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Alfred Wegener was a German scientist who first published the theory of continental drift in the early 20th century. Wegener postulated that the Earth's continents were originally one large land mass that separated over time due to continental drift. The shapes of the continents and fossil/geological data from opposing continents supported this theory but Wegener could never come up with an underlying reason for the motion of the continents. The theory of continental drift eventually evolved into the modern theory of plate tectonics. This holds that the lithosphere (or the Earth's crust) is made up of numerous plates that literally float on the underlying liquid asthenosphere. The motion of the plates and resulting activity at their edges to form mountains, trenches, and volcanoes explains the planet's topography. But there is still some debate on exactly why the tectonic plates move. One theory holds that convection currents in the liquid mantle asthenosphere cause the plates to move. The convection currents arise from either changes in density of the crust or from the movement of the heat from the Earth's core as it radiates outward. Another theory is that the motion of the plates is tied to the rotation of the planet and gravitational effects from the core and possibly the Moon play a role as well.
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