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Chapter 16 Earth’s Surface Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. •Principle of Earth’s uniformity Interpreting surface (James Hutton) • “The present is the key to the past.” • Rocks are changed today by the same processes that changed them in the past • Replaced catastrophic models of previous thinkers • Catastrophic events contribute nonetheless • Volcanoes, earthquakes, meteorite impacts, … Diastrophism • Any process of deformation that changes the Earth’s surface • Produces structures such as plateaus, mountains and folds in the crust • Related to volcanism (the movement of magma) and earthquakes • Basic working theory is plate tectonics Stress and deformation Possible material responses to stress 1. No change 2. Elastic change with recovery 3. Plastic change with no recovery 4. Breaking from the pressure Stress and deformation Cont. Rock variables 1. 2. 3. 4. Nature of the rock Temperature of the rock Speed of stress application Confining pressure Interplay between these variables produces observed rock structures • Sedimentary rocks • Originate from flat sediment deposits • Layers usually horizontal • Folds • Bends in layered bedrock • Result of stress produced plastic strain • Widespread horizontal stress can produce domes and basins Folding Domes Basins Faulting Fault Produced by relative movement on opposite sides of a crack Footwall: mass of rock below the fault Hanging wall: mass of rock above the fault Fault plane: surface between the footwall and hanging wall Faulting Cont. Shows how the hanging wall has moved relative to the footwall. Hanging wall is on the left Footwall to the right Three Classes of Faults • 1. Normal fault • Hanging wall has moved down relative to the footwall • Related features • Graben • Block surrounded by normal faults drops down • Horst • Block surrounded by normal faults is uplifted Other Faults • 2. Reverse fault • Hanging wall moved upward relative to footwall • Result of horizontal compressive stress • 3. Thrust fault • Reverse fault with a low-angle fault plane • Faults provide information on the stresses producing the formation Earthquakes • Quaking, shaking, vibrating or upheaval of the ground • Result from sudden release of energy from stress on rocks • Vibrations are seismic waves • Most occur along fault planes when one side is displaced with respect to the other Causes of earthquakes • Elastic rebound theory • Two plates press tightly together • Friction restricts motion Causes of earthquakes Cont. Stress builds until friction or rock rupture strength is overcome Stressed rock snaps suddenly into new position • Focus • Actual origin of seismic waves Locating and measuring • Epicenter earthquakes • Location on Earth’s surface directly above the focus Locating and measuring •earthquakes Seismometer • Instrument used to detect and measure earthquakes • Detects three kinds of waves 1.P-wave (longitudinal) 2.S-wave (transverse) 3.Surface wave (up and down) Seismic data P-waves travel faster than S-waves Difference in arrival times correlates to distance from earthquake Triangulation used to pinpoint epicenter and focus Earthquake Magnitude • Effects: structural damage to buildings, fires, landslides, displacement of land surfaces, tsunami (tidal wave) • Richter scale • Based on swings in seismograph recordings • Logarithmic scale • Number increases with magnitude of the quake • 3(not felt); 9(largest) Anchorage, March 1964 (9.2) Northridge, January 1994 (6.7) Oakland, October 1989 (6.9) Tsunomi • Very large ocean waves • Generated by strong disturbance in ocean floor • Earthquake, landslide, volcanic explosion • Speeds of up to 725 km/h (459 mi/h) • Wave height can be over 8 m (25 ft) • Very long wavelength of up to 200 km (120 mi) Indian Ocean Tsunami Before and After Before and After •WARNING: GRAPHIC PICTURE After Tsunami • Origin of mountains • Elevated parts of Earth’s crust rising abruptly above the surrounding Mountains • • surface Created by folding and faulting of crust Three basic origins 1. 2. 3. Folding Faulting Volcanic activity Folded and faulted mountains • Domed mountains • Broad arching fold • Overlying sedimentary rocks weather away, leaving more resistant granite peaks Folded and faulted mountains Cont. Fault block mountains Rise sharply along steeply inclined fault planes Weathering erodes sharp edges Volcanic mountains Volcano • A hill or mountain formed by the extrusions of lava or rock fragments from magma below • Structure: vent, crater, lava flow Mt. St. Helens May 18, 1980 Eruption! Ash Clouds Before and After Before and After Spirit Lake Before Spirit Lake After Spirit Lake – 2006 Mt Adams from Mt. St Helens showing tree damage Edge of Blast Growing Lava Dome Growing Lava Dome Hot Spots • Thermal blooms • Maybe up to 2000 miles deep • Plate moves over hot spot • Islands develop over time---depends how long in one area Volcanic Hot Spots Shield Volcano Processes That Tear Down Slow changes resulting in the breakup, crumbling and other destruction of solid rock Includes physical, chemical and biological processes Contributes to 1. The rock cycle 2. Formation of soils 3. Movement of rock materials over Earth’s surface Mechanical weathering Erosion: The process of physically removing weathered materials The physical breakup of rocks without chemical change Disintegration processes Wedging By frost By trees Chemical weathering Decomposition of minerals by chemical reactions 1. Oxidation • Reactions with oxygen • Produces red iron oxides Chemical weathering Cont. 2. Carbonation • Reactions with carbonic acid (carbon dioxide dissolved in water) • Easily dissolves limestone 3. Hydration • Reactions with water • Includes dissolving in water and combining with water Erosion • Mass movement • Erosion caused directly by gravity • Creep • The slow movement of soil down a steep slope • Landslide • Any slow to rapid downhill movement of materials Running water • • Streambed transport: 1. Dissolved materials 2. Suspended materials 3. Rolling, bouncing and sliding along stream bed Streambed evolves over time Streams Youth Landmass recently uplifted Steep gradient, V-shaped valley w/o floodplain Boulders, rapids and waterfalls Maturity Stream gradient smoothed and lowered Meanders over floodplain Old age Very low gradient Broad, gently sloping valleys Sluggish flow; more floods Stream development Cont. Three stages in the aging and development of a stream (A) Youth (B) maturity (C) old age Deltas • Deposits of sediment at the mouth of a river or stream • Stream flow dissipates into an ocean or lake • Erosive and sediment-carrying abilities lost Glaciers • Masses of ice on land that move under their own weight • Form from snow accumulated over a number of years Glaciers Cont. • Alpine glaciers • Form at high elevations • Flow through valleys • Also “valley glaciers” • Continental glaciers • Cover large area of a continent • Today in Greenland and Antarctica Tracy Arm, Alaska Glacier erosion Glaciers: Three mechanisms: 1. Bulldozing • Forms deposits called moraines 2. Abrasion • Produces powdery, silt-sized rock flour 3. Plucking • Glacier water freezes into surrounding rock and pulls it along Wind Considerably less efficient than water or ice Two major processes 1. Abrasion Natural sandblasting Produces ventifacts Shape can depend on prevailing winds 2. Deflation Loose material picked up and carried away by the wind Wind Cont. •Wind-blown deposits •Dunes: low mound or ridge of sand or other sediment •Loess: fine dust deposited over a large area Death Valley Indiana Dunes Great Sand Dunes Loess Hills Scenic Byway Loess Hills