Earth’s crust is divisible broadly into continental crust of
granitic composition and oceanic crust of basaltic composi
tion. Continents comprise 29.22 percent of surface, whereas
34.7 percent of Earth’s surface is underlain by continental
crust (continental crust under submerged continental shelves
accounts for the difference). The continents are in turn divided
into orogens, made of linear belts of concentrated deformation,
and cratons, the stable, typically older interiors of
The distribution of surface elevation is strongly bimodal,
as reflected in the hypsometric diagrams. Continental freeboard
is the difference in elevation between the continents
and ocean floor and results from difference in thickness and
density between continental and oceanic crust, tectonic activity,
erosion, sea level, and strength of continental rocks.
Controls of Deformation
Deformation of the lithosphere is controlled by the strength
of rocks, which in turn is most dependent on temperature
and pressure. Strength increases with pressure and decreases
exponentially with increasing temperature. Because temperature
and pressure both increase downward, a cross section
through the crust or lithosphere will have different zones
where the effects of either pressure or temperature dominate.
In the upper layers of the crust, effects of pressure dominate,
rocks get stronger with depth and fail brittlely throughout
this region. Below about nine miles (15 km), the effects of
temperature become increasingly more important, and the
rocks get weaker, deforming by flowing ductile deformation.
Other important properties that determine how the
lithosphere deforms are composition (e.g., quartz v. olivine in
crust, mantle, continents and oceans), and strain rate. Strain
rate has its greatest variations along plate boundaries, and
most structures develop as a consequence of plate interactions
along plate boundaries.
Structural Geology and Plate Tectonics
The surface of the Earth is divided into 12 major and several
minor plates that are in motion with respect to each other.
Plate tectonics describes these relative motions, which are, to
a first approximation, rigid body rotations. However, deformation
of the plates does occur (primarily in 10s to 100s of
kilometers-wide belts along the plate boundaries), and in a
few places, extends into the plate interiors. Structural geology
deals with these deformations, which in turn give clues to the
types of plate boundary motions that have occurred, and to
the tectonic causes of the deformation.
Plate boundaries may be divergent, convergent, or conservative/
transform. The most direct evidence for plate tectonics
comes from oceanic crust, which has magnetic
anomalies or stripes recording plate motions. However, the
seafloor magnetic record only goes as far back as 180 million
years, the age of the oldest in-place oceanic crust. Any evidence
for plate tectonics in the preceding 96 percent of Earth
history must come from the continents and the study of them
Highly deformed continental rocks are concentrated in
long linear belts called orogens, comparable to those associated
with modern plate boundaries. This observation suggests
that these belts represent former plate boundaries. The
structural geologist examines these orogens, determines the
geometry, kinematics, and mechanics of these zones and
makes models for the types of plate boundaries that created
them. The types of structures that develop during deformation
depend on the orientation and intensity of applied
forces, the physical conditions (temperature and pressure) of
deformation, and the mechanical properties of rocks.
The most important forces acting on the lithosphere that
drive plate tectonics and cause the deformation of rocks are the
gravitational “ridge push” down the flanks of oceanic ridges,
gravitational “trench pull” of subducting lithosphere caused by
its greater density than surrounding asthenosphere, the drag of
traction exerted by the convecting mantle on the overriding
lithosphere, and the resistance of trenches and mountain belts.
At low temperature and pressure and high intensity of
applied forces, rocks undergo brittle deformation, forming
fractures and faults. At high temperature and pressure and
low intensity of applied forces, rocks undergo ductile deformation
by flow, coherent changes in shape, folding, stretching,
thinning, and many other mechanisms.
Different styles of deformation characterize different types
of plate boundaries. For instance, at mid-ocean ridges new
material is added to the crust, and relative divergent motion of
the plates creates systems of extensional normal faults and
ductile thinning at depth. At convergent plate boundaries, one
plate is typically subducted beneath another, and material is
scraped off the downgoing plate in a system of thrust faults
and folds. Along transform plate boundaries, systems of strikeslip
faults merge downward with zones of ductile deformation
with horizontal relative displacements. All types of plate
boundaries have small-scale structures in common, so it is necessary
to carefully examine regional patterns before making
inferences about the nature of ancient plate boundaries.
See also METAMORPHISM; PLATE TECTONICS.
subduction See SUBDUCTION ZONE.