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Chapter 7 Structure of the Earth and Plate
Tectonics (the very short version)
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• Most processes operating near the Earths surface
  are related to conditions deep down
• operate to produce the land forms and features we
  see
• How do we see whats down below?
• For the most part, we dont
• Yet, we have our ways

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• Drill a well obtain actual samples
• Distance from the surface to the center of the
  Earth is about 6456 km (4035 mi)
• Deepest well drilled about 12.3 km (7.7 mi) deep
  deepest mine about 4 km (0.2 0.06)
• Thus, doesnt tell us much about the deep
  interior
• Volcanic activity brings samples from the depths
• However, only from about 200 km (125 mi) deep
• Study of meteorites (later.)

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• Most info by analysis of seismic (earthquake)
  waves
• Measuring the travel times of waves from an
  earthquake, nuclear explosion, or other large
  source, to a number of seismographic stations
• Nuclear tests and mining blasts are best, as the
  exact time and location of the source of the
  energy is known
• And well leave the details out here

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The major boundaries of the Earth
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• Composition of the core
• Metal, most likely iron-nickel, with a few other
  materials
• Based on overall density of the Earth
• Study of meteorites (more on these later)

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• The mantle
• Both layers of the mantle comprise over 82 of
  Earths volume
• What is known is based on experimental data and
  examining material brought to the surface by
  volcanic activity
• i.e., kimberlite pipes likely from about 200 km
  depth, roughly peridotite in composition
• Overall composition thought to be that of the
  mineral olivine

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• The crust
• The part we can touch feel
• Two types, with somewhat different properties

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• Oceanic crust
• 5 to 12 km thick
• Average density 3.0 g/cm3
• Dominantly basalt in composition
• Oldest is about 200 million years

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• Continental crust
• Thicker than oceanic crust, especially beneath
  mountains (up to 60 km)
• Less dense averages 2.7 g/cm3
• Composition averages to granodiorite (similar to
  granite)
• Up to 4 billion years old

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Continental crust
Oceanic crust
A view of the crust
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Plate tectonics

• The current view regarding the dynamics of
  Earths geology
• Many geologists did not accept the theory until
  the 1970s many older geologists still
  resisted
• When I got my degree in the 70s, we had to know
  which prof believed in plate tectonics, and which
  didnt
• Concept took time to be fully developed

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• Fit of the continents
• Jigsaw fit of South America/Africa,
  Greenland/North America/Europe
• Seen by early mapmakers

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• Eduard Suess (not Dr. Suess)
• Austrian scientist, late nineteenth century
• Noted geologic similarities between India,
  Africa, South America
• Developed a theory about breakup of a large
  continent named Gondwanaland

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• Alfred Wegener
• German geographer/meteorologist, early 1900s
• Built on the ideas of Suess he named the
  continent Pangea
• One part was Laurasia (N. Am. Eurasia), the
  other part was Gondwana
• These all surrounded by a large ocean named
  Panthalassa
• Continental drift the continents moved about on
  denser materials below them
• Main objection how can a continent move about
  over the ocean crust?

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Wegeners idea of the breakup
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• Global geography
• Already mentioned based on fit of the coastlines
  we see
• Fit is better at the edges of the continental
  shelve

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Computer fit of the continental shelves (depth of
edge about 3000 ft)
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• Paleoclimate
• What was the weather like in the past?
• Evidence of glaciers at the same time in S.
  America, southern Africa, India, Antarctica,
  Australia, Tasmania
• Grooved rock surfaces
• Reassemble those continents together, the grooves
  match

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• Tropical trees lack annual growth rings
• Coal deposits sometimes have fossil logs lacking
  growth rings
• Most coal deposits now in the U.S., Great
  Britain, Germany, Russia (i.e., not tropical)
• Suggests the areas now having coal deposits were
  once in the tropics

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• Evaporites reefs
• Both form today in areas between 30º north or
  south of the equator
• Ancient deposits are found in areas located well
  north or south of this zone

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• Fossil evidence
• The plant Glossopteris
• Grew on land, found in S. America, Africa,
  India
• Seeds too heavy to transport by wind
• Not likely to develop identically on all 3 if
  separated as they are today

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• Vertebrates
• Several, for example, Mesosaurus
• Aquatic reptile, fossils found only in southern
  Africa southern Brazil
• Skeleton surrounding sediments indicate they
  lived in fresh water could not cross the ocean
• A few other critters also indicate the continents
  were once joined

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• Rock sequences
• In addition to the fossils, look at the rock types

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• Paleomagnetism
• Magnetic properties acquired by rocks when they
  formed
• Earths magnetic field acts as if there was a
  large bar magnet at the center
• Magnetic poles do not coincide with the
  rotational (geographic) axis
• Positions slowly change through time
• Averaging over time, they are essentially the
  same

