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Plate Tectonics

From Continental Drift to Plate Tectonics


  • Until the late 1960s, most geologists believed that the positions of the continents and ocean basins were fixed.
  • Continental drift, a hypothesis that challenged this belief, was first proposed in 1915.
  • After World War II, scientific developments led to the unfolding of the theory of plate tectonics.
  • Plate tectonics refers to the movement of lithospheric plates that shifts continents and causes volcanism, earthquakes, and mountain building.



Continental Drift: An Idea Before Its Time

  • Alfred Wegener
    • First proposed continental drift hypothesis in 1915
    • Published The Origin of Continents and Oceans
  • Continental drift hypothesis
    • A supercontinent, consisting of all of Earth’s landmasses, once existed
    • During the Mesozoic, ~200 million years ago, this supercontinent began fragmenting
    • Wegener named the supercontinent Pangaea, meaning “all lands”


The Great Debate

  • Why was Wegener unable to overturn the established scientific views of his day?
    • Inability to identify a credible mechanism for continental drift
      • Incorrectly proposed the gravitational forces of the Moon and Sun were capable of moving the continents.
      • Incorrectly suggested that continents broke through the ocean crust like icebreakers.
    • There was strong opposition to this hypothesis from all areas of the scientific community, and it was rejected.



The Theory of Plate Tectonics

  • Following World War II, oceanographers with new equipment explored the seafloor
    • Oceanic ridge system winds through all of the major oceans
    • No oceanic crust older than 180 million years old
    • Thin sediment accumulation in the deep oceans
  • These developments and others led to the theory of plate tectonics.


  • Figure 2.9, Rigid Lithosphere Overlies Weak Asthenosphere
    • The lithosphere is comprised of the crust and upper mantle.
    • The asthenosphere is a hotter, weaker region of the mantle under the lithosphere.
    • Two layers move independently of each other.
  • Figure 2.10, Earth’s Major Plates
    • The lithosphere is broken into numerous segments called lithospheric plates.
    • These plates are in constant motion.


  • Plate Movement
    • Plates move as somewhat rigid units relative to each other.
    • Most interactions and deformations occur along plate boundaries.



Types of Plate Boundaries

  • Types of plate boundaries:
    • Divergent plate boundaries
      • plates move apart and new seafloor is created
    • Convergent plate boundaries
      • plates move together, can create mountain belts or recycle oceanic lithosphere
    • Transform plate boundaries
      • plates grind past each other without the production or destruction of lithosphere



Divergent Plate Boundaries

  • Also called constructive plate margins
  • New ocean floor is generated as two plates move apart
  • Most divergent plate boundaries are associated with oceanic ridges
  • Oceanic ridge system is the longest topographic feature on Earth’s surface
    • Exceeds 70,000 kilometers in length
  • Oceanic Ridges and Seafloor Spreading
    • Along the crest of the ridge is a canyon-like feature called a rift valley
    • Seafloor spreading is the mechanism that operates along the ridge system to create new ocean floor.
  • Spreading Rates
    • The average spreading rate is 5 centimeter/year
    • Mid-Atlantic Ridge has a spreading rate of 2 centimeter/year
    • East Pacific Rise has a spreading rate of 15 centimeter/year


  • Continental Rifting
    • Occurs when a divergent plate boundary occurs within a continent
    • A landmass will split into two or more smaller segments
    • A continental rift, an elongated depression, will develop where continental crust sinks.
    • Eventually the depression lengthens and deepens, forming a narrow sea, and then a new ocean basin.
      • Example: East African Rift

Convergent Plate Boundaries and Subduction

  • Two plates move toward each other and leading edge of one slides beneath the other
    • Where lithosphere descends (subducts) into the mantle: subduction zones
    • Deep-ocean trenches are long, linear depressions in the seafloor
      • Produced when oceanic lithosphere descends into the mantle along subduction zones
    • Examples include:
      • Peru-Chili Trench
      • Mariana Trench
      • Tonga Trench
    • Oceanic–continental convergence
      • The denser oceanic slab sinks into the mantle beneath the buoyant continental block
      • At a depth of ~100 kilometers, partial melting is triggered when water from the subducting plate mixes with the hot asthenosphere.
      • This generates magma resulting volcanic mountain chain called a continental volcanic arc.
      • Examples include:
        • The Andes
        • The Cascade Range
      • Oceanic–oceanic convergence
        • When two oceanic slabs converge, one descends beneath the other.
        • As with oceanic–continental convergence, partial melting initiates volcanic activity.
        • If the volcanoes emerge as islands, a volcanic island arc or island arc is formed.
        • Examples include:
          • The Aleutian Islands
          • The Mariana Islands




