Drifting Continents

Evidence for Continetal DriftA large land mass called Gondwanaland is believed to have existed about 250 Ma. This huge contintent included what are no South America, Africa, Antartica, India and Australia. As a single continent, it had the same rocks and fossils and was glaciated at the same time. Gondwanaland broke up in the Jurassic about 167 Ma, the fragments drifting apart to form the continents. The evidence for its former existence can still be seen in the rocks of South America and Africa. Fit of the continents - using the present coastline of Africa and South America does not give an exact fit. This is not surprising because: sea level is constantly changing, 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 continents, there is a gap where there is 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.Rock types - to prove that two rocks on either side of the ocean were once part of the same outcrop they mus: distinctive characteristics - of mineral composition and physical features age determined by radiometric dating Examples of matching rocks include Precambrian cratons. Carboniferous coals and tillites, Permain red sandstones and evaporites and Upper Triassic flood basalts.Mountain ChainsFold mountain chains are linear features hundreds of kilometers long. The map of Gondwanaland shows how one Precambrian fold mountain chain crosses from Africa to South America and back to Africa as a continous - so the two continents must have been joined together in the Precambrian. The trend of fold mountains provides a way to match geology across continents.FossilsIf Africa and South America have always been separated, they would show a different fossil record, especially for animals and plants which have lived 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 Africa and South America.GlaciationIn both South America and Africa, there are sedimentary deposits of angular, poorly sorted and scratched pebbles 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 poisition near the south pole, as ice sheets cannot extend to the equator. Africa and South America are now much further north, so this is clear evidence that the continents have moved.PalaeomagnetismIron-rich minerals in some rocks hold a record of the Earth's magnetic field at the time of their formation. A large number of rocks are collected than dated, and 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. But the magnetic pole cannot significantly change position. So we assume that the north pole stayed fixed, and the continents moved. If the two continents are re-positioned next to each other, the curves match up, and there is then one position for the pole. The curves diverge only afte the continents started to drift apart.

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