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Dipole model of Earths magnetic field
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• Magnetic recording media in the Earth
• Volcanic rocks are best they crystallize
  quickly
• As lava cools, crystals of magnetite often form
• A form of iron oxide that can be magnetized
• As they form, they align with the existing
  magnetic field
• Magnetite grains can also settle in sediments
  their magnetization will also align with the
  existing field
• The grains act as free-floating compass needles
• We actually measure magnetic properties, not the
  grain orientation
• Point to the poles
• Tilt (inclination) of the magnetization indicates
  the latitude at which the rock formed

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Inclination of a freely suspended compass needle
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• Apparent polar wandering paths
• Based on the paleomagnetic data, past pole
  positions were in different positions relative to
  a particular continent
• Appears as if the poles moved about

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Apparent polar wandering
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• Present plate tectonics theory
• Lithosphere
• Crustal rocks the upper, rigid part of the
  mantle
• Broken into several plates
• 7 large ones, 20 smaller
• Asthenosphere
• Weak, partially molten part of the upper mantle
• Lithosphere plates float on this zone

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Earth divisions (not to scale)
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Major tectonic plates
Distribution of earthquakes
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• Three types of plate boundaries
• Divergent
• Transform
• Convergent

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• Divergent boundaries
• Locations where plates move apart
• The trailing edge of a plate
• Earthquakes, volcanic eruptions
• Mid-ocean ridges
• New crust created
• Continents moved about as new material is added
• Seafloor spreading
• May also occur on continent
• Rift valleys (East African Rift)

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Midocean ridges (red) trenches (blue)
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Divergent boundary
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• Transform boundaries
• Basically, a result of moving things around on a
  sphere
• Plates move sideways past each other
• Nothing formed or destroyed
• Earthquakes, generally little igneous activity

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Transform faults along midocean ridges
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San Andreas fault a transform boundary between
plates
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• Convergent plate boundaries
• Two plates move together collide
• The leading edge of a plate
• Earthquakes, compressional mountains, deep-sea
  trenches
• What happens depends on density of the plates
• 3 types of convergent boundaries, 2 associated
  with suduction zones
• Subduction zone
• One plate plunges beneath another
• Sediments other rocks pulled into mantle,
  heated
• Water in the materials causes partial melting

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• Continental-continental crust
• The 2 plates are continental crust
• These rocks are comparatively bouyant, and thus
  are not subducted
• Rocks are folded, faulted, often metamorphosed
• Ex. India colliding with Asia (Himalaya Mtns)

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Continent continent collision
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Collision of India with Asia
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• Oceanic-oceanic crust
• Both plates composed of oceanic crust
• One plate subducts (is pushed) beneath the other
• Deep-sea trenches
• Volcanic arcs develop on the overriding plate

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Volcanic arc
Ocean-ocean crust convergence
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• Continental-oceanic crust
• Continental crust is less dense than oceanic
  crust
• Oceanic crust subducts beneath continent
• Creates a deep-sea trench offshore from an
  on-shore mountain range
• As they collide, the lower part of the
  continental plate undergoes deformation melting
  
• Volcanic activity lava is a mix of granite
  basalt
• Andesite named from extensive flows in Andes
  Mtns
• Some other terms

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• Accretionary prism (or wedge)
• Material scraped off the descending plate
• Contorted metamorphosed
• Melange
• The resulting mass of complexly folded, deformed
  rock in the prism
• Ophiolite suite
• A specific sequence of rocks in the wedge
• Pieces of oceanic crust that were obducted
  (thrust up on continental crust)

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Continent-ocean convergence
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Oceanic rock ages younger nearest midocean
ridge, oldest far away oldest about 200
million years old
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• Seismic evidence
• Location depth of earthquakes in certain spots
• Wadati-Benioff zones
• At convergent boundaries zone slopes at about a
  45º angle

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• Hot spots
• Deep-seated plumes of hot, upwelling mantle rock
• Plate moves over the hotspot, forming new
  volcanoes
• Example Hawaiian Island chain

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Hawaiian Island chain
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Other hotspots
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• Microcontinents
• Small pieces of continental-type crust surrounded
  by oceanic crust
• Fragmented from a larger continent, or formed by
  volcanic activity
• Identified by granitic composition, seismic wave
  velocity, elevation above seafloor, lack of
  earthquakes

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Seychelles Bank, a microcontinent
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• Exotic terranes
• Incorporation of a microcontinent to the margin
  of a larger continent
• The geology of these does not match that of the
  surrounding geology
• Identified on every major landmass
• One bit in the Andes (western S. Am.) appears to
  have originated from the southern U.S. (well see
  this later)
• May also be formed by oceanic crust fragments

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Exotic terranes of western North America well
see these later, too