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The earth's crust is made of up separate plates that ride atop a sea of magma. These plates are constantly shifting and interacting. The phenomena that affect the size and shape of the Earth's continents and ocean basins are subtle and occur over hundreds of thousands of years. (Image source: GIS-based plate boundaries, ETOPO2 global elevation data). To view this animation you may need to download the Macromedia Flash Player or you can view the Printable version.