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Part II: Solar System

Updated: July 14, 2007

The Earth

The Earth

A. The Interior

B. The Surface

C. Atmosphere

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A. Interior of Earth

1. Differentiated Structure2. Seismography3. Composition of layers

3 1a. Interior 4

The earth is “differentiated”•Inner Core: solid iron•Outer Core: liquid iron•Mantle: heavy rock (olivine)•Crust: light rock (granite)

51b. How did it get this way? Differentiation!Heat came from radioactive decay (e.g. Aluminum 26) and/or intense meteor impacts, heated up interior, heavy stuff sank to center. This releases even more gravitational potential energy!

61c. More heat is released as liquid core freezes

Melting point increases with pressure, so the melting point temperature is higher deeper inside the earth. The very center is hence solid.

At current rate, it willtake several billionyears for entire core to become solid.

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72. How do we know about inside the earth?

•We haven’t drilled very far into the earth

•1961 Project Moho went 183 meters into the oceanic crust (which is 5000 meters thick!)

•1970 Russians went 12.3 km, but this was on land where the continental crust is 35 km thick! They did not get to the Basalt layer.

•2007 Japan’s Chikyu Hakken mission will attempt to go 7000 meters, the first to drill deep enough to get sample from the upper mantle!

2a Seismographs 8

•100 AD: Chinese (Zhang Heng) have primitive device

•1880 John Milne invents modern seismograph

(shear waves)

2b Two Basic Wave Types (Andrija Mohorovičić 1909)

S waves can only travel in solids!P can travel in solid, liquid and gas!

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(pressure waves)

(shear waves)

2c Liquid CoreRichard Dixon Oldham (July 31, 1858 – July 15, 1936) was a British geologist who, in 1906, argued that the Earth must have a molten interior as S waves were not able to travel through liquids nor through the Earth's interior.“S Shadow Zone”

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2d Speed of Waves• Beno Gutenberg 1914 (worked with Richter)• determines travel times for waves.• Measures Temp and pressure inside of earth• Measures size of core from P Wave Shadow Zone.

P wave velocities: in granite: 6 km/secin basalt: 7 km/secin peridotite: 8 km/sec

11 2e. Velocities of P and S waves in different layers of the Earth 12

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2f Reflection & Refraction 13

Seismic waves are reflected or bent at the boundaries of different layers inside the earth

2g. Solid Inner Core (1936)• The pressure at the center of the Earth is 3.6 million bars• Solid inner core discovered 1936 by Inge Lehmann from

reflection/refraction of waves (for a time called the Lehmann discontinuity).

• Size not determined until 1960s from shockwaves from underground nuclear tests (echoes bouncing off inner core).

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3a. Composition: The crust• 1909 Andrija Mohorovicic realized that the

velocity of seismic waves is related to the density of the material that they are traveling through.

• He determined the abrupt change in density from the crust to the mantle (The Mohorovicic discontinuity, or “the Moho”

• Upper Crust is Granite• Lower Crust is Basalt

153a. Composition: The crust

• Earth’s interior is layered on a very large scale• CRUST: basalts and granites. 0.4% of Earth’s mass

is in the crust. The crust is ~5 km thick under the oceans and ~40 km thick under continents

• The rigid crust is the uppermost part of the lithosphere (which is ~100 km thick)

• The asthenosphere is a “plastic” or partially molten layer below the crust, and is ~100-200 km thick

Need diagram!

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3b. The Mantel

• MANTLE: made of silicate rocks (dunite, peridotite). 67% of the mass of Earth is in the mantle

• Increasing pressure at greater depth in the mantle causes mineral transitions (atoms are compressed into more dense arrangements). This results in changes in the seismic wave velocities at various depths

• Pressure at the top of the mantle is ~36,000 bars• Pressure at the bottom of the mantle is ~1.3 million

bars

17Inner Core

• In 1996, Xiadong Song and Paul Richards confirmed a prediction that the inner core rotates slightly faster than the rest of the Earth. The magnetic forces of the geodynamo seem to be responsible.

• Over geologic time, the inner core grows as the whole Earth cools. Iron crystals freeze out at the top of the outer core and rain onto the inner core. At the base of the outer core, the iron freezes under pressure taking much of the nickel with it. The remaining liquid iron is lighter and rises. These rising and falling motions, interacting with geomagnetic forces, stir the whole outer core at a speed of 20 kilometers a year or so.

• 2002 evidence of an inner inner core 300 km radius.

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3d. What is the Earth made of ?

