Geology Reference
In-Depth Information
The model of Earth's interior consisting of a core and a
mantle is probably accurate, but not very precise. In re-
cent years, geophysicists have developed a technique called
seismic tomography
that allows them to develop more ac-
curate models of Earth's interior. In seismic tomography,
numerous crossing seismic waves are analyzed much as
CAT (computerized axial tomography) scans are ana-
lyzed. In CAT scans, X-rays penetrate the body and a two-
dimensional image of its interior is formed. Repeated CAT
scans from slightly different angles are stacked to produce
a three-dimensional image.
In a similar manner, geophysicists use seismic waves to
probe Earth's interior. In seismic tomography, the average
velocities of numerous crossing seismic waves are analyzed
so that “slow” and “fast” areas of wave travel are detected. Re-
member that seismic wave velocity depends partly on elastic-
ity; cold rocks have greater elasticity and therefore transmit
seismic waves faster than hotter rocks.
As a result of studies in seismic tomography, a much
clearer picture of Earth's interior is emerging. It has already
given us a better understanding of complex convection
within the mantle and a clearer picture of the nature of the
core-mantle boundary.
Image not available due to copyright restrictions
During the 19th century, scientists realized that the tem-
perature in deep mines increases with depth. More recently,
the same trend has been observed in deep drill holes. This
temperature increase with depth, or
geothermal gradient
,
is approximately 25°C/km near the surface. In areas of ac-
tive or recently active volcanism, the geothermal gradient is
greater than in adjacent nonvolcanic areas, and temperature
rises faster beneath spreading ridges than elsewhere beneath
the seafl oor.
Much of Earth's internal heat is generated by radioactive
decay, especially the decay of isotopes of uranium and tho-
rium and, to a lesser degree, potassium-40. When these iso-
topes decay, they emit energetic particles and gamma rays
that heat surrounding rocks. Because rock is such a poor
conductor of heat, it takes little radioactive decay to build up
considerable heat, given enough time.
Unfortunately, the geothermal gradient is not useful for
estimating temperatures at great depth. If we were simply to
extrapolate from the surface downward, the temperature at
100 km would be so high that, despite the great pressure, all
known rocks would melt. Yet except for pockets of magma,
it appears that the mantle is solid rather than liquid because
it transmits S-waves. Accordingly, the geothermal gradient
must decrease markedly.
Current estimates of the temperature at the base of the
crust are 800 to 1200°C. The latter fi gure seems to be an up-
per limit; if it were any higher, melting would be expected.
Other discontinuities are also present at deeper lev-
els within the mantle. But unlike those between the crust
and mantle or between the mantle and core, these probably
represent structural changes in minerals rather than composi-
tional changes. In other words, geologists think that the mantle
is composed of the same material throughout, but the struc-
tural states of minerals such as olivine change with depth.
At a depth of 410 km, seismic wave velocity increases
slightly as a consequence of such changes in mineral struc-
ture (Figure 8.26). Another velocity increase occurs at about
660 km, where the minerals break down into metal oxides,
such as FeO (iron oxide) and MgO (magnesium oxide), and
silicon dioxide (SiO
2
). These two discontinuities defi ne the
top and base of a
transition zone
separating the upper mantle
from the lower mantle (Figure 8.26).
Although the mantle's density, which varies from 3.3
to 5.7 g/cm
3
, can be inferred rather accurately from seis-
mic waves, its composition is less certain. The igneous rock
peridotite
, containing mostly ferromagnesian silicates, is
considered the most likely component of the upper mantle.
Laboratory experiments indicate that it possesses physical
properties that account for the mantle's density and observed
rates of seismic wave transmissions. Peridotite also forms the
lower parts of igneous rock sequences thought to be frag-
ments of the oceanic crust and upper mantle emplaced on
land. In addition, peridotite occurs as inclusions in volcanic
rock bodies such as
kimberlite pipes
that are known to have
come from depths of 100 to 300 km.
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