Drifting continents

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  • Created by: naomi
  • Created on: 14-05-13 11:24

Evidence of continental drift - fit of the contine

Using prsent coastline of Africa and South America does not give an exact jigsaw fit. This is not surprisingly because:

  • sea level is constantly chaning, so a coastline is a temporary feature
  • deposition and erosion has occurred since the two continents drifted apart 167 Ma
  • where there has been erosion of the continents, there is a gap
  • where there has been deposition of sediment, there is an overlap

There is a much better fit if you use the edge of the continental shelf or a specific depth, like 1000m, or 500m

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evidence of continental drift - rock types

To prove that two rocks on either side of an ocean were once part of the same outcrop, they must have the same:

  • distinctive characteristics - of mineral composition and physical features
  • age determined by radiometric dating

Exampes of matching rocks including Precambrian cratons, Carboniferous coals and tillites, Permian red sandstones and evaporites and Upper Triassic flood basalts

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evidence of continental drift - mountain chains

Fold Mountain chains are linear features hundreds of kilometres long

The map of Gonwanaland shows how one Precambrian fold mountain chain crosses from Africa to South America and back to Africa as a continuous belt - so the two continents must have been joined together in Precambrian

The trend of fold moutains provides a way to match geology across continents

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Evidence of continental drift - fossils

If Africa and South America have always been separated, they should have a different fossil record, especially for animals and plants which lives on land or on shallow sea floors

Such animals and plants would be unable to spread across a wide ocean

During the Carboniferous, land-based reptiles (Mesosaurus) and plants (Glossopteris) are found in both Africa and South America 

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evidence of continental drift - glaciation

In both South America and Africa, there are sedimentary deposits of angular, poorly sorted and scratched pebbles (clasts) in a fine-grained matrix

This is a fossil boulder clay or tillite deposited by an ice sheet that existed during the Carboniferous about 300 Ma

Glacial striations are used to trace the movement of the glaciers to one common source area in central southern Africa

Gondwanaland probably occupied a position near the south pole, as ice sheets cannot extent to the equator

Africa and South America are now much further north, so this is clear evidence that the continents have moved 

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evidence of continental drift - palaeomagnetism

Iron-rich minerals inn some rocks hold a record of the Earth's magnetic field at the time of their formation

A large number of rocks are collected then dated, and the direction of palaeomagnetism measured

This data is then plotted as an apparent polar wandering curve

The curves for South America and Africa suggest that, before 160 Ma, one north pole was in two positions at the same time

In fact, the magnetic pole cannot significantly change position

Instead, if we assume the north pole remained fixed, it must be the continents that have moved 

If the two continents are re-positioned next to each other, the two curves match up, and there is then one position for the pole

The curves diverge only after the continents started to drift apart 

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