Geoscience Reference
In-Depth Information
Figure 12.2 Distance to the cen-
ter of Earth.
The distance to the
center of Earth is about the width
of the North American continent
between Anchorage, Alaska, and
Miami, Florida.
Upper
mantle
Lower
mantle
Outer
core
Miami
Inner
core
Tallahassee
Anchorage
Prince Rupert
Laramie
1220 km
(760 mi)
420 km
(260 mi)
2250 km
(1398 mi)
2230 km
(1385 mi)
which is the approximate distance from Anchorage to
Prince Rupert, British Columbia (Figure 12.2). The inner
core is composed mostly of solid iron, along with a little
bit of nickel. The pressure in this part of Earth is enor-
mous, about 3 million times greater than the air pressure
you feel at the surface, due to the weight of the overlying
material comprising the rest of Earth. As a result, the in-
ner core is solid even though the temperature there is well
above the melting temperature of iron. Rocks simply can-
not melt under such high pressure. Geologists e stimate
that the temperature of the inner core ranges from about
3200°C to 5200°C (5800°F to 9400°F).
Surrounding the solid inner core is the
liquid outer
core
(Figure 12.1). The top of this layer lies about 2900 km
(1800 mi) below sea level. The liquid outer core is ap-
proximately 2250 km (1398 mi) thick, which is about the
same distance as from Prince Rupert, British Columbia, to
Laramie, Wyoming (Figure 12.2). In other words, it would
take two or three days to drive that distance at a reasonable
pace. The outer core is composed of the same material as
the inner core, except the iron is molten; that is, it is mostly
liquid. This variation occurs because the temperatures of the
outer core approach those of the inner core, hot enough to
melt solid rock. The difference between the two layers is
that the outer core has less internal pressure, which allows
rock to melt. The outer core is important because it generates
at least 90% of the Earth's magnetic field and the resulting
magnetosphere that protects Earth from the solar wind. The
magnetic field may exist because the Earth's inner core ro-
tates 19 km/year (12 mi/year), which is faster than the rest
of the planet. This rotational difference is thought to cause
circulation patterns in the outer core that generate electrical
currents. These currents, in turn, may generate the magnetic
field. Scientists are not certain of the exact processes that
lead to the Earth's magnetic field, but circulation of the outer
core is most likely involved.
Working out from the inner core, the next layer encoun-
tered is the
mantle
, which surrounds the core (Figure 12.1).
This part of Earth is composed largely of solid iron, magne-
sium, and silicon oxides and is divided into two parts: the
lower mantle
and
upper mantle
. The lower mantle is about
2230 km (1385 mi) thick—in other words, about the same
distance as from Laramie to Tallahassee, Florida (Figure
12.2). Again, this distance would take two or three days to
drive. Although temperatures in the mantle are still quite
hot, over 1300°C (2370°F), they are much cooler than in
the core. These cooler temperatures, coupled with intense
pressure, cause the lower mantle to be solid. This pressure
lessens gradually, along with temperature, into the upper
mantle.
The upper mantle is approximately 420 km (260 mi)
thick, which is about the same distance as from Tallahassee
to Miami, Florida. In contrast to the lower mantle, which is
solid, the upper mantle consists mostly of viscous nickel;
that is, it is a material like very thick syrup or a slowly flow-
ing plastic. The upper part of the upper mantle is called the
Liquid outer core
The outer part of the Earth's core. This
area is about 2250 km (1400 mi) thick and consists of molten
iron and nickel.
Mantle
The layer of the Earth's interior that lies between
the liquid outer core and the crust. This area is about 2900 km
(1800 mi) thick and consists largely of silicate rock.
