similarity between late paleozoic plant fossils of indian,

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How the Earth Works
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Early Observations

• Fit of the continents ? 1596, Abraham Ortelius,
  Thesaurus Geographicus
• similarities between fossils found on widely
  separated continents ? Antonio Snider -
  Pelligrini, 1858
• Similarity between late Paleozoic plant fossils
  of Indian, Australia, South Africa, Antarctica ?
  Edward Suess, 1885

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• Evidence of glaciation preserved in rocks of
  India, Africa
• Distribution of earthquakes and volcanoes

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Antonio Snider-Pellegrini, 1858
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Explaining the Observations

• Land bridges
• Break-up of early large polar continents by
  gigantic tidal forces,
• Atlantic mid-ocean ridge marks the spot
• Frank Taylor, 1910

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Theory of Continental Drift

• Alfred Wegener
• The Origin of Continents and Oceans, 1915
• break-up of original supercontinent, Pangaea
• theory based on wide range of geological ,
  paleontological, and climatological evidence

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• Mountain ranges and glacial deposits match -up
  when continents are placed together
• Shorelines of continents fit together
• Rock record indicates that global climate belts
  were not always parallel to equator?
• How to explain observations?
• Wegener ? move continents
• Mechanism?

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• Principal proponent Alexander duToit, Our
  Wandering Continents, 1937
• proposes that Pangaea initially rifts apart into
  2 parts
• Laurasia
• Gondwanaland
• later into smaller continents
• Some support in Europe,
• Little support in North America

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Further Observations

• Paleomagnetic evidence 1920 ?
• when magma cools iron-bearing minerals align
  themselves with the Earths magnetic field
• records both intensity and direction
• polar wandering?

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Magnetic reversals 1950s ?

• normal north and south magnetic poles located
  approximately at the north and south geographic
  poles
• at times has completely reversed
• discovered on the continents, later verified in
  ocean basalts

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Vine-Matthews Hypothesis, 1963
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Theory of sea-floor spreading

• Harry Hess, 1962
• Proposes sea-floor spreading to account for
  movement of continents
• continents do not move across oceanic crust
• continents and oceanic crust moving together
• Robert Dietz - adds asthenosphere

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Heezen and Tharp
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• sea-floor separates at oceanic ridges where new
  oceanic crust is being formed by upwelling magma
• as magma cools, newly-formed oceanic crust moves
  laterally away from ridge
• Mechanism? Remember Wegener!

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• Thermal convection cells in mantle
• Hot mantle rises (more buoyant) from the mantle
• Intrudes along rift zone fractures defining
  oceanic ridges
• Cold crust subducted back into the mantle at
  deep-sea trenches
• Reheated and recycled ? completing a thermal
  convection cell

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Imagery courtesy of Stacey Tighe, University of
Rhode Island
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Confirmation

• How could Hesss theory be confirmed?
• Magnetic surveys of the ocean floor revealed that
  the magnetic reversals preserved in the oceanic
  crust were both parallel to and symmetrical with
  oceanic ridges
• drilling indicated ocean basins are geologically
  young features
• oldest oceanic crust, 165,000,000 years old

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• Oldest continental crust, 3.96 billion years
  old
• Rocks on the ocean floor youngest near mid-ocean
  ridge, older as we move away

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How the Earth Works

• The Composition of the Earth

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(Adapted from, Beatty, 1990.)
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W.J. Kious and R.I. Tilling, This Dynamic Earth
The Story of Plate Tectonics, U.S. Geological
Survey, 1996.
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• Consider the composition of the Earth in terms
  of
• chemical makeup
• primary elements
• physical properties

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Chemical Composition
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Core

• Central zone of the Earth
• Begins at a depth of 2,900 km
• Composed of Fe-alloys
• outer core
• liquid (balance of P and T)
• Fe, Ni, S
• inner core
• solid (due to tremendous pressure)
• Fe

