Evidenceĭuring the 1950s, in the mid-atlantic ridge discoveries of sea-floor spreading and magnetic reversal proved that Wegener's theory was real and led to the theory of plate tectonics, though his proposed causes were mistaken. Wegener thought that the forces that moved the continents could be caused by the rotation of the Earth and stellar precession and that same forces made earthquakes and volcanic eruptions. Others argued that Wegener's theory did not explain the forces that would have been needed to move continents to such great distances. Some critics thought that giant land bridges could explain the similarities among fossils in South America and Africa. However, most Earth scientists and palaeontologists did not believe Wegener's theory and thought it was foolish. His Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944. He proposed in 1931 that the Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface. The British geologist Arthur Holmes championed the theory of continental drift at a time when it was unfashionable. Although continental drift explained many of Wegener's observations, he could not find scientific evidence to make a complete explanation of how continents move. Wegener said that because they are less dense, continents float on top of the denser rock of the ocean floor, and move across the ocean floor rock. He believed these similarities could be explained only if these geologic features were once part of the same continent. Also, he said that the rock strata in South Africa and Brazil were similar. For example, he said that there were geological similarities between the Appalachian Mountains in North America, and the Scottish Highlands. Wegener used geologic, fossil, and glacial evidence from opposite sides of the Atlantic Ocean to support his theory of continental drift. The first time was by the mapmaker Abraham Ortelius in the 16th century. The theory had been proposed before, more than once.
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