Plate Tectonics

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Plate Tectonics
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Plate Tectonics

• Plate tectonics is a relatively new idea. Only
  widely accepted since the 1960s!
• It is a revolutionary theory that ties together
  many, seemingly unrelated, observations about the
  Earth and the processes that go on.
• Answers questions like
• Why do mountains and volcanoes occur in chains?
• Why do earthquakes occur and why do they occur
  where they do?
• Why is Earth so different from other planets?
• What is the history of the continents and ocean
  basins? Why are they different?
• How does the earth lose its internal heat?
• What controls the distribution of igneous,
  metamorphic, and sedimentary rocks?

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Plate Tectonics

• Stemmed from the development of two key concepts
• Continental Drift -- plate motion
• Seafloor Spreading -- how the plates move

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Plate Tectonics

• Even early on, people began to hypothesize that
  Earths past geography was different.
• Early suspicions based on the fit of continents
  (like a jigsaw puzzle) and the distribution of
  similar rocks and fossils.

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Fig. 2-2, p. 30
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Continental Drift

• Alfred Wegener (1915) developed the hypothesis.
  Later developed more by Alexander du Toit (1937).
• Wegener proposed that all landmasses were once
  joined together into a supercontinent he called
  Pangea.
• Put together all geologic, paleontologic,
  climatologic, and other data suggesting the
  continents must have been in different
  configurations in the past.

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Pangea
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Continental Drift Evidence

• Geometry of continental margins
• Similarity of rocks and structures
• Correlation of glacial deposits
• Distribution of fossils
• Paleomagnetism and polar wander
• Topography and other features of the seafloor

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Fig. 2-3, p. 31
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• South America and Africa fit almost perfectly.
• Not at the shoreline, but at the true edge of the
  continental margin (at the bottom of the
  continental slope).
• ? South America and Africa were joined in the
  past and split apart when the Atlantic Ocean
  opened

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Fig. 2-4, p. 31
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Fig. 2-5, p. 32
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The now separate Appalachians of North America
and the Greenland, Irish, British, and Norwegian
Caledonides were the same, continuous 430 Ma
mountain chain within Pangea up until about 250
Ma.
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• Paleozoic glacial deposits and features are
  common among the now separate southern
  continents
• But they are now near the equator! Huh?
• Also, the direction of ice movement is from what
  is now the sea towards the interiors! Huh?
• These observations only make sense if the
  southern continents were once joined and were at
  the south pole!

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Fig. 2-6a, p. 32
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Fig. 2-6b, p. 32
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Glossopteris seeds too heavy to be blown across
oceans. Mesosaurus couldnt swim too
far. Lystrosaurus and Cynognathus lived on land.
Fig. 2-7, p. 33
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Fig. 2-8, p. 34
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Paleomagnetism

• Study of the remnant, ancient magnetism recorded
  by some rock-forming minerals (like magnetite).
• Remnant magnetism records the orientation of the
  Earths magnetic field at the time and place the
  rock formed (e.g. the time a sediment lithified
  or metamorphic/igneous rock cooled below certain
  temperature).

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Earth has magnetic field generated by motion of
liquid and solid Fe-Ni core. Earths magnetic
field is like that of a bar magnet, with north
south magnetic poles. These are different than
the geographic poles (rotation axis).
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Fig. 2-9a, p. 34
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Declination (D) angle compass needle makes with
geographic North pole. Inclination (I)
angle compass needle makes with respect to
horizontal (dip). Varies with latitude. D
and I change with time due to plate motions and
magnetic field reversals
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Fig. 2-9b, p. 34
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Curie point Temperature at which magnetization
is frozen in.
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Fig. 2-11, p. 36
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Earths magnetic field reverses itself sometimes.
North magnetic pole becomes south magnetic pole
and vice versa. Reversals are recorded in the
rocks
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Fig. 2-13, p. 37
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Magnetic Stripes
Formed as new ocean crust is formed at the
mid-ocean ridge. We can match the pattern with
the known timescale to get age.
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We can measure magnetic stripes on the
ocean floor. They are symmetric! Suggests
seafloor spreading!
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Fig. 2-12, p. 36
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Fig. 2-14, p. 38
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Apparent polar wander. Remnant magnetization of
old rocks tells what latitude they formed and
relative position of north pole at that
time. Magnetization of rocks from same place, but
of different age, shows apparent motion of the
pole with time.. Best explanation is other way
around the rocks (and the continent they are
on) moved and the pole stayed put. Matching
polar wander paths of same age from different
continents can help us reconstruct past geography.
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Fig. 2-10, p. 35
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Seafloor Spreading

• Solution to the problem of how continents drift
  they dont, really.
• The plates move carrying both the ocean and
  continental crust with them!
• Oceanic crust is made at mid-ocean ridges. New
  seafloor moves laterally away from ridges.
• Oceanic crust is consumed at subduction zones.
  Old seafloor is recycled.
• Plates move as a consequence of both processes.
  Plates carry both the oceans and the continents
  along for the ride.