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• Core-mantle boundary (CMB) defined by the
  Gutenberg discontinuity
• Important thermal boundary above the core-mantle
  boundary (CMB) defined by extreme variation in
  the movement of seismic waves
• Zone of 200 - 300 km, designated D11

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Mantle

• Zone surrounding the core
• 83 of the Earths volume
• lt dense than the core
• entire mantle is composed of two silicates of Fe
  and Mg
• Olivine (Mg, Fe)2SiO4
• Pyroxene (Mg, Fe)SiO3

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• These minerals change their forms at different
  pressures.
• As a result there are boundaries in the mantle
  which represent changes in the phases, or atomic
  packing, of these minerals.
• The two most important phase changes take place
  at depths of 410-km, and 660-km.

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• In the upper mantle (above 660-km) olivine and
  pyroxene are found as peridotite
• In the lower mantle (below 660-km) olivine and
  pyroxene form perovskites and a much smaller
  amount of an oxide, magnesiowüstite

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• The 660-km boundary is important for several
  reasons
• represents the deepest earthquakes
• the phase change of the minerals at this depth
  can be duplicated in the laboratory, temperature
  at that depth must be 1700C

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Crust

• Mantle - Crust boundary established by the
  Mohorovicic discontinuity
• Ranges in thickness from 10 - 70 km
• 2 varieties
• 1. oceanic
• 2. continental

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Oceanic crust

• thin crust under the ocean
• basaltic
• average thickness of about 8 km
• 2-km pillow basalt
• 6-km gabbro
• density of 3.2 g/cm3
• composition rich in Ca, Mg, Fe
• lt 50 SiO2

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Continental

• From 30 - 70 km thick
• thinnest where continents are being pulled apart
• thickest under mountains
• average density 2.7 g/cm3
• granitic
• composition rich in Si, Al, Na, K
• gt 50 SiO2

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Physical Properties
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lithosphere

• rigid outer shell of the Earth
• composed of
• upper mantle
• oceanic crust
• continental crust
• 70 - 125 km thick

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asthenosphere

• Region of the Earths outer shell that lies just
  below lithosphere
• Extends to a depth of 220-km
• behaves plastically

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Theory of Plate Tectonics

• The Modern Synthesis

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The modern synthesis

• The Earths surface is broken into about a dozen
  major plates
• Plates move about horizontally with respect to
  one another
• Plates float or slide on a highly viscous
  layer -- the asthenosphere

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• Plates are made up of both continental and
  oceanic crust, with the continents riding
  passively on the spreading seafloor
• Main action in plate tectonics occurs at the
  edges of the plates

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• in a human time scale these boundaries are marked
  by the presence of volcanoes and the occurrence
  of earthquakes
• over the course of geologic time these boundaries
  are the site of the splitting, shifting and
  crumpling of the continents

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Volcanoes of the World (Smithsonian Institution
Global Volcanism Program)
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Types of Plate Motion
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Divergent Plates

• Spreading boundary
• zone of healed cracks or fissures
• examples
• Rift Valley of eastern Africa
• Mid-Atlantic ridge
• volcanoes, earthquakes

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Heezen and Tharp
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Convergent Plates

• At convergent plate boundaries the response to
  plate collision will vary with the type of crust
  involved
• There are three possible combinations

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Oceanic crust converging with oceanic crust

• subduction of one plate
• formation of an island arc
• volcanoes
• earthquakes
• example ? Japanese Islands

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Oceanic crust converging with continental crust

• Formation of mountain ranges
• volcanoes
• earthquakes
• example ? Andes Mountains

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Continental crust converging with continental
crust

• Overriding of one plate
• formation of mountain ranges
• earthquakes
• example ? Himalayas

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Transform Boundaries

• side-slip boundary
• create fault zones
• earthquakes
• example ? San Andreas

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Cross section by José F. Vigil from This Dynamic
Planet
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50 million years
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150 million years
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250 million years