Geoscience Reference
In-Depth Information
asthenosphere
, which occurs between about 40 km and
250 km (25 mi and 105 mi) below Earth's surface. This part
of the inner Earth is the least rigid portion of the mantle
because it contains scattered zones of high temperature due
to radioactive decay. These zones comprise about 10% of the
asthenosphere and consist of molten rock that moves very
slowly by convection. This process is highly influential on
the Earth's surface, causing earthquakes, volcanoes, and de-
formation of rocks to form mountain chains.
The uppermost layer of Earth is the
lithosphere
(Figure 12.1).
This portion of Earth extends from the surface into the upper-
most part of the asthenosphere at a depth of 70 km (44 mi). It
contains the
crust
, which is the cool, stiff, and brittle exterior of
Earth. The boundary between the crust and the asthenosphere
is very well defined and is called the
Mohorovicic discontinu-
ity
(pronounced Mo-ho-RO-vi-chich; Moho for short), named
for the Yugoslavian scientist who first suggested its presence in
1909. This boundary is known because earthquake waves change
speeds dramatically at the Moho level as a result of the great dif-
ference in density between the mantle and crust.
The crust is the thinnest of the major Earth layers, ranging in
thickness from 8 km to 40 km (about 5 mi to 25 mi). This thick-
ness represents about 1% of the Earth's overall structure and is
equivalent to driving from downtown Miami to a distant suburb
in the metropolitan area. It floats on top of the mantle because its
overall density is less. The crust contains two parts: oceanic crust
and continental crust.
Oceanic crust
is about 8 km (about 5 mi)
thick and consists mostly of a rock called
basalt
, which is very
fine-grained and high in silica, magnesium, and iron; thus, oce-
anic crust is often called
sima
, which is short for silica and mag-
nesium. In contrast,
continental crust
is composed largely of
granite, which is a coarse-grained rock high in silica, aluminum,
potassium, calcium, and sodium. As a result, continental crust is
often called
sial
, short for silica and aluminum. Continental crust
is fundamentally thicker and less dense than oceanic crust, aver-
aging about 40 km (25 mi). It is thicker beneath mountain areas,
where it can reach depths of 50 km to 60 km (31 mi to 37 mi). In
nonmountainous areas, however, continental crust is only about
30 km (19 mi) thick. As you will see in the next chapter, the
difference in composition between continental and oceanic crust
has played a critical role in the overall history of Earth.
With the layers of Earth in mind, take a closer look at the
relationship among the asthenosphere, lithosphere, and crust.
Remember that these layers blend into one another, with the
lithosphere being the transition between the asthenosphere and
crust. Note in Figure 12.1 that the depth of the Moho level is
greater beneath continents than under ocean basins. The rea-
son for this pattern is fairly straightforward, as shown in the
sequence of diagrams in Figure 12.3. Figure 12.3a represents a
period of time during which intrusions of magma rise up from
the mantle into the overlying continental crust. In this way, the
overall elevation and mass of the continental crust increase, per-
haps due to mountain building. This increased mass increases
the pressure on the underlying asthenosphere, which is plastic,
relative to the ocean basin. As a result, the depth of the Moho
level increases beneath the continent.
Over a very long period of time, the elevation of the con-
tinent gradually decreases due to erosion (Figure 12.3b). This
process transports sediments to the continental margin, where
they are deposited. As a result, the mass of the continent slowly
decreases and the underlying asthenosphere begins to bounce
back (or rebound). At the same time, the asthenosphere beneath
the continental margin begins to subside because of the weight of
the new sediments. This process continues until the continent is
nearly leveled (Figure 12.3c), which results in the almost com-
plete rebound of the underlying asthenosphere. At the continental
margin, however, the asthenosphere subsides even more due to
the increased weight of sediments transported from the landmass.
This overall process of subsidence and rebound of the astheno-
sphere is called
isostatic adjustment,
or
isostacy
for short.
KEY COnCEPTs TO REMEMbER AbOUT
THE EARTH's'InTERnAL sTRUCTURE
1.
Earth's interior is divided into seven major layers. These
layers are the inner core, outer core, lower mantle, upper
mantle, asthenosphere, lithosphere, and crust.
2.
The inner core and lower mantle are solid rock bodies
composed mostly of iron. The outer core is liquid iron
and the upper mantle viscous nickel.
3.
The Earth's upper three layers are of most interest to
geographers because these layers contribute directly to
the development of surface landforms and thus affect
Asthenosphere
The layer of very soft rock that occurs in the
upper part of the upper mantle. This region is about 40 km to
250 km (25 mi to 105 mi) below the surface of Earth. The soft
character of this rock allows isostatic adjustments to occur.
Oceanic crust
Basaltic part of the Earth's crust that makes
up the ocean basins. Oceanic crust is about 8 km (5 mi) thick
and is also called sima because it consists largely of silica and
magnesium.
Lithosphere
The outer, solid part of Earth that is about 70 km
(44 mi) thick and includes the uppermost part of the astheno-
sphere and the crust.
Continental crust
Granitic part of the Earth's crust that
makes up the continents. Continental crust averages about
40 km (25 mi) in thickness and is also called sial because it
consists largely of silica and aluminum.
Mohorovicic discontinuity
The boundary between the
Earth's crust and the upper part of the asthenosphere; seismic
waves change speed at this boundary.
