Forces in Earth’s Crust

1. Forces in Earth’s Crust C5S1

2. Stress • A force that acts on an area of rock to change its shape or volume. • A rock’s volume is the amount of space the rock takes up. • Because stress is a force, it adds energy to the rock. The energy is stored in the rock until the rock changes shape or breaks

3. How does stress in the crust change Earth’s surface? • Tension, compression, and shearing work over millions of years to change the shape and volume of rock.

4. Examples of the three types of stress

5. Tension • Pulls on the crust, stretching rock so that it becomes thinner in the middle. Tension occurs where two plates are moving apart. • It is like pulling apart Laffy Taffy

6. Compression • Squeezes rock until it folds or breaks. One plate pushing against another can compress rock. • It is like a giant trash compactor

7. Shearing • Stress that pushes a mass of rock in two opposite directions. • Shearing can cause rock to break and slip apart or to change its shape

8. Kinds of Faults

9. Where are faults usually found, and why do they form? • Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks. There are three main types of faults: normal faults, reverse faults, and strike-slip faults.

10. Normal Fault • A type of fault where the hanging wall slides downward; caused by tension in the crust.

11. Hanging wall • The block of rock that forms the upper half of the fault. • In a normal fault, the fault is at an angle, so one block of rock lies above the other block of rock.

12. footwall • The block of rock that forms the lower half of a fault. • In a normal fault, the fault is at an angle, so one block of rock lies above the other block of rock.

13. Additional information about Normal Faults: • When movement occurs along a normal fault, the hanging wall slips downward. Normal faults occur where plates diverge, or pull apart. For example, normal faults are found along the Owens Valley in California where Earth’s crust under tension

14. Owens Valley Due to fault movement the Owens Valley was created. The two plates a divergent plates which means they are a normal fault. This has created an opening for a river and lake to form within the mountain system

15. Reverse Fault • A type of fault where the hanging wall slides upwards; caused by compression in the crust. • It is opposite of a normal fault. • Movement in a reverse fault produced part of the Rocky Mountains in the western US and Canada. Reverse faults also helped produce the Klamath Mountains in northern California.

16. Strike-Slip Fault • A type of fault where rocks on either side move past each other sideways with little up or down motion.

17. San Andreas Fault line The San Andreas Fault the best example of a strike –slip fault . It is observable from areal pictures.

18. Changing Earth’s Surface

19. What land features result from the forces of plate movement? • Over millions of years, the forces of plate movement can change a flat plain into landforms produced by folding, stretching, and uplifting, Earth’s crust. These landforms include anticlines and synclines, folded mountains, fault-block mountains, and plateaus.

20. Folding Earth’s Crust • Sometimes plate movement causes the crust to fold, like skidding on a rug. When compression happens and the crust bends without breaking. • These bends are called folds in the rock. Anticline and syncline are how geologist describe the high and low parts of these folds

21. Stretching Earth’s Crust • When two normal faults cut through a block of rock, a fault-block mountain forms. When two normal faults form parallel to each other, a block of rock is left laying between them. The hanging wall slips downward and the footwall moves upwards forming a fault-block mountain.

22. Uplifting Earth’s Crust • Forces that uplift mountains also uplift, or rise, plateaus. Some plateaus form when forces in Earth’s crust push up a large, flat block of rock. Plateaus are wider than they are tall.

23. Plateaus • Is a large area of flat land elevated high above sea level. High elevation, low relief.