16-Plate Tectonics

• Plate Tectonics


  A very old idea based on the fit of continents

  
• The shape of the continents suggest that they may have been once part of a supercontinent
• Evidence to support the idea that landmasses were once part of a whole

  • Similarity of rock type and geologic structures
  • Fossil evidence
  • Paleoclimate evidence

  
• Fossil evidence is used to explain the presence of organisms that could not have otherwise shared the same environment or could not swim across oceans
• Paleoclimate evidence includes the orientation of till deposits and glacial striations which indicate the direction of ice movement

  
• Continental drift hypothesis


  Alfred Wegener hypothesized the supercontinent Pangea based on geological evidence

  
• Pangea supercontinent began to break apart around 200 million years ago

  
• Continental drift doesn't provide a viable mechanism and an effective source of power to move continental landmasses, so Wegener's continental drift hypothesis was dismissed
• Seafloor spreading


  Proposed in the early 1960s based on extensive mapping of the ocean floor
• Evidence for seafloor spreading

  • Sea floor has ridges and rises arranged in long structures
  • Iron and magnesium rich lava erupts along these structures
• The age of the seafloor increases with the distance from the oceanic ridges, showing a symmetrical pattern across the mid oceanic structure because of sea-floor spreading

  
• Radiometric dating technology is used to determine the age of the seafloor
• Magnetic polarity reversal


  The liquid iron in the outer core generates Earth's magnetic field, and this field can reverse polarity several times in Earth's history
• The basaltic lava contains the mineral magnetite that aligns itself with the magnetic field, and this pattern of magnetic polarization is recorded in the magnetite minerals on symmetrical patterns

  
• Plate tectonics became the official theory of geology in the late 1960s

  
• Plate tectonics theory


  • Plates movement is classified based on the ways they move with respect to each other: Divergent, Convergent and Transform

  
• Divergent plate boundaries


  1. Where sea floor spreading occurs and plates move away from each others

  2. New oceanic lithosphere is built continuously

  3. Intense volcanism: basalt eruptions from fissures and volcanoes

  4. Low magnitude earthquakes

  
• Convergent plate boundary: Subduction

  1. Where plates collide, the denser plate dives under less dense plate along a depression called the Trench

  2. While subducting, the stress causes many, often very strong earthquakes

  3. Volcanoes form lined up parallel to the trench, forming a volcanic arc

  
• Convergent plate boundaries: Orogeny

  1. When continental lithosphere plates collide, they do not subduct because they are not dense enough; instead, they deform and rocks fold and pile up forming an Orogeny = mountain building

  2. When two or more smaller lithosphere plates add up to a larger plate, this is called accretion

  
• Transform plate boundary


  Plates slide past one another generating high shear stress, with significant earthquakes but no volcanoes form

  
• Transform plate boundaries connect to the other plate boundaries
• Mantle Convection


  The engine that drives the plates, with convection cells in the mantle dragging on the bottom of the lithosphere and moving the plates
• How Pangea broke up

  1. Heat accumulates under large plates of continental lithosphere, making it unstable

  2. Gravity induces rifting, triggering depressurization and the formation of lava
• 4 steps to form an ocean from Pangea breaking up

  • Heat from the mantle produces up-warping
  • Rift valleys form
  • Sea floor spreading begins, forming a narrow "baby" ocean
  • Sea floor spreading fully operational, creating new sea floor
• The East African rift is an example of continental rifting happening now
• Supercontinents


  Geological evidence indicates that Pangea in turn formed from collision of smaller lithospheric plates, and Pangea was not the one and only supercontinent to form in Earth's history

  
• Wilson Cycle


  Outlines the ongoing origin and breakup of supercontinents, and is estimated to take up to 500 million years to complete
• North America was built up of many Wilson cycles, with different geologic regions spanning around 300 million years of history
• North America formed by accretion (addition) of convergent plate margins

  
• Scientists can model what the next supercontinent might look like based on present-day rate and direction of plate movement, if the mantle keeps up its current pattern of convection