Earthquakes

Earth’s Crust In Motion

Guide For Reading

• How does stress forces affect rock?

• Why do faults form and where do they occur?

• How does movement along faults change Earth’s surface?

Earthquakes

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

Earth’s plates create powerful forces that ___ or ___ the rock in the crust.

• squeeze• pull

Stress

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

What is Volume?

• The amount of space an object takes up

Energy is stored in rock until the rock ______________.

• either breaks or changes shape

Shearing

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

Tension

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

Compression

• Stress that squeezes rock until it folds or breaks

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

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

Checkpoint 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

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

Normal Faults

• A type of fault where the hanging wall slides downward

• Tension forces cause normal faults

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

Reverse Faults

• A type of fault where the hanging wall slides up

• Compression forces cause reverse faults

Figure 5: 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

Guide For Reading: Why do faults form and where do they occur?

• Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust so much that the crust breaks

Checkpoint: 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

What is friction?

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

Friction exists because…

• surfaces are not perfectly smooth.

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

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

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

The San Andreas fault in California is a transform boundary that contains ___ stress.

• high

Fault-Block Mountain

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

Folds

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

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 faults

Anticline

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

An example of an anticline is the _________.

• Black Hills of South Dakota

When and how did this location form?

• Black Hills began to form about 65 million years ago

Syncline

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

An example of a syncline is the _____.

• Illinois Basin

This syncline stretches _____ from the western side of _____ through the state of _____.

• 250 kilometers• Indiana • Illinois

Plateaus

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

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