Plate Tectonics

Earth’s Surface

  • Dynamic
  • Surface continually shaped by erosion and mountain building
    • Erosion takes place when precipitation falls on land, wearing away at rocks and landforms
    • Mountain building takes place mostly underground, driven by forces deep within the Earth

Earth’s Internal Structure

  • Three basic layers: core, mantle, crust
    • Each layer has further subdivisions within it
  • Core
    • Inner core is solid (metallic)
    • Outer core is liquid (molten)
    • Very hot (contains some heat left from when the earth first formed and some heat generated by ongoing radioactive decay)
    • This heat escapes outward toward the surface through the mantle
  • Mantle
    • Consists of rock that would feel solid if touched but rock that can move and deform
    • This is behavior that a physicist would describe as elastic – a deformable solid
    • Under heat/pressure from core, the rock of the mantle can move slowly
    • Two parts: lower and upper mantle
    • Upper mantle is the more malleable part (aka asthenosphere)
  • Crust
    • Solid brittle rock that comprises Earth’s surface
    • Very thin relative to the rest of the planet; thin shell of rigid rock around a deformable interior
    • Two types: continental and ocean crust
    • Continental crust: relatively thick and made dominantly of rock with high silica silicate minerals (rock often referred to as felsic – high silica rock); comprises continents
    • Ocean crust: relatively thin and made dominantly of rock with low silica silicate minerals \n (rock often referred to as mafic – low silica rock); comprises oceans
    • Essentially, the continental crust is thick and light; the ocean crust is thin and heavy
    • Under the crust is a layer of solidified mantle rock
    • Combined with the crust, this layer comprises the lithosphere
    • Under the lithosphere is the upper mantle
    • Not one cohesive shell; broken into pieces called plates
    • Crustal plates or lithospheric plates
    • How crustal plates are currently understood by scientists
    • Most larger plates contain both continental and ocean crust
    • Crustal plates move (mobile crust)
    • Plate tectonics: the movement of the crustal plates

Plate Tectonics

  • Many major geological events take place on plate boundaries where they interact with each other
    • A lot of plate tectonics theory centers around the dynamics of plate boundaries
  • Three basic kind of plate boundaries
    • Divergent boundaries: boundaries which occur where plates move away from each other
    • Convergent boundaries: boundaries which occur where plates move toward each other
    • Transform boundaries: boundaries which occur where plates move laterally past each other
Divergence
  • Plates spread apart (“rift”)
  • Divergence is driven from below by hot rising rock in the mantle which concentrates in certain areas of the mantle rather than being evenly distributed
    • In ares of high heat flow, the hot mantle rock rises; the rising rock pushes up on the overlying crust and pushes it apart
    • When the hot rising rock pushes up under a continent, the continent begins to crack and stretch and thins to create a rift valley – a long, narrow valley
    • As the spreading continues, the rift valley floods with water and creates a narrow sea
    • Eventually, the spreading sea opens into a full ocean basin
  • Most divergent boundaries occur in ocean basins and are marked by mid-ocean ridges
    • Mid-ocean ridge: a large linear mountain range on the ocean floor
    • The divergent boundaries form ridges because the ocean crust is being pushed up from underneath and pushed apart
    • In the center of the ridge is a rift where the two diverging plates spread away from each other at a rate of 4-5cm/year
    • In the gap between the two spreading plates hot rock from the underlying mantle

    rises up and solidifies to form new crust – new ocean crust

  • As the plates spread, new crust is continuously added to the back side of the plate
    • All major ocean basins have mid-ocean ridges
  • Eg. mid-Atlantic ridge that runs north to south for the length of the Atlantic ocean
Convergence
  • Plates collide
  • Subduction: when one plate is pushed downward in a convergence
    • This is why convergent boundaries are also called subduction zones
  • New ocean crust is created by divergence and old crust is destroyed by convergence → essentially recycling
  • Three subtypes of convergent boundaries:
    • Ocean crust / ocean crust
    • One plate is subducted under the other plate
    • Marked by a deep ocean trench
    • Deep ocean trench: a linear trench significantly deeper than the rest of the ocean floor
    • Credit: Tarbuck and Lutgens
    • As one plate subducts under the other, the subducting plate experiences friction as well as higher temperatures and pressures, so its crust begins to melt
      • The melting crust produces magma that rises up toward the surface, some of which erupts to form volcanic islands parallel to the deep ocean trench
      • Volcanic island arc: the chain of volcanic islands parallel to the trench
      • Eg. volcanic Japanese islands
    • Ocean crust / continental crust
    • Also marked by deep ocean trenches
    • Ocean crust is almost always subducted since it is heavier
    • Same process for volcano formation may occur, but volcanoes are located on the continent rather than separate islands
    • The force of the plate collisions also causes the formation of mountains
    • Continental crust / continental crust
    • Results in intense mountain building, crustal deformation, and uplift
Transformation
  • One plate slides laterally past another
    • Eg. San Andreas fault of California (Pacific plate/North American plate)
Effects of Plate Tectonics
  • All plate boundaries are sites of crustal movement and geologic activity
  • One exception to this idea is Hawaii, a mid-plate hot spot
    • Under the Pacific plate lies a small area of concentrated heat flow in the mantle
    • When heat rises within the mantle, the mantle’s upper layer begins to rise
    • Rising hot rock generates magma that rises through the crust and erupts to create a volcano on top of the plate and, as the volcano grows, a volcanic Hawaiian island
    • The Pacific plate keeps moving, so the island moves with it
    • Then, a new volcano and a new island form on top of the hot spot and the process continues
    • The current volcanic activity is happening on the southern end of the big island of Hawaii
    • Each island gets progressively older northwestward through the island chain, having moved farther from the hotspot over time