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The Earth

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					The Earth

• Insides
• Plate Tectonics
• How the Philippines
  Came to Be
Parts
The Earth’s Surface




• Has the Earth’s surface always been this way?
• Is it just a coincidence that Africa and the
  Americas seem to have split from each other?
Continental Drift Theory
• First developed by Alfred Wegener (1912)
• Shapes of continents suggest they were once
  joined
• Rocks and fossils on continents on opposite sides
  of oceans are identical and of same age
• Parts of Africa and India were once buried in ice
  sheets; North America and Northern Europe hold
  fossils of tropical organisms
• More evidence in 1960’s:
  – Polar wandering, reversal of magnetic poles
  – Sea floor spreading
Evidence for Continental Drift
                             Similar fossils occur in
                             separate continents




     Geologic histories of
      regions in separate
continents best explained
      by their being once
   adjacent to each other
Continental
Drift
• Theory was initially
  dismissed due to the
  lack of a mechanism
  to cause massive
  continents to move
 Plate Tectonics: Main premises
• Earth’s crust made up of plates (~20)
   – Two kinds: oceanic plates (denser and thinner),
     continental plates
• Plates move as if on a conveyor belt (the mantle),
   – Speed: 10 cm/yr (10,000 kms in 100 million years)
• Mantle flows like a viscous liquid
   – Energy comes from radioactive decay
   – Unequal heating leads to convection currents
   – Plates float on and move with flowing mantle
• Many geologic processes take place near plate
  edges
   – Volcanoes, sea-floor spreading, earthquakes, mountain
     ranges
Mechanism for Continental Drift:
Convection Currents in the Mantle
                          • Parts of the Mantle
                            are hotter than
                            others
                          • Hot spots expand
                             decrease density
                             become buoyant
                             rise like a
              H OT          balloon
                          • Adjacent areas
                            move in to fill
   COOL




                            spaces left behind
                            by rising fluid
                          • Cool spots sink,
                            creating a loop
The Plates




    Geologically active areas are at plate boundaries
        Convergent: Subduction
        zones; destroys crustal
        material




                   Types of Plate
                      Boundaries
Divergent: Ocean
ridges; creates
new crustal            Conservative: Near faults;
material                 translational, no loss nor
                       creation of crustal material
                            Volcanoes
                            • Form along a line (arc) near
                              subduction zones
                            • Water in subducted oceanic
                              plate helps form magma
                            • Associated with trenches,
                              deepest parts of the ocean
• May also form over hot spots in the mantle, or where two
  oceanic plates converge
Sea Floor Spreading




• Occur at mid-oceanic ridges where mantle is moving apart, creating
  openings for magma to be released and form new crust
• Rocks are older the farther away from ridge
 Earthquakes




• Form near plate edges, or where plates are weakest
  (faults) or subject to stresses
• Occur when rocks suddenly give way to pressure
Mountain Ranges
• Form where two continental plates
  push against each other
 The Philippines




• Country lies on a narrow oceanic
  microplate between Eurasian Plate
  and Philippine Plate                  • Part of Pacific Rim of Fire
• Philippine Plate is oceanic, moving   • Near subduction zones, trenches
  northward into Eurasian plate           and faults; highly active
The Philippines
• Plate tectonics explain many
  of its features
  • Rich in limestone due to oceanic
    origin
  • Highly volcanic; most of its
    mountains and lakes are extinct
    volcanoes
  • Rich in mineral resources
  • Earthquake-prone
  • Too young to contain dinosaur
    fossils
  • Rich in diversity – biological and
    linguistic
• In a few tens of millions of
  years, islands will disappear

				
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posted:3/23/2010
language:English
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