Plate Tectonics

Chapter 10, Section 1 - Continental Drift                                                                                                                                                

1912 - Alfred Wegener proposed the idea of Continental Drift.
  • Continental Drift - The hypothesis that a single large landmass broke up into smaller landmasses to form the continents (during the Mesozoic Era, about 200 million years ago), which then drifted to their current locations.
  • Evidence for continental drift:
    • The edges of the continents seem to fit together like puzzle pieces (Africa & South America).
    • Fossils of the same plants/animals were found on different continents (with no land bridges).
      • Mesosaurus skeletons found in Africa and South America.
    • Ages & types of rocks on east coast of S. America and west coast of Africa match closely.
    • Mountain chains that end on one continent seem to continue on another.
      • Appalachians in N. America, mountains in Greenland, Scotland, northern Europe.
    • Evidence of glaciers in southern Africa and S. America.  (Too warm there for glaciers now.)
    • Fossil evidence of tropical/subtropical swamps in area that are now much colder.
  • Wegener's ideas were opposed by most scientists because he could not demonstrate the mechanism by which continents could move.
1947 - A team of scientists set out to map the Mid-Atlantic Ridge (part of a global system of mid-ocean ridges).
  • Mid-Ocean Ridge - A long, undersea mountain range that has a steep narrow valley at its center, that forms as magma rises from the asthenosphere, and that creates new oceanic lithosphere (sea floor) as tectonic plates move apart.
  • Important discoveries...
    • The sediment on the ocean floor gets thinner and younger as you get closer to the mid-ocean ridge.  Sediment farther away is thicker and older.  
      • Sediments dated by studying remains of tiny organisms embedded in it.
    • The ocean floor is very young.
      • Continental rock are up to 4 billion years old.  No oceanic rocks are more than 200 million years old.
      • Radiometric dating shows that rocks closer to the ridge are younger than rocks farther away.
late 1950's - The hypothesis of sea-floor spreading was proposed by Harry Hess (named by Robert Dietz).
  • Sea-Floor Spreading - The process by which new oceanic lithosphere (sea floor) forms when magma rises to Earth's surface at mid-ocean ridges and solidifies, as older,existing sea floor moves away from the ridge.
  • Hess suggested that this was the mechanism for continental drift.
mid-1960's - The study of paleomagnetism provides evidence to support sea-floor spreading.
  • Paleomagnetism - The study of the alignment of magnetic minerals in rock, specifically as it relates to the reversal of Earth's magnetic poles; also the magnetic properties that rock acquires during formation.
  • Iron-rich minerals in magma align with the Earth's magnetic fields.  As the magma hardens, the orientation of the minerals becomes permanent.
  • The orientation of magnetic minerals shows that the Earth's poles have reversed repeatedly throughout Earth's history.
    • Rocks with normal polarity have minerals that point north.
    • Rocks with reversed polarity have minerals that point south.
    • Geomagnetic Reversal Time Scale - The chronological pattern of normal and reversed polarity shown in rocks.
  • Magnetic patterns in sea-floor rocks mirror each other on opposite sides of the mid-ocean ridge, with youngest rocks closest to the ridge.
    • Shows that new rock forms at the center of the ridge, then moves away in opposite directions.
  • Later, similar patterns were found in continental rocks.

Chapter 10, Section 2 - The Theory of Plate Tectonics                                                                                                                          

Plate Tectonics - The theory that explains how large pieces of the lithosphere, called plates, move and change shape.

Lithosphere - The solid, outer layer of Earth that consists of the crust and the rigid upper part of the mantle.  (See more in Chapter 2.)

Asthenosphere - The solid, plastic layer of the mantle beneath the lithosphere.  (See more in Chapter 2.)

Tectonic Plates
  • Earth has 15 major plates and several smaller ones.
  • Earthquakes and volcanoes often occur along the edges of plates, so can be used to locate plate boundaries.
  • Plates can be formed of continental crust, oceanic crust, or both.
    • Continental crust is mainly silicate rock (granite), so is lighter.
    • Oceanic crust contains more iron and magnesium (basalt), so is denser.

