Terrestrial Planetary Geology: Processes in the Earth

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Terrestrial Planetary GeologyProcesses in the
Earth
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Results of 2nd Exam

• Raw mean 56.81 so curve of 18 pts
• Curved mean 74.81 ? 11.19
• Distribution High, 102 Low, 54 90
  10 80 26
• 70 26
• 60 26
• lt 60 8
• Random guessing 52 (after curve) -- no one did
  quite that badly

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Tides

• are due to the DIFFERENTIAL force of GRAVITY.

M and D are the mass and diameter of object on
which tides are raised m the mass of the object
raising tides and r the distance between their
centers. TWO HIGH TIDES a day because of Earth
rotating through BOTH the FRONT and BACK bulges.
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Lunar Tides Raised on Earth
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Lunar Solar Tides

• SPRING (strong) TIDES new moon and full moon
  the Sun's weaker tide and the Moon's stronger
  one add up
• NEAP (weak) TIDES first and third quarter moon
  the Sun's weaker tide partially cancels the
  Moon's stronger one

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Tides Affect the Earths Rotation
Energy is dissipated by oceans rubbing against
crust the Earth's spin slows down (but only 1.5
ms/century!) About 500 Myr ago, the day was 22
hours long and the year had 397 days!
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Conservation of Angular Momentum

• as the Earth's rotation slows, decreasing its
  angular momentum (AM), the Moon's orbital AM
  increases--the Moon moves slightly further away!
• Eventually no more total solar eclipses!

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Earths Differentiated Interior
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CRUST

• Mostly made of ROCKS.
• The main rock classes are Igneous (of
  volcanic origin) Sedimentary (compressed _at_
  ocean, lake bottoms) Metamorphic (either igneous
  or sedimentary placed under so much pressure
  that their crystalline structure changes)
• Composition mostly SiO2 (quartz-like), Al2O3
  (granite-like) and other elements combined w/
  Silicon,Aluminum and Oxygen
• Oceanic Crust Covers 3/5 of earth, averages 7
  km thick, ? 3.0 g cm-3 all lt 200,000,000 yr
  old
• Continental Crust Ave. 36 km thick ? 2.8 g
  cm-3 some very old, up to 4 Gyr

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MANTLE

• Plastic rocks --solid but will yield mostly
  Fe, Mg, Si, O (basalt-like) About 2900 km
  thick, contains most of earth's mass ??? 5 g
  cm-3 T 1800 K Heated mainly from below,
  convection currents are present.
• RECALL MODES OF HEAT TRANSPORT Radiation
  microscopic (photon) mode works best in vacuum,
  well in low density gases Conduction
  microscopic (molecules bumping) mode works best
  in solids, then liquids, then gases Convection
  macroscopic (large-scale) motions of blobs of
  matter works in liquids, gases and (very
  slowly) in plastics

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Interior Density and Temperature
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CORE

• Metallic mostly Fe (iron), Ni (nickel), (little
  S/O) In toto, about 3500 km radius or 1/6 of
  Earth's volume.
• OUTER CORE liquid, about 2200 km thick
  ??? 11 g cm-3 T 4000 K Temperature
  wins
• INNER CORE solid, about 1300 km radius ?c
  13 g cm-3 Tc 5200 K Pressure wins
  over temp.

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DIFFERENTIATED via MELTING

• Separation into metallic core and rocky mantle
  could be explained either if
• Core formed first, mantle and crust added later,
  OR
• Entire Earth formed together but was molten early
  on the denser (metallic) liquid sinks to the
  center.
• This 2nd hypothesis fits the evidence better.
  COLLISIONAL HEATING (formation) plus RADIOACTIVE
  HEATING (fission) plus continued FRICTIONAL
  HEASTING (differentiation) can nicely explain
  current temperatures. Mostly RADIOACTIVE HEAT
  now.

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Idea Test

• Recall the formula for tidal forces
• Now, imagine the Earth had a second Moon, called
  Nemoon at twice the distance of the real Moon.
  Also imagine that the mass of Nemoon is 4 times
  that of the Moon. You would then conclude that
  the tides due to Nemoon would be about _____
  times as high as our actual tides.
• A. 1/8
• 1/2
• 1
• D. 2
• E. 4

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Idea Test

• Recall the formula for tidal forces
• Now, imagine the Earth had a second Moon, called
  Nemoon at twice the distance of the real Moon.
  Also imagine that the mass of Nemoon is 4 times
  that of the Moon. You would then conclude that
  the tides due to Nemoon would be about _____
  times as high as our actual tides.
• A. 1/8
• 1/2 Ratio is 4/23 4/8
• 1
• D. 2
• E. 4

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THE ACTIVE EARTH

• TECTONIC ACTIVITY The Earth's CRUST is
  divided into PLATES.
• CONTINENTAL DRIFT Africa and S. America had
  similar shapes and rock formations. CONVECTION
  in the MANTLE drags the plates.
• SEAFLOOR SPREADING at OCEANIC RIDGES where new
  crust solidifies over hot-spots. REVERSALS in
  Magnetic Polarity are frozen into newly
  solidified rocks this allows dating of rocks and
  plotting of drift back some 500 Myr. These
  reversals occur roughly every 500,000 years.

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The Earths Crustal Plates
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Plate Tectonics Seafloor Spreading
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Magnetic Reversals Date Oceanic Crust

• Magnetic Fields are Frozen in Rocks as N and S
  poles move and switch, these fields can date when
  rocks solidified from magma.

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Plate Tectonics Where Plates Collide

• Earthquakes either stick, then slip (TRANSFORM
  FAULTS) or Oceanic is SUBDUCTED under
  Continental (melts and rejoins the mantle).
• MOUNTAINS built from folding in COLLISIONS.
• VOLCANOES most likely at plate boundaries too.

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Mantle Convection Drives Plate Tectonics
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Earths Magnetic FieldMagnetic Field Generated
in Liquid Iron Outer CoreCharged particles
trapped in the Van Allen belts
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Earths Magnetosphere is BIG
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Magnetosphere Protects us from Solar Storms

• When charged particles penetrate the
  atmosphere AURORA BOREALIS AURORA AUSTRALIS
  ("Northern/Southern Lights") (the light is
  from recombination emission lines of Nitrogen
  and Oxygen)
• Solar storms disrupt communications, power grids.