Convergent Plate Boundaries

  • Continental–continental convergence
    • Continued subduction can bring two continents together.
    • Less dense, buoyant continental lithosphere does not subduct.
    • This results in continental collision and produces mountain belts of deformed rocks.
    • Examples include:
      • The Himalayas
      • The Alps
      • The Appalachians



Transform Plate Boundaries

  • Also called a transform fault
  • Plates slide horizontally past one another, without production or destruction of lithosphere.
  • Most occur on the seafloor joining two spreading center
    • Known as fracture zones
  • Can move oceanic ridges toward subduction zones
  • A few transform faults cut through continental crust
    • Examples include:
      • The San Andreas Fault
      • The Alpine Fault of New Zealand



Changing Plate Boundaries

  • Although Earth’s total surface area does not change, the size and shape of individual plates are constantly changing.
    • Plate boundaries migrate
    • Plate boundaries are created and destroyed
  • Breakup of Pangaea
    • Formation of the Atlantic Ocean basin
    • India collided with Asia to form the Himalayas



  • Plate Tectonics in the Future
    • Geologists use present plate motions to extrapolate plate movements into the future.
      • Baja and southern California will eventually slide past the North American Plate
      • Africa will continue to collide with Eurasia



Testing the Plate Tectonics Model

  • Evidence from Ocean Drilling
    • Some of the most convincing evidence has come from drilling directly into the ocean floor.
      • Hundreds of holes were drilled through layers of sediments on the ocean floor and the basaltic crust
      • Sediments increase in age with distance from the ridge crest
      • Sediments are almost absent on the ridge crest and thickest furthest from the spreading center
    • Pattern of distribution and thickness provided additional verification of seafloor spreading


  • Evidence from Hot Spots and Mantle Plumes
    • A mantle plume is a cylindrically shaped upwelling of hot rock.
    • The surface expression of a mantle plume is an area of volcanism called a hot spot.
    • As a plate moves over a hot spot, a chain of volcanoes, known as a hot-spot track, forms.
    • The age of each volcano indicates how much time has elapsed since it was over the mantle plume.
    • Examples include:
      • Hawaiian Island chain
      • Yellowstone


  • Evidence from Paleomagnetism
    • Basaltic rocks contain magnetite, an iron-rich mineral influenced by Earth’s magnetic field.
    • When the basalt cools below the Curie point, the iron-rich minerals become magnetized and align with the existing magnetic field.
    • The magnetite is then “frozen” in position and, like a compass needle, indicates the position of the north pole at the time of rock solidification.
    • This is referred to as paleomagnetism or preserved magnetism.
  • Apparent Polar Wandering
    • The apparent movement of the magnetic poles indicates that the continents have moved.
    • It also indicates North America and Europe were joined in the Mesozoic.



  • Magnetic Reversals and Seafloor Spreading
    • Earth’s magnetic field reverses polarity periodically
    • During a magnetic reversal, the north pole becomes the south pole, and vice versa.
      • Rocks that exhibit the same magnetism as the present magnetic field exhibit normal polarity.
      • Rocks that exhibit the opposite magnetism exhibit reverse polarity.
    • Once this concept was confirmed, researchers established a timescale for these occurrences, called the magnetic time scale.


How is the Plate Motion Measured

  • Geologic Measurement of Plate Motion
    • Dates of ocean floor from hundreds of locations gathered by ocean-drilling ships
    • By knowing the age of a sample and distance from the ridge axis, an average rate of plate motion can be calculated
    • Combined with paleomagnetism data to make maps of the age of the ocean floor




  • Measuring Plate Motion from Space
    • Global Positioning System (G P S) data are collected at numerous sites over years
    • Measure plate motions to the millimeter



What Drives Plate Motions


  • Convection is the way heat transfers through liquids and gases.
  • Forces That Drive Plate Motion:
    • The subduction of cold, dense oceanic lithosphere is a slab-pull
    • Elevated lithosphere at oceanic ridges will slide down due to gravity, causing the ridge-push


Plate-Mantle Convections

  • Although not fully understand, researchers agree on the following:
    • Plate tectonics and convective flow in the mantle are part of the same system.
    • The energy source for plate tectonics is Earth’s internal heat.
  • As a result, many models have been proposed although we will examine one type

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