Plate Boundaries by 0wAQ9jf


									Plate Boundaries
3 main types of boundaries
• Divergent (plates
  diverge, or move away)

• Convergent (plates
  converge, or come

• Transform (plates slide
  past each other)
Divergent boundary
• Plates move away (or
  spread) from each other.
• New crust is created here.
  – Magma forces plates apart.
• Example:
  – Mid-Atlantic Ridge
  – Great Rift Valley in
     East Africa
Geologic phenomena at divergent boundaries:
* = unique to divergent boundaries
  – Iceland* (formed right on top of Mid-
    Atlantic Ridge)
  – Volcanoes
  – Earthquakes
  – Mountains
     • Fault-block mountains (one side is
       dropping lower as crust spreads)
  – Mid-Ocean Ridges*
  – Rift Valleys*
  – Spreading Zones*
     • Crust is spread out and stretched as plates
       move apart.
Mid-Ocean Ridges
• New crust created here.
• See “Evidences of Plate
  Tectonics” Notes for more
Rift valleys (divergent)
   •A deep valley formed by the two plates
   moving away from each other.

  •Crust warps downward, spreads, eventually
  “breaks” (boundary comes to surface)
           Example: Great Rift Valley in East Africa
Convergent boundary
• Plates move toward each other and
• Crust is destroyed here.
• Subduction: Where one plate sinks
  underneath another and is forced
  into the mantle (then melted)
 Geologic phenomena at convergent
– Volcanoes
– Earthquakes
– Mountains
   • Folded mountains (crust
     crumples/folds upward)
– Trenches/subduction
– Volcanic Island Arcs
Types of Convergent boundaries
• Ocean plate collides
  with a continent.
• Ocean plate sinks.
• Forms a trench.
  – Trench: also called a
    subduction zone,
    where one plate
    subducts under the
Continent-Oceanic, continued
   • Examples:
      – Juan de Fuca plate and
        North American plate
         • Juan de Fuca plate
           subducts under N.A. plate;
           created the Cascade
           Mountain Range in
           Northern CA, OR, and
   • Nazca plate and South
     American plate
      – Subduction of Nazca
        plate formed Andes
        Mountains on west coast
        of S.A.
Types of Convergent boundaries,
• Two continents
• Pushes the crust
  upward to form
Continent-Continent Example
• The Himalayas and
  Mount Everest—
  formed by Indian
  Plate and Eurasian
  Plates colliding.
Types of Convergent boundaries,
• Two ocean plates collide.
• The older one sinks under the newer one.
• Creates volcanic island arc *Not
Oceanic-Oceanic examples
 – Mariana Islands
   (volcanic!): Formed by
   Philippine plate and
   Pacific plate
 – Tonga Trench in South
   Pacific (Pacific plate
   subducting under
   Australian plate)
    • Fastest moving plate: 24
Transform boundary
• Plates move past each other laterally.
• Neither plate is destroyed.
• Example:
  – San Andreas Fault (formed by North
    American and Pacific plates)
Geologic phenomena associated with
Transform Boundaries:
 • Earthquakes
 • Some warping of crust
Why Plates Move
  • Convection is the driving force behind plate
    How It Works:
       1. Magma in the mantle is heated by the core.
       2. Heated magma rises toward crust.
       3. As magma moves away from heat source, it begins
          to cool.
       4. Cool magma sinks back down toward core, creating
          a convection cell that helps move crust.

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