Robert W. ChristophersonCharlie ThomsenChapter 12Tectonics, Earthquakes, and VolcanismEarth’s Major PlatesFigure 11.17Arrows represent 20 million years of movement.Spreading Ocean Floor: Oceanic ridges (~64,000 km in length and ~1,000 km in width. NAZCARPACIFIC COCOSNORTH AMERICAEURASIAINDO-AUSTRALIAARCTICAFRICASOUTH AMERICACrustal FormationFigure 12.5Movement of tectonic plates is not smooth due to friction. The tectonic forces, gravity and the weight of overlaying rock can generate three types of stress: tension, compression and shear.Stress causes strain. Strain can be expressed as folding or faultingCrustal Deformation ProcessesStress and StrainFigure 12.7FoldingFigure 12.8Normal FaultFigure 12.11Reverse FaultFigure 12.11Strike-Slip FaultFigure 12.11San Andreas fault rift.San Andreas FaultFigure 12.12Oceanic Plate-Continental Plat collisionOceanic Plate-Oceanic Plate collisionContinental Plate-Continental Plat collisionOrogenesis (Mountain Building)Oceanic-Continental CollisionFigure 12.16Oceanic-Oceanic CollisionFigure 12.16Continental-Continental CollisionFigure 12.16India is moving 2 m/yr northward.The sudden release of energy at faulting that causes the quake of the Earth. Focus: subsurface where the seismic waves is initiated. The corresponding place at the surface is called Epicenter.Foreshock, and Aftershock: shocks occur before (foreshock) or after (aftershock) the main shock. Foreshocks does not necessarily followed by a main shock.EarthquakesAnatomy of an EarthquakeFigure 12.20Reliable forecasting can save lives, but it remains elusive. False alarms can cause enormous anxiety and uncertainty for civilians.First successful forecast: Feb 1975, Chinese scientists forecastan Earthquake is immanent and evacuated 3 million people in Haicheng, an earthquake struck 6 hours later. But 17 months later, a M 7.4 struck Tangshan in the wee hours killing ~250,000 people with no warning. So is this past year Sichuan Earthquake (M8.1) killing more than 80,000. The Sumatera Earthquake in 2004 caused tsunami killing 230,000 people.Challenges: we cannot detect exacting what is happening underground. Indicators are not reliable.Earthquake ForecastingRichter Scale and Moment Magnitude ScaleRichter scale established by Charles Richter in 1935. Scale is logarithmic. Each whole number is a 10 fold increase in the measured wave amplitude (31.5-fold increase in energy).Moment magnitude scale was established in 1993, which considers the amount of fault slippage produced by earthquake. It is more accurate for large quakes than Richter scale.Earthquake Intensity and Magnitude Buildup and Release of StressFigure 12.21Historical SeismicityFigure 12.23Volcanic SettingsFigure 12.25A volcano forms at the end of a central vent rises from the asthenosphere and uppermost mantle through the crust into a volcanic mountain.Cinder cone: a small cone-shaped hill with truncated top.Caldera: a large, basin-shape depressingThree settings:1. Along subduction boundaries2. Along sea-floor spreading centers3. At hot spots where individual plumes of magma rise to the crustHawaiian Basaltic LavaFigure 12.24Hawaiian Volcanic ActivityFigure 12.28Effusive Eruption: Low-viscosity magma, less than 50% silica, rich in iron and magnesiumExplosive EruptionFigure 12.36Magma more viscous, 50-75% silica and rich in aluminum. Magma tends to block magma conduit, causing magma accumulation in magma chamber with pressure and eventually explode.Destroys everything on its way, causing life and property damage.Powerful volcanoes can send ashes into the stratosphere, such as Mountain Pinatobu eruption in 1991 causing global cooling and dimming.Volcanic ashes are of high health risk material in the air.Volcano