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

Section 1: Earth’s Crust in Motion How Do Stress Forces Affect Rock?. The movement of earth’s plates creates 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

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

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  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 Affect the Earth’s Crust? • Deformation – any change in the volume or shape or 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. a. Most faults lie between the surface and a depth of 70 kilometers b. Focus- point below Earth’s surface where rocks break and move c. Epicenter- point above Earth’s surface directly above the focus

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

  6. 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

  7. 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

  8. Normal Fault

  9. 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

  10. Reverse Fault

  11. 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

  12. 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

  13. 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 the theEpicenter

  14. 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

  15. 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

  16. 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

  17. 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

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

  19. 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

  20. 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

  21. 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

  22. 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

  23. Earthquake damage in Charleston

  24. February 21, 1916 Asheville, NC - the most intense earthquake in NC history, measuring a 6 on the Mercalli scale. June 5, 1998 an earthquake in Moorseville, NC measured 3.2 on the Richter scale, but there was no reported damage.

  25. Mantle- directly above the outer core • 1. Plasticity is the property of a solid with the ability to flow like a liquid at very, very slow rates. • a. High temperature and pressure allow the rock to flow like a liquid • b. This also allows the plates of Earth (lithosphere) to move on the mantle.

  26. MANTLE • The mantle is composed of silicon, • oxygen, iron and magnesium. • 3. Moho- boundary between the crust and • the mantle. • Change in the speed of seismic waves moving through the Earth led to its discovery. • b. Discovered in 1909 by a Yugoslav • scientist, AndrijaMohorovicic.

  27. 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

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