Geology Reference
In-Depth Information
Figure 3.3. The lithosphere of Earth is divided into some seven major plates and more than a dozen smaller segments, most of which are in
constant motion. Collisions and differential movement of plates lead to compression and tension and the formation of structural features,
such as faults and folds in crustal rocks (courtesy of the US Geological Survey).
tends to be more silicic (silica-rich) in comparison with
zones of up-welling convection.
In some places, upward convection is concentrated in
one place, forming a plume, or hotspot. The resulting
volcanism can be ma
c if the plume rises through oceanic
crust, or silicic if the plume rises through continental crust.
As will be seen in
Section 3.3
, the composition of the
erupted magma has a signi
cant in
uence on volcanic
eruptions and the resulting landforms.
As one can imagine, individual lithospheric plates do not
move at the same rate, which causes them to deform by
processes such as tension and compression. Coupled with
plate collisions, this tectonic activity results in a variety of
structures, including faults, joints, and folds. Breaking of
rocks (
Fig. 3.4
) can occur as (a) normal faults,whichresult
from tensional stresses; (b) reverse faults, which result from
compressional forces (thrust faults are reverse faults
involving very-low-angle fault planes); and (c) strike
-
slip
faults in which displacement is principally in horizontal
directions. The San Andreas fault in California is an example
of a strike
-
slip fault. Both normal and reverse faults involve
primarily vertical displacements of rocks along the fault
plane. Joints also involve fracturing of rocks, but, unlike
faulting in which rocks shift along a fault plane, jointing
involves just separation of rocks away from the fracture.
Figure 3.4. Diagrams of features re
ecting structural deformation
of rocks.
Some rocks subjected to compression yield by bending
into folds, rather than breaking along faults. Folds of several
types can occur (
Fig. 3.4
), some of which produce topo-
graphy that directly re
ects the form of the fold. Thus, in
