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The Solid Earth:Plate Tectonics

Plate TectonicsWhy study this in this class?• Tectonic processes make volcanoes -

ultimate source of carbon dioxide, water,and other gases to the atmosphere– Weathering of mountains controls atmospheric

carbon dioxide, makes soils.

• Tectonic processes make mountainranges– Mountain ranges perturb atmospheric

circulation.

• Tectonic processes move continents– Continental positions influence ocean

circulation, energy input

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Plate Tectonics• Plate tectonic theory : Earth’ssurface composed of ridgid, platesthat are mobile and interact witheach other.

• Revolutionized earth science inthe 1960’s.

• Grew out of older ideas ofcontinental drift, normallyattributed to Alfred Wegener in1924.

Global Plate System

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Continental DriftSnider-Pelligrini (1858)

http://pubs.usgs.gov/publications/text/historical.html

Wegener’s Hypothesis

Ocean Crust

Continental Crust

Continental Drift

Evidence• Shapes of continents• Antarctic fossils records• African glacial deposits

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Understanding of theStructure of the EarthLead to Acceptance of

the Theory of Plate Tectonics

Understanding Earth structure arosefrom the study of earthquakes

Earthquakes result from movement within theEarth, which is transmitted through the earthas waves, like a sound or water wave. Theinitial movement occurs at a single point(Focus). The waves result in further motionwhich we experience at the surface.

Focus

Epicenter

Body Waves

Surface Waves

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Body Waves

1) Primary- Wave or Compression Wave: wavemoves in same direction as original force.

2) Secondary-Wave or Shear Wave: wave movesperpendicular to original force.

Primary-Wave or Compression Wave

wave moves in same direction as original force

FORCE

DIRECTION OF WAVE

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Secondary-Wave or Shear Wave

wave moves perpendicular to original force

DIRECTION OF WAVE

FORCE

Seismic waves tell us aboutEarth’s interior

Seismic wave speed depends on: rock type,how deep they travel (due to pressureincrease).

This causes two important effects:• Seismic waves are bent, or refracted asthey travel deeper in the earth (due toincreasing in speed as they go deeper).

• Seismic wave paths are shifted ordisrupted where the structure changes(e.g., core/mantle boundary, etc.).

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Why do waves bend?

Slower Velocity

Faster Velocity

Slower Velocity

Faster Velocity

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Slower Velocity

Faster Velocity

Slower Velocity

Faster Velocity

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Slower Velocity

Faster Velocity

Slower Velocity

Faster Velocity

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Earth Structure and Seismic Waves

Mantle Solid

SolidP-waves LiquidSolid

Earth Structure and SeismicWaves

MantleS-waves

SolidLiquidSolid

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Internal Structure of the Earth

Upper Mantel Asthenosphere 3.3 g/cm3

Lower Mantel

Outer Core 10.7 g/cm3

Molten Iron

InnerCore13.5 g/cm3

Solid Iron

Crust dO = 3.0 g/cm3, dL = 2.7 g/cm3

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Composition of the EarthConstraints: Observations of surface rocks,comparison with meteorite composition,inference from seismology.

Crust: Igneous, sedimentary, and metamorphicrocks, primarily silicate and carbonate minerals.(Silicon, oxygen, potassium, sodium, calcium,aluminum, carbon).

Mantle: Silicate minerals (silicon, oxygen,calcium, iron, magnesium, aluminum).

Core: Iron, Nickel (~ 6%), Sulfur.

Earth has a Magnetic Field

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• The internal earth is too hot to function assolid bar magnet.

• But, a rotating electrical conductor cangenerate a magnetic field.

• Heat generated in the earth, and magneticfields from the sun, generate convection in theliquid outer core. This motion is thought tocreate the earth’s magnetic field.

• Figure 6-6

Origin of the Magnetic Field

Magnetic Field and Reversals

S

N

NORMAL POLARITY REVERSED POLARITY

N

S

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History of the Magnetic Field:Last 5 Million Years

0 1.0 2.0 3.0 4.0 5.0

Age (Ma)

NORMAL REVERSED

Back to Plate Tectonics

The periodic reversals in Earth’s magnetic field provided evidence tosupport the theory of Plate Tectonics

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Mid-Ocean Ridges

Magnetic Record at Mid-Ocean Ridges

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Sea Floor Spreading Moving Plates

Driving Mechanism-Mantle Convection

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Total Picture

Back to Climate

Plate Tectonics

Wilson Cylces

500 million year cyclefor continental drift

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Wilson Cycles• Asthenosphere is not same temp. Everywhere

• Continents are drawn toward cold region

• Continents colide

• Supercontinent forms

• Athenosphere gets hot under continent

• Continents again move toward cold region

• Supercontinent breaks apart

N NN

Timing of Wilson Cycles

• Plates move at ~40 km/million years

• Circumference of Earth ~40,000 km

• Plates must travel half way, 20,000 km to meet

• Cycle Time = 20,000 km 40 km/million years = 500 million years

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Continental Drift

Continental Drift

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Continental Drift

Continental Drift

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Continental Drift and Climate

• Continents change latitude

• Coast lines change

• Mountain ranges form

• Ocean currents change