Plate Boundaries
  • Divergent Boundaries
    • Between plates that are moving away from each other.
    • Occur at mid-ocean ridges.
    • Magma rises to the surface, cools, and forms new lithosphere.  (See sea-floor spreading in 10.1.)
  • Convergent Boundaries
    • Between plates that are colliding.
    • Three types of collisions...
      • Continental crust collides with continental crust.  The edges of the plates crumple, forming mountains, such as the Himalayas.
      • Continental crust collides with oceanic crust.  The oceanic crust moves under the continental crust (subduction).
        • This forms a trench at the subduction zone.
        • Heat from this movement, along with fluids added to the plate, cause rock to melt and push upwards to form volcanic mountains, such as the Andes.
      • Oceanic crust collides with oceanic crust.  One subducts under the other, forming a trench and and island arc, such as Japan or the Aleutian Islands.
  • Transform Boundaries
    • Between plates that are sliding past each other, such as at the San Andreas Fault.  (No magma involved.)
    • Fracture zones - transform boundaries that connect segments of mid-ocean ridges.

Causes of Plate Motion
  • Mantle Convection
    • Convection - The movement of fluids due to differences in temperature.
    • This is the primary force behind plate tectonics.
  • Ridge Push
    • Rock forming at mid-ocean ridges tends to start out light, and become denser.  This causes it to slide down the slope, pushing the plates away from the ridge.
  • Slab Pull
    • As a plate moves into the mantle at a subduction zone, it exerts a pull on the rock behind it.

Chapter 10, Section 3 - The Changing Continents                                                                                                                                   


The continents did not always look as they do now.

Continents contain...
  • Cratons - Large areas of stable rock, older than 540 million years.
    • Shields - Rocks within cratons that have been exposed at the surface.
  • Terranes - Pieces of lithosphere with unique geologic histories, and which may be part of a larger piece of lithosphere (such as a continent).
    • Contain fossils that differ from those found in neighboring terranes.
    • Have major faults along their boundaries.
    • Have magnetic properties which generally don't match those of neighboring terranes.
Most continents consist of cratons surrounded by terranes.

Rifting - The process by which Earth's crust (oceanic or continental) breaks apart.
  • Why do continents break apart?  Answer uncertain, but here's the theory...
    • Continental crust is high in silica => acts like an insulator, trapping heat from below.
    • As heat builds up, continental lithosphere thins and weakens until it finally begins to break apart.
Accretion - The process by which terranes are added to continents.
  • At subduction zones, terranes can get scraped off of the subducting plate and added to the overlying (continental) plate.
  • This can form mountains, or just add to the surface area of the continent.
  • Major mountain chains form when large terranes and continents collide.  (Himalayan Mountains => collision of India and Asia)

Effects of Continental Change
  • Changes in climate due to...
    • location of continent (distance from equator).
    • location relative to oceans and other continents.
    • mountain ranges (affect rainfall - "rain shadow").
  • Arrangement of continents affects the movement of air and water around the Earth.
  • Changes in living things due to...
    • Natural selection due to local climate.
    • Divergent evolution as populations are isolated.

The Supercontinent Cycle
  • The process by which supercontinents form and break apart over millions of years.
  • Pangaea - The supercontinent that formed 300 million years ago and that began to break up 200 million years ago.
  • Panthalassa - The single, large ocean that covered Earth's surface during the time that Pangaea existed.
    • Tethys Sea cut into one side of Pangaea.  (Will eventually form Mediterranean Sea.)
  • Break-Up of Pangaea
    • 200 mya - Separated into two continents...
      • Laurasia (North America and Eurasia).
      • Gondwanaland (South America, Africa, Australia, Antarctica).
    • 150 mya - Atlantic Ocean started to form from rifting.
      • Current continents separated and started drifting towards current positions.
    • 50 mya - India collides with Asia.
  • In the future, the continents will eventually come back together to form a new supercontinent.



Resources
  • Pg. 286-287 #6-23, 33-35.