Chapter 8: Plate Tectonics

1
Chapter 8 Plate Tectonics

• 8.1 Earth has several layers
• 8.2 Continents change position over time
• 8.3 Plates move apart
• 8.4 Plates converge or scrape past each other

2
8.2 Continents change position over time

• Before, you learned
• Earths main layers are the core, mantle, and
  crust
• The lithosphere and asthenosphere are the topmost
  layers of Earth
• The lithosphere is made up of tectonic plates
• Now, you will learn
• How the continental drift hypothesis was
  developed
• About evidence for plate movement from the sea
  floor
• How scientists devloped the theory of plate
  tectonics

3
Core, Mantle, Crust
Litho stone or rock Asthenes weak

• Lithosphere crust and very top of mantle
  solid, most rigid layer
• Asthenosphere hotter, softer rock in the upper
  mantle (just below the lithosphere) can flow
  like hot tar

Less dense materials rise
Denser materials sink
4
Continents join together and split apart

• As far back as the 1500s, map makers noticed the
  western coast of Africa and the eastern coast of
  South America seemed to fit together
• 1912, German scientist Alfred Wegener proposed
  the hypothesis continental drift

5
Evidence for Continental Drift

• Fossils
• Fossils of an ancient (270 million years ago)
  reptile Mesosaurus were found in South America
  AND western Africa, but no where else in the
  world
• Explanation the continents were once joined
• Climate
• Greenland today is mostly covered in ice, yet
  tropical plant fossils are found there
• South Africa is warm, but rocks were deeply
  scratched by ice sheets
• Geology
• Kinds of rocks that make up the continents those
  found in Brazil match those in western Africa
• Limestone layers in the Appalachian Mountains
  (NA) exactly like Scotlands Highlands

6
Pangaea and Continental Drift

• Huge supercontinent over 200 million years ago
• Pangaea, all lands
• Centered over where Africa lies today
• But how?

7
The theory of plate tectonics explains how plates
and their continents move

• Mid-1900s scientists proved tectonic plates
  move
• Evidence from the sea floor
• Huge underwater mountain ranges mid-ocean ridges
  circling earth like baseball seams

8
The theory of plate tectonics explains how plates
and their continents move

• Evidence from the Sea Floor
• Sea-Floor Spreading the ridges form along cracks
  in the crust, melted rock rises through these
  cracks, cools, and forms new oceanic crust

9
The theory of plate tectonics explains how plates
and their continents move

• Evidence from the sea floor
• Age of the sea floor youngest rocks cloest to
  the ridge, oldest rocks farther away
• Oldest ocean floor is young 160 to 180 my old
  continental crust much older 4 billion yrs
• Ocean Trenches
• Sea floor spreads, then dense oceanic crust sinks
  into the asthenosphere (upper mantle) into huge
  trenches (like deep canyons)
• Old crust destroyed as new crust forms Earth
  remains the same size

10
Causes of Plate Movement

• Tectonic plates rest on the asthenosphere layer
  of soft, hot rock
• Moves by convection heat transfer by the
  movement of a material
• Hot soft rock rises, cools, and sinks, then is
  heated and rises again convection current slow
  few cm/yr
• Slab pull gravity pulls the edge of a cool,
  dense plate into the asthenosphere the entire
  plate is dragged along
• Ridge push material from a mid-ocean ridge
  slides downhill from the ridge

11
(No Transcript)
12
Putting the Theory Together

• Theory of plate tectonics the Earths
  lithosphere is made up of huge plates that move
  over the surface of the Earth
• One plate could not shift without affecting the
  others nearby
• Plates can move apart, push together, or scrape
  past each other
• Most major earthquakes, volcanoes, and mountain
  ranges appear where tectonic plates meet

13
(No Transcript)
14

• List of animations!
• http//www.wwnorton.com/college/geo/egeo/animation
  s/ch2.htm