Chapter 2 - Chapter 2 Plate Tectonics Continental drift A...

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Chapter 2: Plate Tectonics I) Continental drift:  A) Pangaea: single supercontinent once existed B) Began to fragment into smaller continents which gradually “drifted” to their current positions C) Evidence:  1) Fit of South America and Africa a) Not perfect because of erosional and depositional processes b) True outer boundary of a continent is the seaward edge of its continental shelf 2) Geologic distribution of fossils: Identical fossil organisms found from rocks in South American  and Africa a) Mesosaurus: aquatic fish-catching reptile fossils found only in South America and Africa b) Could be a land bridge but modern seafloor maps show no land bridge of this size could  have existed c) Glossopteris: plant with large seeds  i) Grew only in sub polar climates ii) When landmasses were joined they were located closer to the south pole 3) Rock types and structural similarities a) Rocks found in particular region on one continent correspond in age and type found in  adjacent positions on adjoining continent b) Mountain belts that terminate at the coastline: when landmasses connected formed a  nearly continuous belt 4) Ancient climates (paleoclimatic evidence) a) Ancient glacial deposits indicate ice sheets covered extensive areas of Indian and  Southern Hemisphere—layers of glacially transported sediments of same age found in  southern Africa, South America, Australia i) Evidence of late Paleozoic glaciation lies within 30 degrees of the equator in  subtropical and tropical climates ii) Southern continents joined together and located near south pole b) Vast coal deposits in Northern landmasses: geography would have been nearer to the  equator D) Rejection of continental drift: Could not identify mechanism capable of moving continents across  the globe, Wegener suggested: 1) Gravitational force that moon and sun exert on Earth to produce the tides—tidal forces would  affect Earth’s outermost layer which would slide as detached continental fragments over the  interior 2) Larger and sturdier continents broke through the oceanic crust  E) Problems with continental drift: 1) Main idea correct but many incorrect details 2) Could not be tested II) New evidence: paleomagnetism A) Today magnetic poles align closely with the geographic poles
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B) Earth’s magnetic field: invisible lines of force pass through the planet and extend from one  magnetic pole to the other C) Rocks contain iron-rich minerals that serve as fossil compasses 1) When heated above Curie point they lose their magnetism 2) When iron-rich grains cool below Curie point they become magnetized in direction of existing  magnetic lines of force 3)
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