Plate Tectonics

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Plate Tectonics

• Week 4

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1)There are 3 main types of rock each type
represented by a photo below. What are the three
main categories (types) of rocks? (in any
order).2) Identify the two types of dating in
geology.
Quiz 3
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TECTONICS

• From the Greek tecton
• to build, or one who constructs (architect)
• Major paradigm Theory of plate tectonics
• Theory states Earth's surface is made up of
  rigid plates
• In geology, this theory is the equivalent of
  General Relativity and quantum theory in physics,
  or evolution in biology

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The Plate Tectonic Theory

• As with all theories it is not perfect but it
  explains much of the behavior of the solid Earth.
• Science is not absolute. Science is not a series
  of facts, but more like a way of looking at
  problems and revealing the facts.
• None of the theories used in science are
  complete!
• General Relativity, Quantum Theory, Newtonian
  Mechanics, and Evolution all have problems at
  some level
• Does this mean they are wrong or worthless?

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History of Plate Tectonics

• First published by Alfred Wegener as Continental
  Drift
• Wegeners observations (some were recognized by
  others as early as 1620)
• 1) The eastern coastline of South America fits
  the western coastline of Africa

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Continental Drift

• First published by Alfred Wegener
• Wegeners observations (some were recognized by
  others as early as 1620)
• The eastern coastline of South America fits the
  western coastline of Africa
• 2) Evidence of glacial deposits were found in
  South America, Africa, India, Antarctica and
  Australia

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Alternate interpretations ?
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Continental Drift

• First published by Alfred Wegener
• Wegeners observations (some were recognized by
  others as early as 1620)
• 3) Evidence of warmer climates was found in the
  northern hemispheres
• Coal deposits, sand dunes and fossil coral reefs
  were found in North America, Southern Europe and
  Northwestern Africa

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Worlds coal deposits
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Continental Drift

• First published by Alfred Wegener
• Wegeners observations (some were recognized by
  others as early as 1620)
• 4) The distribution of fossils
• Fossils of land reptiles and plants were found in
  South America, Africa, India, Antarctica and
  Australia

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Same fossils on different continents
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Continental Drift

• First published by Alfred Wegener
• Wegeners observations (some were recognized by
  others as early as 1620)
• 5) Mountain Chains were continuous across the
  oceans
• - The rocks in the Appalachian Mountain Belt, for
  example, are very similar to those found in
  British isles, Scandinavia and Africa

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Continental Drift

• Wegners conclusion from observations
• All the continents were once part of a larger,
  unified continent, or supercontinent that he
  called Pangea

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Breakup of Pangea
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Supercontinent
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Continental Drift

• What happened to Wegner?
• Wegner was severely criticized by others who
  wanted to know how continents could drift.
• He suggested two possibilities, but physicists
  proved both were impossible.
• Since his mechanisms were wrong, many believed
  his theory was wrong despite his large body of
  evidence that continents were once joined.
• Wegner disappeared in Greenland in 1930 and his
  theory did too for almost 30 years

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Continental Drift is Revived!!

• World War II technology
• ocean floor exploration, sonar magnetometers
• International Geophysical Year (IGY) 1957-58
• Worldwide Standardized Seismic Network 1963

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Worldwide Standardized Seismic Network
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Earths magnetic field has TWO poles
It is DIPOLAR
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Each atom behaves like a little dipole
In a permanent magnet all atomic dipoles line up
with each other
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• Just listen, dont write this down!
• Magnetic materials like iron, nickel etc. can
  become permanent magnets under an external
  magnetic field
• Molten rock (or lava), contains tiny crystals of
  magnetic minerals (e.g. magnetite)
• When the lava is too hot (temp gt580ºC), the
  magnetite crystals are not aligned with each other

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• The temperature above which a magnet loses its
  magnetic properties is called its CURIE POINT
  (580ºC for magnetite).
• As the lava cools below 580ºC, the crystals of
  magnetic minerals line up with each other and
  with the lines of earths magnetic field at that
  time and location

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• The minerals lock into position pointing to the
  direction of the earths magnetic pole and the
  cold lava becomes a permanent magnet
• Since this alignment is permanent, it provides a
  record of the orientation of the earths magnetic
  field lines at the time when the rock cooled

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• From the orientations of magnetic minerals that
  formed millions of years ago, the ancient
  locations of earths magnetic poles can be found
• OBSERVATION
• The past positions of the earths magnetic poles
  (paleopoles) DO NOT coincide with the present
  pole position, but change progressively with time
  (MAGNETIC POLAR WANDER)

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The apparent polar wandering paths for Eurasia
and North America DO NOT coincide
(Fig. 19.10)
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Continental drift hypothesis is the only way this
observation can be explained
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Apparent polar wandering paths for continents
have the same shape during the time they were
part of Pangaea and changed only when they moved
away from each other
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Questions

• Where does the Earths Magnetic field come from?
• In the past, the Earths magnetic pole has
  switched periodically (magnetic reversals)
• How do we know this?

