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Deformation of the Crust Chapter 11 Deformation • Bending • Tilting • Breaking Isostasy • When the force of gravity (weight) of the lithosphere equals the buoyancy of the asthenosphere • What happens when lithosphere becomes thinner? Thicker? • Isostatic adjustments Isostatic Adjustments • Mountainous regions • Regions of deposition (causes ocean floor to sink – subsidence) • Glaciers (growth or retreat) • What happens to ocean floor when glaciers grow? Types of Stress • Compression • Squeezes rock together to occupy smaller areas • Can bulge up above crust or deeper into crust • Boundary type? • Tension • Pulls rock apart causing it to become thinner • Boundary type? • Shear • Parts of the rock are pushed separate directions • Can break, bend, or twist rocks • Boundary type? Types of Strain • Permanent Strain • Brittle (breaks) • Ductile (bends) Factors Affecting Strain • • • • • • Composition of rock Temperature Pressure Amount of stress Type of stress Rate of stress Folds • Bend in rock resulting from stress • Compression or shear stress Features of a Fold • Limbs – sloping side of fold • Hinge – bend of the rock • Axial plane – plane at which fold could be split into symmetrical halves Types of Folds • Anticline • Compression • Arch-shaped • Oldest rocks in center of fold • Syncline • Compression • Bowl-shaped • Youngest rocks in center of fold • Monocline • Both limbs horizontal • One part of crust moves up, the other moves down When Rocks Break: • Fracture – no movement along break • Fault – motion of the rock along the fault plane Types of Faults • Normal • Hanging wall moves downward relative to footwall • Divergent boundary • Step-like formations, Great Rift Valley • Reverse • Hanging wall moves upward relative to footwall • Thrust fault – fault plane nearly horizontal • Rockies and Alps • Strike-slip • Rocks on both sides of fault plane move horizontally • Transform boundaries Sizes of Faults • Small fault – only in small area and through few layers of rock • Large fault system – large area, several layers of rock • San Andreas Fault Earthquakes Chapter 12 Elastic Rebound Theory • Fault is usually locked by friction • Stress is added and builds up until crust deforms • Rock fractures and snaps back into its original shape • Earthquake Anatomy of an Earthquake • Focus – where movement first occurs • Shallow (most damage) – 70 km deep • Intermediate – 70-300 km deep • Deep – 300-650 km deep • Epicenter – point on Earth’s surface directly above focus Seismic Waves • Body waves • P waves – (primary, compression) • fastest and first to be detected • Particles move back and forth parallel to wave action • Can travel through solids, liquids, and gases • S waves – (shear, secondary) • Particles move in side to side direction perpendicular to direction of wave • Can only travel through solids Seismic Waves • Surface Waves • • • • Form when P & S waves reach surface Slowest moving Cause greatest damage Love – particles move side to side and perpendicular to wave direction • Rayleigh – elliptical and rolling motion Seismic Waves and Earth’s Interior • Waves speed up 30 km down because mantle is more dense • Speed changes of waves tells where layers are Shadow Zones Earthquake Environments • Convergent Oceanic Environments • Overriding plates scrapes over subducting plate • Divergent Oceanic Environments • Spreading motion causes earthquakes along ridge • Continental Environments • Stress added to rock by plates colliding, diverging, or sliding past one another Earthquakes Away from a Fault • New Madrid, Missouri (1811-1812) • Caused damage as far as South Carolina • Caused Mississippi River to flow backwards • Ancient fault zone (600 myo) Measuring Earthquakes • Seismograph • Measures vertical motions, horizontal motions E-W, horizontal motions N-S • Seismogram traces earthquake motion • Which are recorded 1st? 2nd? Last? Locating an Earthquake • Need measurements from 3 stations • Difference in time P and S waves arrive (lag-time) • Draw circles (radius = distance to epicenter from station) • Point where circles intersect is location of the epicenter Earthquake Measurement • Magnitude • Amount of ground movement • Richter Scale • Moment Magnitude Scale (more accurate for large earthquakes) • Largest ever recorded: 9.5 • Felt by humans: 2.5 • Intensity • Measures effects of earthquake • I – XII (XII is total destruction) • II is felt by only a few people at rest; delicately suspended items may swing. Tsunamis • Earthquake with epicenter on ocean floor • Sudden drop or rise in seafloor causes water to suddenly drop or rise forming waves • Underwater landslides may also cause them Predicting Earthquakes • Past earthquakes • Seismic Gaps – area along a fault where there has been a big earthquake in the past but none recently • Foreshocks – tiny earthquake seconds or weeks before earthquake • Changes in Rocks – sensors detect any change in the strain of the rock