Geoscience Reference
In-Depth Information
chemically reacted as gases in the solar nebula to form
high-molecular-weight compounds that were condens-
able, although less condensable (more volatile) than
rock-forming elements. When meteorites and asteroids
collided with the Earth, they brought with them volatile
compounds and rock-forming elements. Whereas some
of the volatiles vaporized on impact, others have taken
longer to vaporize and have been outgassed ever since
through volcanos, fumaroles, steam wells, and geysers.
Earth's first atmosphere likely contained primarily
hydrogen (H) and helium (He), the most abundant ele-
ments in the solar nebula. During the formation of the
Earth, the sun was also forming. Early stars are known
to blast off a large amount of gas into space. This out-
gassed solar material, the
solar wind
,waspreviously
introduced as an extension of the sun's corona. During
the birth of the sun, nuclear reactions in the sun that
fuse hydrogen to helium were enhanced. The resulting
blast increased solar wind speeds and densities to much
higher values than today. This early stage of the sun is
called the
T- Tauri stage
after the first star observed at
this point in its evolution. The enhanced solar wind is
believed to have stripped away the first atmosphere not
only of the Earth, but also of all other planets in the solar
system. Additional H and He were lost from the Earth's
first atmosphere after escaping the Earth's gravitational
field. As a result of these two loss processes (solar wind
stripping and gravitational escape), the ratios of H and
He to other elements in the Earth's atmosphere today
are less than are the corresponding ratios in the sun.
During the formation of the Earth's core, between
4.6 and 4.0 b.y.a., core temperatures were higher than
they are today, and the only mechanism of heat escape
to the surface was
conduction
,which is the transfer of
energy from molecule to molecule. Because conduction
is a slow process, the Earth's internal energy could not
transfer to the surface and dissipate easily, so its tem-
perature increased until the entire body became molten.
In this state, the Earth's surface consisted of
magma
oceans
,ahot mixture of melted rock and suspended
crystals. When the Earth was molten,
convection
,the
mass movement of molecules, became the predominant
form of vertical energy transfer from the core to the
surface. Convection occurred because temperatures in
the core were hot enough for core material to expand
and float to the crust, where it cooled and sank down
again. This process enhanced energy dissipation from
the Earth's core to space. After sufficient energy dissi-
pation (cooling), the magma oceans solidified, creating
the Earth's crust. The crust is estimated to have formed
3.8 to 4.0 b.y.a., but possibly as early as 4.2 to 4.3 b.y.a.
(Crowley and North, 1991). The core cooled as well,
but its outer part, the
outer core
,remains molten. Its
inner part, the
inner core
,issolid.
Figure 2.7 shows temperature, density, and pres-
sure profiles inside the Earth today. The Earth's crust
extends from the topographical surface to about 10 to
75 km below continents and 8 km below the ocean
floor. The crust itself contains low-density, low-melting-
point silicates. The continental crust contains primar-
ily granite, whereas the ocean crust contains primarily
basalt.
Granite
is a type of rock composed mainly of
quartz [SiO
2
(s)] and potassium feldspar [KAlSi
3
O
8
(s)].
Basalt
is a type of rock composed primarily of plagio-
clase feldspar [NaAlSi
3
O
3
-CaAl
2
Si
2
O
8
(s)] and pyrox-
ene (multiple compositions). The densities of both gran-
ite and basalt are about 2,800 kg m
−
3
.
Below the Earth's crust is its mantle, which con-
sists of an upper and lower part, both made of iron-
magnesium-silicate minerals. The upper mantle extends
from the crust down to about 700 km. At that depth,
a density gradation occurs due to a change in crystal
packing. This gradation roughly defines the base of the
upper mantle and the top of the lower mantle. Below
700 km, the density gradually increases to the mantle
core boundary at 2,900 km.
The outer core extends from 2,900 km down to
about 5,100 km. This region consists of liquid iron and
nickel, although the top few hundred kilometers con-
tain liquids and crystals. The inner core extends from
5,100 km down to the Earth's center and is solid, also
consisting of iron and nickel, but packed at a higher
2.3.1. Solid Earth Formation
The rock-forming elements that reached the Earth
reacted to form compounds, each with different melting
points, densities, and chemical reactivities. Dense and
high melting point compounds, including many iron-
and nickel-containing compounds, settled to the center
of the Earth, called the
Earth's core
.Table 2.2 shows
that the total Earth today contains more than 34 per-
cent iron and 2 percent nickel by mass, but the Earth's
crust
(its top layer) contains less than 7 percent iron and
0.1 percent nickel by mass, supporting the contention
that iron and nickel settled to the core. Low-density
compounds and compounds with low melting points,
including silicates of aluminum, sodium, and calcium,
rose to the surface and are the most common compounds
in the Earth's crust. Table 2.2 supports this hypothesis.
Some moderately dense and moderately high-melting-
point silicates, such as those containing magnesium or
iron, settled to the Earth's
mantle
,which is a layer of
Earth's interior between its crust and its core.
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