Solid Earth

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Solid Earth

• Earth Convection
• Plate Tectonics

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"But is it not well-known that heat increases one
degree for every seventy feet you descend into
the earth? Which gives a fine idea of the central
heat. All the matters which compose the globe are
in a state of incandescence even gold, platinum,
and the hardest rocks are in a state of fusion.
What would become of us?" "Don't be alarmed at
the heat, my boy." "How so?" "Neither you nor
anybody else know anything about the real
state of the earth's interior. All modern
experiments tend to explode the older theories.
Were any such heat to exist, the upper crust of
the earth would be shattered to atoms, and the
world would be at an end."
Jules Verne, Journey to the Center of the
Earth 1864
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Himalayas and the Tibetan Plateau elevation model
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Himalayas and the Tibetan Plateau Shuttle
photograph
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Himalayas and the Tibetan Plateau Shuttle
photograph
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Mt. Everest
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Mt. Everest from the shuttle
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Mt. Rainier
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Mt. Rainier
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Hawaii
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Hawaii
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Origins of the theory

• Taylor, 1910, first proposed an idea with ancient
  land masses and movement of continents
• Wegener, 1914, proposed Pangaea, based on the
  similarity of coastlines and fossil evidence
• This idea almost died with Wegener in 1930

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Basic theory of solid earth circulation

• The earths core is extremely hot
• More than 5000 K
• Source of heat is radioactive decay
• Temperatures are observed to increase with depth
  in rock
• 15-75 degrees C per km
• A conductive process
• Magma reaches the earths surface through
  convection

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Convection

• Kind of odd to think of convection in a solid
• Yet glaciers, a solid, can bend and deform
• Very hot rocks can convect at very slow rates

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The basic theory convection and conduction
Convection
Conduction
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Solid crust, plastic core

• Kind of like a pot of simmering clam chowder
• Forms a skin on top
• Moving around underneath
• The outer surface, the lithosphere, is about the
  thickness of a light bulb compared to the bulb
  itself
• The lithosphere floats on top of the
  asthenosphere, a weaker, hotter, more pliable
  layer

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Conceptual diagram of the lithosphere and the
asthenosphere. Other features continent plates
are less dense than the ocean plates and thus
tend to float on top
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How do we know this happens?

• Magnetism
• From studies of the magnetic orientation of rocks

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The earth is a giant magnet caused by the
rotation of a magnetic body. In this case,
convection of liquid iron.
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• Magnets will exhibit an inclination from the
  horizontal as a function of latitude
• Equator no inclination
• Pole 90 degrees
• Some kinds of rocks will maintain a signature of
  this magnetic inclination

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For the mineral magnetite Above the Curie point
of 580 degrees C, no magnetic orientation is
possible. Once cooler below 580, if there is a
magnetic field present, the atoms will orient in
the direction of the magnetic field
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Paleomagnetists observed this phenomenon in the
1950s wandering of the apparent poles People
concluded that perhaps Wegener was
correct. Perhaps the continents, not the poles,
had moved
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Sea floor further evidence. People began to
notice that parallel bands of the sea floor had
reversed polarity Thus the sea floor contained a
record of past magnetic conditions
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Three lines of evidence

• Wanderings of the apparent pole
• Sea floor polarity
• Volcano distribution
• Led to increased interest in the theory of plate
  tectonics in the 1950s

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Age of the ocean sea floor based on polarity data
and radioactive dating of lava
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Global distribution of earthquakes
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Plate tectonics

• The data from all these sources gives geologists
  the ability to infer the size and movement of
  plates

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Oceanic and continental plates
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Oceanic and continental plates
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Paleomagnetic and satellite data can both
precisely show how fast the plates are moving.
Both approximately the same speed
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Boundaries between plates

• There are three general kinds of plate boundaries
• Divergent
• Convergent
• Transform
• Each has unique geological consequences

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Divergent margins Begin on continents Create rift
valleys (Rio Grande and Africa) Spread the
continents apart Filled with ocean over
time Characterized by shallow earthquakes
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Convergent margins The subduction of the ocean
lithosphere into the asthenosphere. Creates many
kinds of volcanoes, island chains, and mountain
ranges. Deep focus earthquakes occur as a result
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Convergent margins

• Ocean/Ocean
• One sinks beneath the other
• Produces island arcs
• Ocean/Continent
• Ocean plate goes under the continent
• Produces mountain ranges like the Sierras and the
  Andes
• Also produces major ocean trenches, the deepest
  in the world

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Collision zones A kind of convergent
margin Occurs after the oceans are forced out of
the way and continents collide. Produces the
worlds largest mountain ranges Alps, Urals,
Himalayas, and Appalachians are all collision
ranges
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Balance of convergent/divergent

• One of the early problems with plate tectonics
  theory was that it seemed the earth must be
  expanding
• Plates are produced at divergent margins at
  roughly the same rate they are consumed at
  convergent margins

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What drives the plate movements?

• Some think it is a push from the divergent zones
• Others think there is a pull on the slabs from
  the subduction zones
• No one really knows, and it is probably a
  combination

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Transform faults The movement of two plates past
each other Most famous is the San Andreas fault
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Other evidence of plate tectonics

• Hot spots, such as those forming the Hawaiian
  Islands
• Not at a margin
• Formed by plate movement over a permanent hot
  spot
• Dating material on the islands tells us how fast
  the plates are moving

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Age of island in the Hawaiian chain
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Effects of plate tectonics

• Major rain shadows
• Tibetan Plateau
• Eastern Washington and Oregon
• Existence of many island nations
• Sumatra and Java in Indonesia
• Distribution of plants and animals, both on land
  and in the ocean
• Earthquake and volcano distribution

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Recycling time of the continents

• Most evidence suggests that supercontinents will
  reconverge every 500 million years
• Hard to prove because the oldest ocean sediments
  are only 200 million years old