• Crust and Mantle:– Silicate rocks– Silicon (Si) is an abundant element

• Si + Oxygen + metals (Fe, Mg, Al, etc.) = silicate rocks

• Interior (Core and Inner Core)• Atmosphere: Gases (oxygen, nitrogen, etc.)• Hydrosphere: Water• Magnetosphere: Charged atomic particles from the

Sun

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B. Earth’s Surface

1. Plate Tectonics2. Continental Drift3. Volcanoes4. Rocks

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Forces that Shape the Face of the Earth

• The world-wide pattern of earthquakes shows that they occur along lines in the Earth’s crust

• These lines define the margins of segments of the Earth’s crust

• These are called tectonic plates

Tectonism

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The distribution of earthquakes defines severalplates into which the Earth’s crust is divided

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Fig 7-6, p.157

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Fig 7-8, p.160

The San-AndreasFault runningthrough California.

This representsmotion along themargin of twosub-plates, andis the source of manydangerousearthquakes.

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Forces that Shape the Face of the Earth

– Plates move a few cm per year– Earthquakes– Sea-floor spreading (few cm per year). 2 km2 of

new sea floor per year. Enough to resurface ocean floors every 100 million years

– Subsidence of crust at continent margins– Vertical uplift of mountains (few mm per year)

Tectonism -- continued

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Fig 7-7b, p.159

This shows the subduction of an oceanic crustalplate under a continental mass. Friction of thesubducting plate with the thicker land mass causesrock melting; the melted rock (magma) rises alongcracks and fissures, and erupts as volcanoes.

27 Subduction and volcanism on the NW coast of North America

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Convective motionin the mantle

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Tectonism is drivenby the heat in the

Earth’s deepinterior

The ocean floorspreads as new

lavas come up fromthe mantle

Volcanoes risewhere ocean floorcrust is subducted

below the continents

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What would happen if the Earth cooled ?

• Plate tectonic motions would stop• The magnetic field would collapse, and Earth

would be unprotected from the solar wind• Life would cease to exist, except perhaps deep

in the surface rocks• The atmosphere and oceans would evaporate

into space• What other consequences can you imagine ?

31 Sea-floor spreading at the Mid-Atlantic Ridge

The Mid-Atlantic Ridge is a fracture zone that runs through the middle of the Atlantic Ocean,from a point near Antarctica toa point north of Iceland.

This Ridge is the site of ocean-floorspreading, a process that makesnew ocean floor and moves the two plates (North American Plateand Eurasian Plate) laterally,west and east.

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The Mid-Atlantic Ocean Ridge 33

Fig 7-7a, p.159

Lava rising along the long fracture that constitutesthe Mid-Atlantic Ridge forces the two plates on theleft and right to move laterally to make room for the newly forming crust.

This continuing replacement of the Atlantic Oceanfloor ensures that the oceanic crust is very young.

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Crustal platesclose to home:the Juan de FucaPlate is movingeast under theNorth AmericanPlate, causing the Cascadevolcanoes fromN. Californiato BritishColumbia

Earthquakes and eruptionsof Mt. St. Helens are themost recent consequences of this motion

352. Continental

Drift

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39 Rates of plate tectonic motion

How can we measure this?

1. Date ocean sediments (determine age of sediments and measure distance from ridge)

2. Magnetic reversals (date magnetic reversals and measure distance from ridge - wider stripes indicate faster plate motion)

3. Satellite laser ranging techniques (bounce laser beams off a satellite and to a station on another plate - time to reach station is proportional to distance traveled)

4. Hot spots (provide a fixed reference point to allow absolute motion to be determined)

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Rates of plate tectonic motionWhat do we find?

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Rates of plate tectonic motionWhat do we find?

43 Rates of plate tectonic motionWhat do we find?

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3. Volcanoes3 50

Forces that Shape the Face of the Earth• Volcanism

– 10-30 volcanoes erupt every year– New land is created– Lava flows cover hundreds of km2

– Gases (H2O, sulfur dioxide, CO2) enter the atmosphere

51 Heimaey, Iceland, 1973 Tolbachik, Kamchatka, 1975 52

Fig 7-5, p.157Lava flow in Hawaii

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Fig 7-CO, p.152

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The Hawaiian Island Chain has been building on a Pacific Basin Hot Spot for 50 Million Years

55Mid-OceanVolcanoes

(e.g., Hawaii)

A pocket of liquid rock (magma) remains stationaryin the upper mantle, whilethe Pacific plate slowlymoves across it.

Continued eruptions from the hot spot build volcanicmountains (eventuallyislands) in a chain, as thePacific plate moves along.

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574. Rocks 58

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The History of the Earth61

• Earth is a complex system of interacting elements –

• Lithosphere• Atmosphere• Hydrosphere• Biosphere• Magnetosphere

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Notes

• Clean Up Part B• Need overview of california and its faults,

local geography of hayward etc.• 1906 earthquake stuff?

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