby melting formed buoyant masses of
molten rock that rose toward the surface,
where they solidified to produce a primitive
crust. These rocky materials were enriched
in oxygen and “oxygen-seeking” elements,
particularly silicon and aluminum, along
with lesser amounts of calcium, sodium,
potassium, iron, and magnesium. In addition, some heavy metals such as gold, lead,
and uranium, which have low melting
points or were highly soluble in the ascending molten masses, were scavenged from
Earth’ s interior and concentrated in the
developing crust. This early period of chemical segregation established the three basic
divisions of Earth’ s interior—the iron-rich
core; the thin primitive crust; and Earth’ s
largest layer, called the mantle, which is
located between the core and crust.
An important consequence of this early
period of chemical differentiation is that
large quantities of gaseous materials were
allowed to escape from Earth’ s interior, as
happens today during volcanic eruptions.
By this process a primitive atmosphere
gradually evolved. It is on this planet, with
this atmosphere, that life as we know it
came into existence.
Following the events that established
Earth’ s basic structure, the primitive crust
was lost to erosion and other geologic
processes, so we have no direct record of its
makeup. When and exactly how the continental crust—and thus Earth’ s first landmasses—came into existence is a matter of
ongoing research. Nevertheless, there is
D I D Y O U K N O W ?
The light-year is a unit for measuring
distances to stars. Such distances are so
large that familiar units such as
kilometers or miles are cumbersome to
use. One light-year is the distance light
travels in one Earth year—about 9.5
trillion km (5.8 trillion mi)!
with high melting points to condense into tiny particles that began to coalesce (join
together). Materials such as iron and nickel and the elements of which the rock-forming
minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and
rocky clumps that orbited the Sun (Figure 1.23). Repeated collisions caused these masses to
coalesce into larger asteroid-size bodies, called planetesimals, which in a few tens of millions
of years accreted into the four inner planets we call Mercury, Venus, Earth, and Mars
(FIGURE 1.25). Not all of these clumps of matter were incorporated into the planetesimals.
Those rocky and metallic pieces that remained in orbit are called meteorites when they
survive an impact with Earth.
As more and more material was
swept up by these growing planetary
bodies, the high-velocity impact of
nebular debris caused their temperature to rise. Because of their relatively
high temperatures and weak gravitational fields, the inner planets were
unable to accumulate much of the
lighter components of the nebular
cloud. The lightest of these, hydrogen and helium, were eventually
whisked from the inner solar system
by the solar winds.
At the same time that the inner
planets were forming, the larger,
outer planets (Jupiter, Saturn,
Uranus, and Neptune), along with
their extensive satellite systems, were
also developing (Figure 1.25).
Because of low temperatures far from
the Sun, the material from which
these planets formed contained a
high percentage of ices—water,
carbon dioxide, ammonia, and
methane—as well as rocky and
metallic debris. The accumulation of
ices accounts in part for the large size
and low density of the outer planets.
The two most massive planets,
Jupiter and Saturn, had a surface
gravity sufficient to attract and hold
large quantities of even the lightest
elements—hydrogen and helium.
Formation of Earth’ s
Layered Structure
As material accumulated to form
Earth (and for a short period afterward), the high-velocity impact of
nebular debris and the decay of
radioactive elements caused the
temperature of our planet to steadily
increase. During this time of intense
heating, Earth became hot enough
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Sun
FIGURE 1.25 The planets drawn to scale.
22
that iron and nickel began to melt. Melting
produced liquid blobs of heavy metal that
sank toward the center of the planet. This
process occurred rapidly on the scale of
geologic time and produced Earth’ s dense
iron-rich core.
The early period of heating resulted in
another process of chemical differentiation,
whereby melting formed buoyant masses of
molten rock that rose toward the surface,
where they solidified to produce a primitive
crust. These rocky materials were enriched
in oxygen and “oxygen-seeking” elements,
particularly silicon and aluminum, along
with lesser amounts of calcium, sodium,
potassium, iron, and magnesium. In addition, some heavy metals such as gold, lead,
and uranium, which have low melting
points or were highly soluble in the ascending molten masses, were scavenged from
Earth’ s interior and concentrated in the
developing crust. This early period of chemical segregation established the three basic
divisions of Earth’ s interior—the iron-rich
core; the thin primitive crust; and Earth’ s
largest layer, called the mantle, which is
located between the core and crust.
An important consequence of this early
period of chemical differentiation is that
large quantities of gaseous materials were
allowed to escape from Earth’ s interior, as
happens today during volcanic eruptions.
By this process a primitive atmosphere
gradually evolved. It is on this planet, with
this atmosphere, that life as we know it
came into existence.
Following the events that established
Earth’ s basic structure, the primitive crust
was lost to erosion and other geologic
processes, so we have no direct record of its
makeup. When and exactly how the continental crust—and thus Earth’ s first landmasses—came into existence is a matter of
ongoing research. Nevertheless, there is
D I D Y O U K N O W ?
The light-year is a unit for measuring
distances to stars. Such distances are so
large that familiar units such as
kilometers or miles are cumbersome to
use. One light-year is the distance light
travels in one Earth year—about 9.5
trillion km (5.8 trillion mi)!
with high melting points to condense into tiny particles that began to coalesce (join
together). Materials such as iron and nickel and the elements of which the rock-forming
minerals are composed—silicon, calcium, sodium, and so forth—formed metallic and
rocky clumps that orbited the Sun (Figure 1.23). Repeated collisions caused these masses to
coalesce into larger asteroid-size bodies, called planetesimals, which in a few tens of millions
of years accreted into the four inner planets we call Mercury, Venus, Earth, and Mars
(FIGURE 1.25). Not all of these clumps of matter were incorporated into the planetesimals.
Those rocky and metallic pieces that remained in orbit are called meteorites when they
survive an impact with Earth.
As more and more material was
swept up by these growing planetary
bodies, the high-velocity impact of
nebular debris caused their temperature to rise. Because of their relatively
high temperatures and weak gravitational fields, the inner planets were
unable to accumulate much of the
lighter components of the nebular
cloud. The lightest of these, hydrogen and helium, were eventually
whisked from the inner solar system
by the solar winds.
At the same time that the inner
planets were forming, the larger,
outer planets (Jupiter, Saturn,
Uranus, and Neptune), along with
their extensive satellite systems, were
also developing (Figure 1.25).
Because of low temperatures far from
the Sun, the material from which
these planets formed contained a
high percentage of ices—water,
carbon dioxide, ammonia, and
methane—as well as rocky and
metallic debris. The accumulation of
ices accounts in part for the large size
and low density of the outer planets.
The two most massive planets,
Jupiter and Saturn, had a surface
gravity sufficient to attract and hold
large quantities of even the lightest
elements—hydrogen and helium.
Formation of Earth’ s
Layered Structure
As material accumulated to form
Earth (and for a short period afterward), the high-velocity impact of
nebular debris and the decay of
radioactive elements caused the
temperature of our planet to steadily
increase. During this time of intense
heating, Earth became hot enough
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Sun
FIGURE 1.25 The planets drawn to scale.