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Discovery of Sea-Floor Spreading

• Sea floor mapping revealed a massive, continuous
  mountain chain running through the center of the
  Earths ocean basins, called the mid-oceanic ridge
  system (MOR)
• The ocean floor was NOT flat as previously
  believed. High ridges and deep trenches exist on
  the ocean floor

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A section of the East Pacific Rise
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Discovery of Sea-Floor Spreading

• Sediments progressively thickened away from the
  MOR
• Magnetometers detected patterns on the sea floor
  symmetric stripes on either side of a ridge

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Discovery of Sea-Floor Spreading
Magnetic patterns on the sea floor
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Discovery of Sea-Floor Spreading
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• Sea-Floor Spreading hypothesis by Harry Hess
  (1962) provided a mechanism for Continental Drift
• Vine-Matthews hypothesis a year later explains
  marine magnetic anomalies and provides proof of
  seafloor spreading
• Age of the igneous rocks on the ocean floor
  progressively increases away from a ridge was
  proven in following years.

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• Continental drift
• Sea-Floor spreading
• PLATE TECTONICS THEORY

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What does plate tectonics explain?

• Much of the topography of Earth Where the high
  mountains are (Andes, Himalayas etc) Shape of
  seafloor (mid-ocean ridges, deep trenches)
• Locations of most earthquakes and volcanoes

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Why is it so important?

• Future motions of plates
• Where earthquakes are likely to occur
• Plate Tectonics gives a framework in which to
  formulate hypotheses that can be tested.

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Break??
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Plate Tectonic Theory

• The Earths surface is covered by about 12
  lithospheric, or tectonic plates
• Tectonic plates move relatively slowly (cm/year)
• Most geological activity (earthquakes and
  volcanoes) occurs at or near plate boundaries

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A sure bet for a test question Know what plate
you live on and what is happening to it
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Plates around North America
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Review of The Earths Layers

• Crust
• Lithosphere
• Asthenosphere
• Mantle
• Core
• outer core is liquid
• inner core is solid

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Plate Boundaries

• There are 3 types of plate boundaries
• 1)Divergent boundaries
• Plates move apart
• Volcanic activity is always present
• Earthquakes are shallow
• Two types of divergent boundaries
• Oceanic
• Continental

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Divergent Plate Boundary
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animation
Fig. 6-7, p.130
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New sea floor is created!

• ocean

continent
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Plate Boundaries

• 3 types of plate boundaries
• 1)Divergent boundaries
• 2) Convergent boundaries
• -Plates move towards one another
• Three types of Convergent Boundaries
• Ocean-ocean
• Ocean-continent
• Continent-continent

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Convergent Plate Boundary
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Ocean-continent convergence
During ocean-continent convergence, oceanic
lithosphere always subducts beneath continental
lithosphere Continental volcanic arc
formed Example Andes mountains
animation
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Ocean-ocean convergence

• Subduction one oceanic plate subducts (dives)
  beneath the other
• Volcanic island arc is formed
• Example Aleutian Islands

animation
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Ocean-ocean convergence

• Which ocean plate subducts?

animation
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Continent-continent convergence
Subduction does not occur (continental crust is
too buoyant to be subducted) Mountain building
occurs without volcanism Example Himalayas, Alps
animation
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The Rocky Mountains

• The Rockies are not near a plate boundary, which
  is where mountain chains are usually found.
• Why are they where they are?

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Plate Boundaries

• 3 types of plate boundaries
• 1)Divergent boundaries
• 2) Convergent boundaries
• 3) Transform boundaries
• -Plates slide horizontally past each other
• Little or no volcanic activity
• Shallow earthquakes
• Example oceanic transform faults San Andreas
  Fault

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Transform Plate Boundary
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Transform boundary
animation
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Why do tectonic plates move?

• Convection in the mantle

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Why do tectonic plates move?

• Convection in the mantle
• Ridge push, or gravity

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Why do tectonic plates move?

• Convection in the mantle
• Ridge push, or gravity
• Slab pull

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Why do tectonic plates move?
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Mantle Plumes

• Rising column of plastic mantle rock
• Can be in the center of a plate or on a plate
  boundary
• Form hot spots

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BIG ISLAND OF HAWAII
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Mauna Kea
? 14,000 feet
Mauna Loa
? 14,000 feet
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14,000 feet above sea level
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14,000 feet above sea level
18,000 feet below sea level
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TALLER THAN MOUNT EVEREST!
14,000 feet above sea level
18,000 feet below sea level
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Other hot spots?

• Yellowstone
• Iceland

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How Plate Movements Affect Earth Systems

• Volcanoes
• Earthquakes
• Mountain Building
• Migrating Oceans and Continents

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Summary

• Earths surface is covered by about 12
  lithospheric plates.
• The plates move slowly (2.5-15 cm/year).
• Most volcanism and earthquake activity occurs at
  or near plate boundaries.
• Plate interiors are relatively quiet geologically
  (some exceptions).

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Isostasy
Fig. 6-16, p.138
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Lake Bonneville
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