7-1    Continental drift:

       Earlier this century, Alfred Wegener, a climatologist, proposed continental drift. A super continent Pangaea broke about 200 Ma Fig 7.2

Evidence:

1. Jigsaw puzzle of the continents plotted based on continental shelf, not the coast line.

   • Continental shelf: extension of the continent in ocean

2. Similarity in fossils: Mesosaurus, a type of reptile, found only in S. America and S. Africa. Fossil fern, Glossopteris, whose seed can't be blown across the ocean, found in every continents.

3. Rock types of structures: in Appalachians and mountains in British Isles and Scandinavia, rocks with similar age and structure were found.

4. Ancient climate: Glaciation in Africa and S. America and Australia. Similar age and types of till. But Northern hemisphere was warm weather, resulting coal formation. Fig 7.7.

So, he proposed key in south pole.

7-2    Plate tectonics:

• The Earth crust consists of ~ 20 rigid plates. These plates are in constant motion. Fig 7.8.

• Differ from continental drift: plates in oceans, too.

• Plate boundary (Fig 7.9):

• Features in divergent boundaries Fig 7.12:

  1. Upwaping: earlier stage, magma rise, surface under tension, faults form.

  2. Rift valley: due to further tension, rock slab sinks, valley forms, current: East African Rift.

  3. Linear sea: further spread apart to form sea, like red sea.

  4. Mid-ocean ridge: tension secondary, but upward move of magma pushed plates away.

  5. Newer spreading center: Red Sea! On continent: East Africa Rift!

• Features in convergent boundaries:

  1. Arc an trench system - oceanic - continental convergence. Oceanic plate heavier and sink. Where? S. America, Andes mountains, Cascade range (younger), Sierra Nevada (older)

  2. Oceanic- oceanic convergence: island arc formed instead of volcanic arc. Where? Japan, Philippines. 

  3. Continental - continental convergence: India-Himalaya. Fig 7.13 c.

  

• Features in transform boundaries:

  • The rate of spreading of mid-ocean ridge differs. Sliding one plate against another horizontally will offset the difference. No creation nor destruction of plates.

New Evidences:

1. Paleomagnetism: Earth has magnetic N and S poles and geographic N and S poles. Magnet points to magnetic poles. Fe has no magnetism above 580 degree, called Curie point. Thus, after magma cools, magnetism of Fe in rocks develops.

• The arrow indicate the magnetic N pole. Does the N pole change position with time? No. So the rock in plates changes. Thus, plates change their position.

• Polar wondering: When construct directions, magnetic poles move. What happened is the move of the plates. See Fig. 7.18.

• Location of the pole stayed the same, but the magnetic field reversed its polarity periodically, i.e. field change directions with time. The normal and reverse position are systematic at the MOR, indicating rocks formed at the same age were pushed away to both sides.

1. 

2. Earthquake: Earthquake active regions corresponds to plate boundaries. Shallow ==> mid ==> deep earthquake follows the same direction as the subducting slab.

3. Ocean Drilling: younger and thinner sediments near the MOR and thicker and older sediments near the subduction zone. Also, in oceanic basin, rocks younger than 160 Ma.

4. Hot spot: fied in position beneath the crust. WHen plate move through, volcanoes form. Thus, tell the directions of plate movement. Fig. 7.25.

7-3    Pangaea reconstruction (7.26):

• Before 200 Ma Pangaea.

• 150 Ma, break into 2: Laurasia and Gandwanaland

• 100 Ma, major plates form

7-4    Driving force:

Convection Fig.7.27:

Homework:

• Read chapter summary on p.212.

• Use your own word to explain the key terms on page 212.

• Answer the review questions on page 212.