2Earthquakes—large vibrations that move through rock or other Earth materials Elastic rebound—when rocks strain and then break, the broken pieces snap back.Rocks deform slowly over long periods of timePotential energy builds up in themEnergy is released suddenly when rocks break and move.The movement causes vibrations that move through Earth.
3Fault—the surface of a break in rock Normal fault—caused by tension forces, rock above the fault moves down compared to rock below the faultReverse fault—caused by compression forces, rock above the fault moves upward compared to rock below the faultStrike-slip fault—caused by shear forces, rock on either side of the fault moves past one another in opposite directions
4Seismic waves—when strained rock’s potential energy is released, it moves outward from the fault in seismic wavesFocus—the point inside Earth where movement along a fault first occurs and energy is releasedEpicenter—the point on Earth’s surface located directly above the focus
5Seismic waves start at the focus and travel away in all directions Primary waves—cause rock to move back and forth in the same direction the waves are movingSecondary waves—cause rock to vibrate at right angles to the direction the waves are movingSurface waves—slowest, largest, most destructive waves
6Measuring earthquakes Seismograph—instrument that records and earthquake’s vibrationsIf seismic wave arrival times are recorded from three stations, the epicenter can be determinedRichter scale—measures an earthquake’s size, or magnitude, based on the heights of lines representing the amount of energy released through seismic waves recorded on a seismograph
7Earthquake damageModified Mercalli intensity scale—measures an earthquake’s intensity based on the amount of structural and geologic damageMost earthquake damage is caused by surface wavesTsunamis—when an earthquake occurs on the ocean floor, the sudden movement pushes against the water and creates powerful waves that can travel thousands of kilometers
8Predicting earthquakes Seismic-safe structures are able to stand up against an earthquake’s vibrations.Many high-rise buildings stand on huge steel and rubber springsUnderground water and gas pipes are replaced with pipes that will bend.Highways have cement pillars with spiral reinforcing rods around themPredicting earthquakesLong-range forecasts predict whether an earthquake is likely to occur in a given area within 30 to 100 years.
10Volcanoes—cone shaped hills or mountains formed by magma eruptions When magma flows onto Earth’s surface through a vent, it is called lavaTephra—bits of rock or solidified lava dropped from the air after an explosive eruptionSome volcanoes form where Earth’s plates collideOne plate subducts, or is forced underneath, the otherPart of the plate that is forced underneath melts, forming magma chambers
11How forceful an eruption is depends on the composition of the magma Avalanches of hot, glowing molten rock that flow in cushions of hot gases down a side of a volcano are called pyroclastic flowsHow forceful an eruption is depends on the composition of the magmaMore silica makes magma thicker and more resistant to flowWater vapor trapped in the magma becomes steam and creates pressureThe type of lava and gases contained in the lava determine the type of volcano that forms
12Four types of volcanoes Shield volcanoes—basaltic lava, which flows easilyForms a broad volcano with gently sloping sidesLargest type of volcanoForm where Earth’s plates are separating and magma is forced upward between platesCinder cone volcanoes—high gas content in the magmaExplosive, but short-lived, eruptionsForm a small cone of volcanic material from tephra
13Composite volcanoes—made of alternating layers of lava and tephra Steep-sided mountainsForm where Earth’s plates are colliding and being forced underneath each other, or subduction zonesFissure eruptions—magma that is very fluidOozes from cracks in Earth’s surfaceMagma flows freely across the land, as flood basaltsMost of Earth’s crust beneath the ocean is flood basalts
14Earthquakes and Volcanoes Section 3Earthquakes, Volcanoes, and Plate Tectonics
15Earth’s crust is broken into plates that move around. Most volcanoes form where plates are colliding or moving apart.Divergent plate boundaries—where move apartLong cracks called rifts form between themFissure eruptions are common where plates separateConvergent plate boundaries—where plates collide and denser plates subduct, or are forced underneath less dense platesMost earthquakes occur where plates are colliding or moving apart.