Earth’s surface

Chapter 16

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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, …

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• Force tending to

compress, pull apart or deform a rock

• Three stress forces1. Compressive stress

• Plates moving together2. Tensional stress

• Plates moving apart3. Shear stress

• Plates sliding past each other

Strain• Adjustment to stress• Three strain types

1. Elastic strain• Returns to original

shape2. Plastic strain

• Molded or bent• Do not return to

original shape3. Fracture strain

• Rock cracks or breaks

Stress and deformation

Possible material responses to stress

1. No change2. Elastic change with recovery3. Plastic change with no

recovery4. Breaking from the pressure

Rock variables1. Nature of the rock2. Temperature of the rock3. Speed of stress application4. 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 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– Stress builds until friction

or rock rupture strength is overcome

– Stressed rock snaps suddenly into new position

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

1. P-wave (longitudinal) - body

2. 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 SurfaceWeathering

• Slow changes resulting in the breakup, crumbling and other destruction of solid rock

• Includes physical, chemical and biological processes

• Contributes to1. The rock cycle2. Formation of soils3. 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– Reactions with oxygen– Produces red iron oxides

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

• Most important of all gravitational erosion processes

• Three stream transport mechanisms1. Dissolved materials2. Suspended materials3. 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/ess05/sci/ess/earthsys/canyon/index.html

• Plate tectonics, Seismograph, and Seismometer – http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/plateintro/

index.html and earthquake prediction: http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/japan/index.html

– http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/seismograph/index.html

– http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/seismometer/index.html

• Rock Cycle Animation– http://www.teachersdomain.org/resources/ess05/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 and

Antarctica

Glacier erosion

Three mechanisms1. 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 processes1. 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