Download Movement of the Earth ’ s Crust

Movement of the Earth’s Crust
Earth’s Changing Surface

There are two major sections of the
crust: the continental crust (32 km
thick) and the oceanic crust (8 km
thick).
Deformation

Stress pushes and pulls on the Earth’s
crust. As the rocks of the crust
undergo stress, they slowly change
shape and volume. They also move up
or down or sideways. The movement
causes the rocks to break, tilt or fold,
This is caused deformation.
Stress

There are three basic types of stress,
each of which deforms the crust in a
different way. The three types of stress
are compression, tension and shearing.
Compression

Compression squeezes the rocks of the
crust. This often causes the particles in
the crustal rocks to move closer
together, making the rocks denser and
smaller in volume. As the rocks are
compressed, they are pushed both
higher up and deeper down.
Tension

Tension pulls on the rocks of the crust,
causing them to stretch out over a
larger area. A rock under tension
becomes thinner in the middle than at
the ends. As the volume of the rock
increases, its density decreases.
Shearing

Shearing pushes rocks of the crust in
two opposite directions. This causes
the rocks to twist or tear apart. During
shearing, then, rocks are not
compressed or stretched. They bend or
break apart.
Joints

Compression, tension and shearing can
change a rock’s volume, its shape or
both. These stresses can cause the
rocks to fracture or crack. If the rocks
fracture along numerous flat surfaces
which show no displacement, the cracks
are called joints. Such rocks may break
into blocks when the different sets of
joints cross one another.
Faulting

Stress sometimes causes rocks to
break. A break or crack along which
rocks move is called a fault. The rocks
on one side of the fault slide past the
rocks on the other side of the fault.
The movement can be up, down or
sideways. Earthquakes often occur
along fault lines.
Hanging Wall

The block of rock above the fault is
called the hanging wall.
Foot Wall

The block below the fault is called the
foot wall.
Normal Fault

Stress can cause hanging wall to move
up or down along a fault. If tension is
acting on a fault, the hanging wall will
move down relative to the foot wall. If
this occurs, the fault between the two
blocks is called a normal fault.
Reverse Fault

If compression is acting on a fault, the
hanging wall will move up relative to
the foot wall, this type of fault is called
a reverse fault.
Thrust Fault

A special type of reverse fault is a
thrust fault. A thrust fault is formed
when compression causes the hanging
wall to slide over the foot wall. Thrust
faults are special because they are
almost horizontal, whereas regular
reverse faults and normal faults are
almost vertical.
Thrust Faults

Thrust faults carry rocks many km from
their original position. Rocks are
usually severely bent also. Thrust faults
also mix up the order of the layers in
rock pushing older rocks on top of
younger rocks. The Lewis Overthrust
Fault in Glacier National Park in
Montana is an example.
Lateral Fault

Stress does not cause blocks of crustal
rock to move only up and down.
Shearing will cause the blocks of rock to
slide horizontally past each other. One
block moves to the left or right in
relation to the other block. The fault
along which the blocks move
horizontally past each other is called a
lateral fault.
Faulted Mountains and Valleys

When there are many normal faults in
one area, a series of mountains and
valleys may form. Mountains formed by
blocks of rock uplifted by normal faults
are fault-block mountains. A vast
region in western North America called
the Cordilleran Mountain region
contains many.
Rift Valleys

Valleys also form when mountains form.
Rift valleys are formed when the block
of land between two normal faults
slides downward. One example of a rift
valley is Death Valley in California. It is
a long narrow valley 87 meters below
sea level.
Rift Valley Formation
Folding

Sometimes when stress is applied to the
rocks of the crust, the rocks bend but
do not break. A bend in a rock is called
a fold.
Anticline and Syncline

A rock can fold wither upward or
downward. An upward fold in a rock is
called an anticline. A downward fold in
a rock is called a syncline. The
Appalachian Mountains are made up of
many anticlines and synclines.
Anticline and syncline
Fault or Fold
A number of factors determine whether
rocks will fault or fold:
 Temperature
 Pressure
 Rock type
 How the stress is applied

Folded Mountain
Faulted Mountain
Temperature

One factor is temperature. If they
become extremely hot during
compression, they are more likely to
fold than to fault.
Pressure

Another factor that affects whether
rocks will fault or fold is pressure. The
greater the pressure applied to the
rocks, the more likely they are to fold
rather than to fault.
Rock Type

Rock type is yet another factor that
determines whether rocks will fault or
fold. Some types of rocks break easily
when stress is applied. Such fragile
rocks are said to be brittle. Other
rocks, such as rock salt, bend easily
under stress and are said to be ductile.
How the Stress is Applied

If the stress is applied gradually, the
rocks will usually fold. If the stress is
applied suddenly, the rocks will usually
fault.
Plateaus

A plateau is a large area of flat land
that is raised high above sea level. A
plateau is wider than it is tall. Although
plateaus are often raised up by the
same processes that form mountains,
the rock layers in a plateau remain flat.
Plateau Formation
Plateaus can be formed by:
 Vertical Faulting
 Fold
 Rivers

Plateau Formation by Vertical
Faulting

Plateaus may be formed is through
vertical faulting, such as the Colorado
Plateau.
Colorado Plateau
Plateau Formation by Fold

One way a plateau may be formed is by
a slow, flat-topped fold, such as the
Appalachian Plateau.
Appalachian Plateau
Plateau Formation by Molten
Rock Flows

Plateaus can also be formed by a series of
molten rock flows on the surface of the
Earth,such as the Columbia Plateau.
Columbia Plateau
Plateau Formation by Rivers

Rivers often carve one large plateau
into many smaller plateaus or deep cut
valleys. One of the most spectacular is
the Grand Canyon.
Grand Canyon
Domes

Lava often flows onto the surface to
form a plateau. Sometimes, magma
pushes upward but does not reach the
surface. The stress caused by the
magma causes the rock layers above it
to fold upward, forming an uplifted
area. The magma cools and forms
hardened rock. The uplifted area
formed by rising magma is called a
dome.
Dome Mountains

Domes that have been worn away in
places form many separate peaks called
dome mountains. The Black Hills of
South Dakota and Wyoming are dome
mountains.
Black Hills of South Dakota
Mount Rushmore
The Floating Crust

Because the mantle is denser than the
crust, the solid rocky crust floats on the
mantle. The floating crust exerts a
downward force on the mantle. The
mantle also exerts a force. A balance
exists between the downward force of
the crust and the upward force of the
mantle. The balancing of these two
forces is called isostasy.
Isostasy
Balancing Act

Low-lying regions such as Norway,
Sweden and Finland have risen since
the ice has melted. Crustal rock can
also sink. The Mississippi River has
dropped millions of tons of mud and
sand particle into the Gulf of Mexico.
The addition of materials has caused
the crust on the Gulf floor to sink but
the depth of water has not changed.