DEFINITION OF CHARACTERISTICS OF THE CRUST

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 continents.

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

(structural geology).

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.

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