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Earthquakes An Earthquake is… the shaking and trembling that results from the movement of rock beneath Earth's surface The movement of Earth's plates produces strong forces that squeeze or pull the rock in the crust Stress • Force that acts on rock to change its shape & volume • Adds energy to rock until it breaks or changes shape • Causes deformation • Rocks can break…creating a fault • Causes earthquakes Stress There are three different types of stress that occur on the crust, shearing, tension, and compression These forces cause some rocks to become fragile and they snap Some other rocks tend to bend slowly like road tar softened by the suns heat Shearing • Pushes rocks in opposite directions Tension • Pulls, stretching rock so it becomes thinner in the middle • 2 plates move apart Compression • Squeezes rocks until it folds or breaks Faults A fault is a break in the crust where slabs of crust slip past each other. The rocks on both sides of a fault can move up or down or sideways When enough stress builds on a rock, the rock shatters, creating faults Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust too much so the crust smashes Strike-Slip Faults Shearing creates this fault Rocks on both sides of the fault slide past each other sideways Forms transform boundary Strike-Slip Faults Normal Faults Tension forces in Earth's crust causes these types of faults Normal faults are at an angle, so one piece of rock is above the fault, while the other is below the fault The above rock is called the hanging wall, and the one below is called the footwall When movement affects along a normal fault, the hanging wall slips downward Occurs at divergent boundaries Normal Faults Reverse Faults Compression forces produce this fault Same structure as a normal fault, but blocks move in opposite directions Hanging wall slides up over footwall Occurs at convergent boundaries Reverse Faults Friction • Force that opposes the motion of one surface across another • Low friction: rocks slide without sticking • Medium friction: sides of the fault jam together & then jerk-free to produce small earthquakes • High friction: rocks lock & don’t move, stress increases until it overcomes friction, produces major earthquakes How Do Mountains Form? 1.) By Faulting- when two normal faults form parallel to each other, a block of rock in between them moves upward as hanging wall slips down (fault-block mountain) How Do Mountains Form? 2.) Folding: when 2 plates collide, compression forces cause the rock to fold or bend Anticlines and Synclines An anticline is a fold in a rock that arcs upward A syncline is a fold in a rock that arcs downward These folds in rocks are found on many parts of the earths surface where compression forces have folded the crust Anticline Syncline Plateaus The forces that elevate mountains can also raise plateaus, a large area of flat land elevated high above sea level How Earthquakes Form
