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Earth’s surface Chapter 16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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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, …
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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
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Stress and strain Stress Strain
Force tending to compress, pull apart or deform a rock Three stress forces Compressive stress Plates moving together Tensional stress Plates moving apart Shear stress Plates sliding past each other Strain Adjustment to stress Three strain types Elastic strain Returns to original shape Plastic strain Molded or bent Do not return to original shape Fracture strain Rock cracks or breaks
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Stress and deformation
Possible material responses to stress No change Elastic change with recovery Plastic change with no recovery Breaking from the pressure Rock variables Nature of the rock Temperature of the rock Speed of stress application Confining pressure Interplay produces observed rock structures
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Folding Sedimentary rocks Folds 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
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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
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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
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Other faults Reverse fault Thrust fault
Hanging wall moved upward relative to footwall Result of horizontal compressive stress Thrust fault Reverse fault with a low-angle fault plane Faults provide information on the stresses producing the formation
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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
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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
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Locating and measuring earthquakes
Focus Actual origin of seismic waves Epicenter Location on Earth’s surface directly above the focus Seismometer Instrument used to detect and measure earthquakes Detects three kinds of waves P-wave (longitudinal) - body S-wave (transverse) - body Surface wave (up and down)
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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
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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)
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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)
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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 Folding Faulting Volcanic activity
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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
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Volcanic mountains Volcano
A hill or mountain formed by the extrusions of lava or rock fragments from magma below Structure: vent, crater, lava flow
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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,…
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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 The rock cycle Formation of soils Movement of rock materials over Earth’s surface Erosion The process of physically removing weathered materials
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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
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Chemical weathering Decomposition of minerals by chemical reactions
Oxidation Reactions with oxygen Produces red iron oxides Carbonation Reactions with carbonic acid (carbon dioxide dissolved in water) Easily dissolves limestone Hydration Reactions with water Includes dissolving in water and combining with water
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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
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Running water Most important of all gravitational erosion processes
Three stream transport mechanisms Dissolved materials Suspended materials Rolling, bouncing and sliding along stream bed Streambed evolves over time
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Stream development Youth Maturity Old age 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
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Some cool multimedia Grand Canyon Formation
Plate tectonics, Seismograph, and Seismometer and earthquake prediction: Rock Cycle Animation
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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
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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 and Antarctica
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Glacier erosion Three mechanisms Bulldozing Abrasion Plucking
Forms deposits called moraines Abrasion Produces powdery, silt-sized rock flour Plucking Glacier water freezes into surrounding rock and pulls it along
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Wind Considerably less efficient than water or ice Two major processes
Abrasion Natural sandblasting Produces ventifacts Shape can depend on prevailing winds 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
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