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Fig. 2-15, p. 38
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Oceanographic studies show existence of
continuous, 65000 km mid-ocean ridge with
symmetric magnetic stripes. Also no granitic
material in the oceans all young, thin, mafic
crust.
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Ocean crust is very young! Must be continually
consumed and replenished.
Seafloor gets younger towards mid- ocean
ridge! Ages are symmetric just like
the magnetic stripes! Must be seafloor
spreading!
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Plate Boundaries

• Divergent boundaries
• Convergent plate boundaries
• Subduction zones
• Collision zones
• Transform boundaries

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Table 2-1, p. 42
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Divergent Boundaries

• Plates move away from each other
• Mid-ocean ridge spreading centers
• Create new oceanic crust
• Gentle, basalt lava volcanoes
• Shallow (lt10km) small to medium earthquakes
• Examples Iceland, mid-Atlantic Ridge
• Continental rifts
• Tears continent in two to create an ocean
• Examples Rio Grande Rift, East African Rift

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Fig. 2-22, p. 47
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Fig. 2-18a, p. 44
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Fig. 2-18b, p. 44
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Fig. 2-17a, p. 43
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Fig. 2-17b, p. 43
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Fig. 2-17c, p. 43
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Fig. 2-17d, p. 43
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Convergent Boundaries

• Subduction Zones
• Oceanic crust vs. oceanic crust
• Oceanic crust vs. continental crust
• Collision Zones
• Continental crust vs. continental crust

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Subduction Zones

• Destroys oceanic part of a plate as ocean crust
  is subducted under other plate and melts
• Associated with deep trenches
• Explosive volcanoes
• Island arcs (Philippines, Aleutians)
• Volcanic arcs (Andes, Cascades)
• Large, deep earthquakes (m7, lt700km)

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Fig. 2-19a, p. 45
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Fig. 2-19b, p. 45
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Fig. 2-20a, p. 46
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Fig. 2-20b, p. 46
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Collision Zones

• Large mountain belts (orogens) and plateaux.
  Fold and thrust belts.
• Some initial volcanism. Mostly plutonism
  (intrusive igneous activity)
• Many deep, strong earthquakes
• Extensive deformation and metamorphism as rocks
  are folded and faults and continents are fused
  together
• Examples Himalayas, Alps

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Fig. 2-21a, p. 46
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Fig. 2-21b, p. 46
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Cratons
Accreted Terraines and Mountain Belts
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India vs. Asia Began ca. 45 Ma Still going
on Made Himalaya and Tibet
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Transform Boundaries

• Plates slide past each other, side-by-side, with
  horizontal motion.
• Transform one type of motion or location of
  motion to another.
• Few volcanoes
• Strike-slip earthquakes with variable depths
• Examples seafloor fracture zones San Andreas
  Fault zone

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Fig. 2-23a, p. 47
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Fig. 2-23b, p. 47
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Fig. 2-23c, p. 47
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Fig. 2-24, p. 48
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Plate Motions

• Why do plates move?
• How fast to plates move? In what direction?
• How have they moved in the past?
• How do we know?

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Hot Spots

• Plumes of hot material from the mantle
• They are likely stationary with respect to plate
  motion
• They can be used as reference points to show
  absolute plate movement

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Fig. 2-25, p. 49
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Fig. 2-16, p. 39
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Measuring Plate Motions

• Hot spot tracks age and spacing gives rate
• Magnetic stripes age of stripes and width gives
  rate
• Paleomagnetism APW paths
• Reconstructions based on geologic correlations
• VLBI, GPS modern plate velocities

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Fig. 2-26a, p. 50
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Fig. 2-26b, p. 50
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GPS plate velocities (modern)
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What Causes Plate Tectonics?

• Earth is a heat engine. It has internal heat
  that is being released. The most effective way
  to do that, given the material properties of the
  mantle at mantle P and T conditions is
  convection.

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Fig. 2-27a, p. 50
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Fig. 2-27b, p. 50
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What Drives Plate Motion?

• Plates are moved about. How.
• Basal drag. The motion of the asthenosphere
  carries the plates along
• Ridge push. Spreading at the mid-ocean ridge
  pushes the rest of the plate from the side.
• Slab pull. Subduction at subduction zones pulls
  the cold dense plate down into the mantle.
• Gravity. Ocean crust has a slope, from high at
  mid-ocean ridge to low at subduction zone.
  Plates just go downhill.

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Fig. 2-28, p. 51
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Wilson Cycles

• Idea that supercontinents form over and over
  again in earth history.
• There may be a cycle of about 500 million years
  over which they form, break up (100 million years
  later), and subsequently re-form

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I
II
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III
1. Rift open a continent 2. New ocean with
passive margins 3. Passive margins break 4.
Subductions zones form 5. Oceans consumed 6.
Continents collide 1. Rift open a
continent Rinse and repeat ?
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Supercontinents

• Columbia - all continents (?) 1.5 billion years
  ago (?)
• Rodinia all continents 1 billion to 600
  million years ago
• Gondwana, Larentia, Baltica, etc. smaller
  supercontinents 600 to 300 million years ago
• Pangea all continents 300 to 230 million years
  ago

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Consequences of Plate Tectonics

• Distribution of earthquakes and volcanoes
• The rock cycle
• Natural resources
• Evolution of Life
• Climate

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Plate Tectonics and Resources

• Hydrothermal activity at plate boundaries
  (subduction zones and mid-ocean ridges) tends to
  concentrate metallic mineral deposits.
• Zinc, copper, sulfides, etc.

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Fig. 2-29a, p. 52
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Fig. 2-29b, p. 52
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Plate Tectonics and Life

• Motion of continents changes the geography of
  oceans and continents.
• Contributes to changing habitats, environments,
  climate (changes in ocean, atmospheric
  circulation).
• Contributes to isolation/mixing of populations
  (changes in physical barriers) ? speciation
  results.
• Contributes to increase/decrease of ecological
  diversity ? increase/decrease in number of
  species
• Plate tectonics recycles material necessary for
  life
• Life may have begun in divergent plate
  boundaries
• Humans originated in a divergent plate boundary

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Fig. 2-30a, p. 53
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Fig. 2-30b, p. 53
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Plate Tectonics and Climate