Earths Crust in Motion EQ How do stress

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

EQ: How do stress forces affect 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 EQ: How do stress forces affect rock? • 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 EQ: 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 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

EQ: How do stress forces affect rock? 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? EQ: How do stress forces affect rock? • 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

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 EQ: 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?

EQ: How do stress forces affect rock? Syncline • Syncline: A downward fold in rock formed by tension in Earth’s crust

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