Geology Reference
In-Depth Information
Chapter Summary
Folded and fractured rocks have been deformed or
strained by applied stresses.
Stress is compression, tension, or shear. Elastic strain
is not permanent, but plastic strain and fracture
are, meaning that rocks do not return to their origi-
nal shape or volume when the deforming forces are
removed.
Strike and dip are used to defi ne the orientation of
deformed rock layers. This same concept applies to other
planar features such as fault planes.
Anticlines and synclines are up- and down-arched folds,
respectively. They are identifi ed by strike and dip of the
folded rocks and the relative ages of rocks in these folds.
Domes and basins are the circular to oval equivalents of
anticlines and synclines, but they are commonly much
larger structures.
The two structures that result from fracture are joints
and faults. Joints may open up, but they show no
movement parallel with the fracture surface, whereas
faults do show movement parallel with the fracture
surface.
Joints are very common and form in response to com-
pression, tension, and shear.
On dip-slip faults, all movement is up or down the dip
of the fault. If the hanging wall moves relatively down it
is a normal fault, but if the hanging wall moves up it is a
reverse fault. Normal faults result from tension; reverse
faults result from compression.
In strike-slip faults, all movement is along the strike
of the fault. These faults are either right-lateral or left-
lateral, depending on the apparent direction of offset of
one block relative to the other.
Oblique-slip faults show components of both dip-slip
and strike-slip movement.
A variety of processes account for the origin of moun-
tains. Some involve little or no deformation, but the large
mountain systems on the continents resulted from defor-
mation at convergent plate boundaries.
Subduction of an oceanic plate beneath another oceanic
plate or beneath a continental plate causes an orogeny. At
an oceanic-oceanic boundary a volcanic island arc intruded
by plutons forms, whereas at an oceanic-continental
boundary a volcanic arc forms on the continental plate. In
both cases deformation and metamorphism occur.
Some mountain systems are within continents far from
a present-day plate boundary. These mountains formed
when two continental plates collided and became sutured.
Geologists now realize that orogenies also involve
collisions of terranes with continents.
Continental crust is characterized as granitic, and it is
much thicker and less dense than oceanic crust that is
composed of basalt and gabbro.
According to the principle of isostasy, Earth's crust fl oats
in equilibrium in the denser mantle below. Continen-
tal crust stands higher than oceanic crust because it is
thicker and less dense.
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Important Terms
anticline (p. 249)
basin (p. 251)
compression (p. 247)
continental accretion (p. 264)
deformation (p. 246)
dip (p. 248)
dip-slip fault (p. 255)
dome (p. 251)
elastic strain (p. 247)
fault (p. 253)
fault plane (p. 253)
fold (p. 249)
footwall block (p. 253)
fracture (p. 248)
geologic structure (p. 248)
gravity anomaly (p. 265)
hanging wall block (p. 253)
isostatic rebound (p. 269)
joint (p. 252)
monocline (p. 249)
normal fault (p. 255)
oblique-slip fault (p. 257)
orogeny (p. 258)
plastic strain (p. 248)
principle of isostasy (p. 265)
reverse fault (p. 255)
shear stress (p. 247)
strain (p. 246)
stress (p. 246)
strike (p. 248)
strike-slip fault (p. 256)
syncline (p. 249)
tension (p. 247)
terrane (p. 264)
thrust fault (p. 255)
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