Plate tectonic theory revolutionised the study of earth science. As soon as maps of the Atlantic Ocean were produced, people noticed that the continents either side seemed to fit togther remarkably well – the bulge of Souht America fitting into the indent below west Africa. Fracis Bacon had noted this fit as early as the 17th Century butit did not attract any serious attnetion as no one thought the continents could move.
The theory of plate tectonics
In 1912, a German, Alfred Wegener, published his theory that a single continent existed about 300 million years ago. He named this supercontient Pangaea, and maitntianed that it had later split into to the two continents of Laurasia in the north and Gondwanaland in the south. Today’s continents were formed from further splitting of these two masses. Wegener published this theory of continental drift and claimed that it was supported by several pieces of evidence that these areas were once joined.
· The fit of South America and Africa
· Evidence of the glaciation of the late Carboniferous period (290 million years ago), deposits from which are found in South America, Antarctica and India. The formation of these deposits cannot be explained by their present position; they must have been formed together and then moved. There are also striations (scratches) on rocks in Brazil and west Africa which point to a similar situation
· Rock sequences in northern Scotland closely agreed with those found in eastern Canada, indicating that they were laid down under the same conditions in one location
· Fossil brachiopods found in Indian limestones are comparable with similar fossils in Australia
· Fossil remains of the reptile Mesosaurus are found in both South America and southern Africa. It is unlikely that the same reptile could have developed in both areas or that it could have migrated across the Atlantic
· Fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica
Development of the theory
Wegener’s theories were unable to explian how continental movement could have taken place and his ideas gained little ground. From the 1940s onwards, however, evidence began to accumulate to show that Wegener could have been correct.
The mid-Atlantic ridge was discovered and studied. A similar feature was later discovered in the Pacific Ocean.
xamination of the ocean crust either side of the mid-Atlantic ridge suggested that sea-floor spreading was occurring. the evidence for this is the alternating polarity of the rocks that form the ocean crust. Iron particles in lava erupted on the ocean floor are aligned with the Earth’s magnetic field. As the lava’s solidify, these particles provide a permanent record of the Earth’s polarity at the time of eruption (palaeomagnestism). However, the Earth’s polarity reverses at regular intervals (approx. every 400,000 years). The result is a series of magnetic ‘stripes’ with rock aligned alternatively towards the north and south poles. The striped pattern, which is mirrored exactly on either side of a mid-oceanic ridge, suggest that the ocena crust is slowly spreading away from this boundary.Moreover, the oceanic crust gets older with distance from the mid-oceanic ridge.
Sea-floor spreading implies that the Earth must be getting bigger. As this is not the case, then plates must be being destroyed somewhere to accomodate the increase in their size at mid-oceanic ridges. Evidence of this was found with the discovery of huge oceanic trenches where large areas of ocean floor were being pulled downwards.