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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