Alfred Wegener and Pangaea

1
Alfred Wegener and Pangaea

• Pangaea is the name given to the single giant
  continent in a proposal by Wegener (1912).
• The theory of continental drift states that the
  continents were once a single landmass that
  drifted apart and are still doing so.

2
Evidence for Continental Drift

• Wegener tried to support his theory with
  evidence
• Maps - continents apparently fit together like a
  jigsaw-puzzle
• Plant and animal fossils - coal in Antarctica

3
Evidence for Continental Drift

• Why was his theory not accepted?
• No explanation for how drift occurs
• He was a meteorologist, not a geologist!
• Plate tectonics theory - explanation for how
  continents move, making Wegeners theory widely
  accepted.

4
New Technology and Seafloor Knowledge

• Sonar 1925 first seafloor map of ocean ridges
• New features discovered
• Mid-ocean ridges mountain ridges on ocean floor
• Rift valleys valley in center of MOR
• Trenches
• Scientists observed patterns in features

5
Ridges and Trenches
6
Seafloor Spreading

• Seafloor constantly being created and destroyed
• New crust at rift valley
• Crust destroyed at trenches

7
Evidence for Seafloor Spreading

• Thin sediment in ridges
• Age of rock at ridges much younger than
  continents
• Patterns of magnetic polarity reversal

8
Seafloor Age
9
Types of Movement

• Divergent plates moving apart (rift valleys in
  MOR)
• Convergent plates coming together
• Transform plates slide past each other

10
Island Arcs

• Convergent ocean plates
• Subduction of denser plate

11
Subduction Zones

• Ocean plate into continent
• Trench, mountains, volcanoes formed

12
Convergent Continents

• Mountains form

13
Hot Spots

• Magma chambers in mantle push through surface of
  crust
• Crust moves over mantle forming island chain

14
Plate Movement

• Convection is the primary force driving seafloor
  spreading andcontinental drift.
• Convection currents form as hot material rises
  and cold material sinks. This occurs in the
  mantle.
• This creates a current that moves the plates away
  from each other at the divergent boundaries,
  toward each other at the convergent boundaries,
  and past each other at the transform boundaries.
• A second driving force comes from seafloor
  spreading.
• As new seafloor forms, the platestend to slide
  away fromthe elevated mid-ocean ridges.

The Unifying Theory Plate Tectonics
Chapter 11 Pages 11-26 11-27
15
Plate Movement (continued)

• Predicted changes over the next 50 million years
• The Baja Peninsula will have moved past and apart
  from the North American Plate.
• Southern California will pass San Francisco as it
  moves to the northwest.
• A new sea will form in eastern Africa.
• Australia will move toward Eurasia/the Equator.
• The Mediterranean Sea will close as Africa pushes
  towards Europe.
• The Atlantic and Indian Oceans will continue to
  grow while the Pacific will become smaller.

The Unifying Theory Plate Tectonics
Chapter 11 Pages 11-28 11-29