Geology Reference
In-Depth Information
Furthermore, fragments of mantle rock in kimberlite pipes,
thought to have come from depths of 100-300 km, appear
to have reached equilibrium at these depths at a temperature
of approximately 1200°C. At the core-mantle boundary, the
temperature is probably between 2500 and 5000°C; the wide
range of values indicates the uncertainties of such estimates.
If these fi gures are reasonably accurate, the geothermal gra-
dient in the mantle is only about 1°C/km.
Because the core is so remote and its composition uncer-
tain, only very general estimates of its temperature are possi-
ble. Based on various experiments, the maximum temperature
at the center of the core is thought to be 6500°C, very close to
the estimated temperature for the surface of the Sun!
deposits, most rocks of the continental crust have densities
between 2.5 and 3.0 g/cm
3
, with the average density of the
crust being about 2.7 g/cm
3
. P-wave velocity in continen-
tal crust is about 6.75 km/sec, but at the base of the crust,
P-wave velocity abruptly increases to about 8 km/sec. Con-
tinental crust averages 35 km thick, but its thickness varies
from 20 to 90 km. Beneath mountain ranges such as the
Rocky Mountains, the Alps in Europe, and the Himalayas
in Asia, continental crust is much thicker than it is in ad-
jacent areas. In contrast, continental crust is much thinner
than average beneath the Rift Valleys of East Africa and in a
large area called the Basin and Range Province in the western
United States and northern Mexico. The crust in these areas
has been stretched and thinned in what appear to be the ini-
tial stages of rifting (see Chapter 2).
In contrast to continental crust, oceanic crust is sim-
pler, consisting of gabbro in its lower part and overlain
by basalt. It is thinnest, about 5 km, at spreading ridges,
and nowhere is it thicker than 10 km. Its average density
of 3.0 g/cm
3
accounts for the fact that it transmits P-waves
at about 7 km/sec. In fact, this P-wave velocity is what one
would expect if oceanic crust is composed of basalt and
gabbro. We present a more detailed description of the oce-
anic crust's composition and structure in Chapter 9.
Our main concern in the latter part of this chapter is Earth's
interior; however, to be complete, we must briefl y discuss
the crust, which along with the upper mantle constitutes the
lithosphere.
Continental crust is complex, consisting of all rock
types, but it is usually described as “granitic,” meaning that
its overall composition is similar to that of granitic rocks.
With the exception of metal-rich rocks such as iron ore
Chapter Summary
Approximately 80% of all earthquakes occur in the circum-
Pacifi c belt, 15% within the Mediterranean-Asiatic belt,
and the remaining 5% mostly in the interior of the plates
and along oceanic spreading ridges.
The two types of body waves are P-waves (primary
waves) and S-waves (secondary waves). P-waves are the
fastest seismic waves and travel through all materials,
whereas S-waves are somewhat slower and can travel only
through solids. P-waves are compressional (expanding
and compressing the material through which they travel),
whereas S-waves are shear (moving material perpendicu-
lar to the direction of travel).
Earthquakes are vibrations caused by the sudden release
of energy, usually along a fault.
The elastic rebound theory is an explanation for how
energy is released during earthquakes. As rocks on oppo-
site sides of a fault are subjected to force, they accumulate
energy and slowly deform until their internal strength is
exceeded. At that time, a sudden movement occurs along
the fault, releasing the accumulated energy, and the rocks
snap back to their original undeformed shape.
Seismology is the study of earthquakes. Earthquakes are
recorded on seismographs, and the record of an earth-
quake is a seismogram.
An earthquake's focus is the location where rupture
within Earth's lithosphere occurs and energy is released.
The epicenter is the point on Earth's surface directly
above the focus.
■
■
■
■
■
Rayleigh (R-waves) and Love waves (L-waves) move
along or just below Earth's surface.
An earthquake's epicenter is determined using a time-
distance graph of the P- and S-waves to calculate how
■
■
■
Search WWH ::
Custom Search