Earth’s Crust in Motion

EQ: How do stress forces affect rock?

Earthquakes

• Earthquake: The shaking that results from the movement of rock beneath Earth’s surface

EQ: How do stress forces affect rock?

• Two powerful forces that change rock in the Earth’s crust:– Stress– Friction

Stress

• Stress: A force that acts on rock to change its shape or volume

EQ: How do stress forces affect rock?

What is Volume?

• The amount of space an object takes up

EQ: How do stress forces affect rock?

Energy is stored in rock until the rock either breaks or changes

shape

EQ: How do stress forces affect rock?

Shearing

• Shearing: Stress that pushes a mass of a rock in opposite, horizontal directions

EQ: How do stress forces affect rock?

Tension

• Tension: Stress that stretches rocks so that it becomes thinner in the middle

EQ: How do stress forces affect rock?

Compression

• Stress that squeezes rock until it folds or breaks

EQ: How do stress forces affect rock?

Figure 2: If shearing continues to tug at the slab of rock in B, what will happen to the rock?

• The rock will break; the two parts will move in opposite directions

EQ: How do stress forces affect rock?

Deformation

• Deformation: A change in the volume or shape of Earth’s crust

• Most changes in the crust occur so slowly that they can not be observed directly

EQ: How do stress forces affect rock?

Checkpoint (Page 55): How does deformation change Earth’s surface?

• It causes it to:• Bend• Stretch• Break• Tilt• Fold• Slide

Guide For Reading: How does stress forces affect rock?

• The three kinds of forces that affect rock are:

• Shearing – The rocks break and slip apart

• Tension– The rock stretches and becomes thin in the

middle

• Compression– The rock squeezes until it folds or breaks

• These stresses work over millions of years to change the shape and volume of rock

Faults

• A break in the Earth’s crust where slabs of rock slip past each other

• Faults occur when enough stress builds up in rock

• Rocks on both sides of the fault can move up or down, or sideways

EQ: How do stress forces affect rock?

Strike-Slip Faults

• A type of fault where rocks on either side move past each other sideways with little up-or down motion.

• Shearing causes these types of faults

EQ: How do stress

forces affect rock?

Normal Faults

• A type of fault where the hanging wall slides downward

• Tension forces cause normal faultsEQ: How do stress forces affect rock?

Hanging Wall & Footwall

• Hanging wall: The block of rock that forms the upper half of a fault

• Footwall: The block of rock that forms the lower half of a fault

EQ: How do stress forces affect rock?

Reverse Faults

• A type of fault where the hanging wall slides up

• Compression forces cause reverse faults EQ: How do stress

forces affect rock?

Checkpoint (Page 57): What are the three types of fault? What force of deformation produce each?

• Strike-slip faults• Produced by shearing

• Normal faults• Produced by tension

• Reverse faults• Produced by compression

Why do faults form and where do they occur?

• Plate move and they compress, pull or shear the crust so much it breaks

• Faults occur along plate boundaries

EQ: How do stress forces affect rock?

Which half of the reverse fault slid up and across to form this mountain, hanging wall or the footwall? Explain.

• The hanging wall slipped up and across. If the footwall had moved up, the fault would be called a normal fault

EQ: How do stress forces affect rock?

What is friction?

• A force that opposes the motion of one surface as it moves across another surface

• It exists because rocks are not perfectly smooth

EQ: How do stress forces affect rock?

Describe what occurs when the friction along a fault line is low.

• The rocks on both sides of the fault slide by each other without much sticking

EQ: How do stress forces affect rock?

Describe what occurs when the friction along a fault line is moderate.

• The sides of the fault jam together

• From time to time they jerk free

• Small earthquakes occur EQ: How do stress forces affect rock?

Describe what occurs when the friction along a fault line is high.

• Both sides of the fault lock together and do not move

• The stress increases until it is strong enough to overcome the force of friction

• Larger and/or more frequent earthquakes will occur

EQ: How do stress forces affect rock?

Fault-Block Mountain

• A mountain that forms where a normal fault uplifts a block of rock

EQ: How do stress

forces affect rock?

How does the process of a fault-block mountain begin?

• Where two plates move away from each other, tension forces create many normal faults

• When two of these normal faults form parallel to each other, a block of rock is left lying between them

• As the hanging wall of each normal fault slips downward, the block in between moves upward

• When a block of rock lying between two normal faults slides downward, a valley forms

EQ: How do stress forces affect rock?

Folds

• A bend in rock that forms where part of Earth’s crust is compressed

EQ: How do stress forces affect rock?

How does the compression of two plates cause an earthquake?

• The collisions of two plates can cause compression and folding of the crust

• Such plate collisions also lead to earthquakes, because folding rock can fracture and produce faultsEQ: How do stress

forces affect rock?

Anticline

• Anticline: An upward fold in rock formed by compression of Earth’s crust

EQ: How do stress forces affect rock?

Syncline

• Syncline: A downward fold in rock formed by tension in Earth’s crust

EQ: How do stress forces affect rock?

Plateaus

• A large area of flat land elevated high above sea level

EQ: How do stress forces affect rock?

Guide For Reading: How does movement along faults change Earth’s surface?• Over millions of years, fault

movement can change a flat plain into a towering mountain range

• Mountain ranges can form from:• Fault – block mountain• Folding• Anticlines & Synclines• Plateaus