Chapter 16 Earths surface Copyright The Mc GrawHill

Chapter 16 Earth’s surface Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

Interpreting Earth’s surface • Principle of uniformity – “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 • The 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 strain Stress Strain • • Force tending to compress, pull apart or deform a rock Three stress forces 1. Compressive stress • Plates moving together 2. Tensional stress • Plates moving apart 3. Shear stress • Plates sliding past each other Adjustment to stress Three strain types 1. 2. 3. Elastic strain • Returns to original shape Plastic strain • Molded or bent • Do not return to original shape Fracture strain • Rock cracks or breaks

Stress and deformation Possible material responses to stress 1. 2. 3. 4. No change Elastic change with recovery Plastic change with no recovery Breaking from the pressure Rock variables 1. 2. 3. 4. Nature of the rock Temperature of the rock Speed of stress application Confining pressure Interplay produces observed rock structures

Folding • 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 – Anticline: arch-shaped structure – Syncline: trough-shaped

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

Classes of faults • 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 • Reverse fault – Hanging wall moved upward relative to footwall – Result of horizontal compressive stress • Thrust fault – Reverse fault with a lowangle 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 – Stress builds until friction or rock rupture strength is overcome – Stressed rock snaps suddenly into new position

Locating and measuring earthquakes • Focus – • Epicenter – • Actual origin of seismic waves Location on Earth’s surface directly above the focus Seismometer – – – Instrument used to detect and measure earthquakes Detects three kinds of waves 1. 2. P-wave (longitudinal) - body S-wave (transverse) - body 3. Surface wave (up and down) http: //www. geo. mtu. edu/UPSei s/waves. html

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

Measuring earthquake strength • 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 measured so far)

Tsunami • 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)

Origin of mountains • Mountains – Elevated parts of Earth’s crust rising abruptly above the surrounding surface – Created by folding and faulting of crust – Three basic origins 1. Folding 2. Faulting 3. Volcanic activity

Folded and faulted mountains • Domed mountains – Broad arching fold – Overlying sedimentary rocks weather away, leaving more resistant granite peaks • 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

Other features • Most magma remains underground • Cools and solidifies to form intrusive rocks • Batholith – Large amount of crystallized magma – Stock: small protrusion from a batholith – Batholith intrusions can cause hogbacks • Related processes: dikes, sills, laccoliths, …

Tearing Down Earth’s Surface Weathering • 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 • Erosion – The process of physically removing weathered materials

Mechanical weathering • The physical breakup of rocks without chemical change • Disintegration processes – Wedging • By frost • By trees – Exfoliation • Reduced pressure effect • Fractures caused by expansion of underlying rock

Chemical weathering Decomposition of minerals by chemical reactions 1. Oxidation – – 2. Carbonation – – 3. Reactions with oxygen Produces red iron oxides Reactions with carbonic acid (carbon dioxide dissolved in water) Easily dissolves limestone 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 • • Most important of all gravitational erosion processes Three stream transport mechanisms 1. Dissolved materials 2. Suspended materials 3. Rolling, bouncing and sliding along stream bed • Streambed evolves over time

Stream development • 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

Some cool multimedia • Grand Canyon Formation – • http: //www. teachersdomain. org/resources/ess 05/s ci/ess/earthsys/canyon/index. html Plate tectonics, Seismograph, and Seismometer – http: //www. teachersdomain. org/resources/ess 05/sci/ess/earthsys/plateintro/i ndex. html and earthquake prediction: http: //www. teachersdomain. org/resources/ess 05/sci/ess/earthsys/japan/inde x. html – http: //www. teachersdomain. org/resources/ess 05/sci/ess/earthsys/seismogra ph/index. html – http: //www. teachersdomain. org/resources/ess 05/sci/ess/earthsys/seismome ter/index. html • Rock Cycle Animation – http: //www. teachersdomain. org/resources/ess 05/sci/ess/earthsys/rockcycle/ index. html

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 (5 -3500) • Alpine glaciers – Form at high elevations – Flow through valleys – Also “valley glaciers” • Continental glaciers – Cover large area of a continent – Today in Greenland Antarctica

Glacier erosion 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-blown deposits – – Dunes: low mound or ridge of sand or other sediment Loess: fine dust deposited over a large area