Section 1: Earth’s Crust in Motion How Do Stress Forces Affect Rock?

1. Section 1: Earth’s Crust in MotionHow Do Stress Forces Affect Rock? • The movement of earth’s platescreates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress • Stress– a force that acts on rock to change its shape or volume • An earthquake is the shaking and trembling that results from the movement of rock beneath earth’s surface

2. How Does Stress Effect the Earth’s Crust? • Deformation – any change in the volume or shape of earth’s crust • Three kinds of stress in the crust: • Shearing – stress that pushes a mass of rock in two opposite directions • Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle like warm bubble gum • Compression – squeezes rock until it folds or breaks like a giant trash compactor

3. What Is a Fault? • Fault – a break in earth’s crust where slabs of crust slip past each other; These usually occur at plate boundaries

4. What Kind of Faults Are There? • Three Kinds: • Strike-slip faults • Normal Faults • Reverse Faults

5. What Are Strike-slip Faults? • Strike-slip faults • Shearing forces cause rocks to slip past each other sideways with little up and down Motion; • Ex. San Andreas fault in California

6. What Are Normal Faults? • Normal faults • Tension forces cause the rocks to form the fault at an angle • One block is above the fault • Hanging wall – the half of the fault that lies above • Footwall – the half of the fault that lies below • Ex. Rio Grande rift valley

7. What Are Reverse Faults? • Reverse faults • compression forces cause the rocks to move towards each other • Same structure as normal fault but the blocks move in opposite direction; hanging wall move up • Ex. Appalachian Mountains and Mount Gould in Glacier National Park

8. A miner walks on the foot wall and looks up at the hanging wall! B A Hanging wall moves down Hanging wall moves up What type of fault? What type of fault? Normal Fault Reverse Fault

9. How Are Mountains Effected by These Forces? • Fault-block mountains – normal faults uplift a block of rock • Folding – bends in the rock that form when compression shortens and thickens part of the earth’s crust. Ex. Himalayas

10. How Are Mountains Effected by These Forces? (Continued) • Anticlines– a fold upward into an arch • Syncline– a fold downward into an arch • Plateaus– a large area of flat land elevated high above sea level

11. Section 2: Measuring QuakesHow Does the Energy of an Earthquake Travel Through Earth? • Earthquakes – most begin in the lithosphere • Focus – the point beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake • Epicenter – the point on the earth’s surface directly above the focus

12. What Are Seismic Waves? • Seismic Waves – vibrations that travel through Earth carrying the energy released during an earthquake • They move like ripples on a pond • They carry the energy of an earthquake away from the focus, through Earth’s interior, and across the surface • The energy is greatest at theEpicenter

13. What Are the Different Kinds of Seismic Waves? • Three categories: • P waves • S waves • Surface waves • P waves and S waves are sent out from the focus; Surface waves develop when the waves reach the surface

14. What Are P Waves? • P waves are primary waves • The first waves to arrive • Earthquake waves that compress and expand the ground like an accordion • Cause buildings to contract and expand

15. What Are S Waves? • S waves are secondary waves • Earthquake waves that vibrate from side to side as well as up and down • These waves shake the ground back and forth • Shake structures violently • Cannot move through liquids

16. What Are Surface Waves? • When P waves and S waves reach the surface some are transformed into surface waves • Surface waves move more slowly than P waves and S waves • Produce the most severe ground movements • Can make the ground roll like ocean waves or shake buildings from side to side

17. How Do Scientists Detect Seismic Waves? • Seismograph – records the ground movements caused by seismic waves as they move through the Earth

18. How Do Scientists Measure Earthquakes? • There are at least 20 different measures for rating earthquakes, three are: • Mercalli • Richter • Moment Magnitude • Magnitude – a measurement of earthquake strength based on seismic waves

19. What Is the Mercalli Scale? • Rated earthquakes according to their intensity • Intensity: strength of ground motion in a given place • Not a precise measurement • Describes how earthquakes affect people, buildings, and the land surface

20. What Is the Richter Scale? • A rating of the size of seismic waves as measured by a particular type of seismograph • Accurate measurements for small, nearby earthquakes not large, distant earthquakes

21. What Is the Moment Magnitude? • A rating system that estimates the total energy released by an earthquake • Can be used to rate earthquakes of all sizes, near or far • Below 5.0 – little damage • Above 5.0 – great destruction

22. How Do Scientists Locate the Epicenter? • Geologists use seismic waves • P waves arrive first • S waves arrive close behind • Scientist measure the difference in arrival times • The farther away an earthquake is the greater the time between their arrival • Scientists draw three circles using data from seismographs set at different stations to see where they intersect – the epicenter