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Continental drift: 1912

Alfred Wegener, a German geophysicist, gives a lecture in which he proposes a radical theory. He begins from a geographical oddity often previously commented on - the neat way the coast of south America fits, like a piece of a jigsaw puzzle, into the coast of West Africa. He elaborates by pointing out how identical types of rock and fossil are found in different continents.

Wegener's hypothesis, to explain this state of affairs, is that all our present land masses were at one time joined together as one; and that they have reached their present positions by a process of continental drift.

Wegener's hypothesis provokes violent controversy, with strong views expressed for and against. But by the time of his death, in 1930, the consensus of scientific opinion is against such a radical notion.

However, from the 1950s confirmation seems to come from several areas of research - into the earth's magnetic field in past times, and into the ocean beds - and the new science of plate tectonics evolves, providing a feasible explanation of how continental drift can occur. More recently satellite observations have provided direct evidence. The Atlantic is growing wider at the rate of about 7 cm every year.

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A spreading seafloor: from the 1960s

During the 1960s there are major changes in scientists' understanding of the behaviour of the earth's crust. It is discovered that the crust beneath the oceans is thinner than beneath the land, and that it is composed of much younger rock formations. It is also proved that in parts of the oceans there are submerged mountain ranges with great rift valleys in them.

The explanation is that molten material (or magma) from the earth's mantle is forced up through these rifts to form new ocean floor. Where this happens, the seafloor spreads and the continents drift apart - by a process known as Plate tectonics.

This History is as yet incomplete.

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