Earth’s Crust In Motion 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 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