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
  • 
  • 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
  • 
  • 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