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