Geology Reference
In-Depth Information
outer core varies from 9.9 to 12.2 g/cm
3
. At Earth's cen-
ter, the pressure is equivalent to approximately 3.5 million
times normal atmospheric pressure.
The core cannot be composed of minerals common at
the surface because, even under the tremendous pressures at
great depth, they would still not be dense enough to yield an
average density of 5.5 g/cm
3
for Earth. Both the outer and
inner cores are thought to be composed largely of iron, but
pure iron is too dense to be the sole constituent of the outer
core. It must be “diluted” with elements of lesser density.
Laboratory experiments and comparisons with iron meteor-
ites indicate that perhaps 12% of the outer core consists of
sulfur and possibly some silicon, oxygen, nickel, and potas-
sium (Figure 8.21).
In contrast, pure iron is not dense enough to account
for the estimated density of the inner core, so perhaps 10-
20% of the inner core consists of nickel. These metals form
an iron-nickel alloy thought to be suffi ciently dense under
the pressure at that depth to account for the density of the
inner core.
When the core formed during early Earth history, it
was probably entirely molten and has since cooled so that
its interior has crystallized. Indeed, the inner core con-
tinues to grow as Earth slowly cools, and the liquid of the
outer core crystallizes as iron. Recent evidence also indi-
cates that at present the inner core rotates faster than the
outer core, moving approximately 20 km/yr relative to the
outer core.
hundred kilometers from an earthquake's epicenter were re-
cording two distinct sets of P- and S-waves.
From his observations, Mohorovi ˇi ´ concluded that a
sharp boundary separates rocks with different properties
at a depth of about 30 km. He postulated that P-waves be-
low this boundary travel at 8 km/sec, whereas those above
the boundary travel at 6.75 km/sec. When an earthquake
occurs, some waves travel directly from the focus to a seis-
mic station, whereas others travel through the deeper layer
and some of their energy is refracted back to the surface
(
Figure 8.25). The waves traveling through the deeper layer
(the mantle) travel farther to a seismic station, but they do so
more rapidly and arrive before those that travel more slowly
in the shallower layer.
The boundary identifi ed by Mohorovi ˇi ´ separates the
crust from the mantle and is now called the
Mohorovi ˇi ´
discontinuity
, or simply the
Moho
. It is present every-
where except beneath spreading ridges. However, its depth
varies: Beneath continents, it ranges from 20 to 90 km, with
an average of 35 km; beneath the sea fl oor, it is 5 to 10 km
deep.
◗
and Composition
Although seismic wave velocity in the mantle increases with
depth, several discontinuities exist. Between depths of 100
and 250 km, both P- and S-wave velocities decrease markedly
(
Figure 8.26). This 100- to 250-km-deep layer is the
low
-
velocity zone
, which corresponds closely to the
asthenosphere
,
a layer in which the rocks are close to their melting point and
are less elastic, accounting for the observed decrease in seis-
mic wave velocity. The asthenosphere is an important zone
because it is where most magma is generated, especially un-
der the ocean basins. Furthermore, it lacks strength, flows
plastically, and is thought to be the layer over which the
plates of the outer, rigid
lithosphere
move.
◗
Another significant discovery about Earth's interior was
made in 1909 when the Yugoslavian seismologist Andrija
Mohorovi ˇi ´ detected a seismic discontinuity at a depth of
about 30 km. While studying the arrival times of seismic
waves from Balkan (part of southeastern Europe) earth-
quakes, Mohorovi ˇi ´ noticed that seismic stations a few
200 km
Direct wave
400 km
Epicenter
Focus
Moho
Refracted wave
◗
Figure 8.25
Seismic Discontinuity Andrija Mohoroviˇi´ studied seismic waves and detected a
seismic discontinuity at a depth of about 30 km. The deeper, faster seismic waves arrive at seismic
stations fi rst, even though they travel farther. This discontinuity, now known as the Moho, is between
the crust and mantle.
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