Transcript Earthquake
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