Oceans have been created and oceans have been destroyed many times in the Earth’s history by plate tectonicsPlate tectonics. In this Chapter we shall meet Alfred WegenerAlfred Wegener and learn about his early theory of continental driftContinental drift and its problems. Rivals to Wegener’s theory were the shrinking Earth theory and the expanding Earth theory, but they had problems of their own. We will meet Marie TharpMarie Tharp and her discovery of the great rift valley that encircles the globe. The “Rosetta Stone” for plate tectonicPlate tectonics theory was the discovery of magnetic “stripes” on the ocean floor created by episodic reversals of the Earth’s magnetic field. The disintegration of Pangaea 170 million years ago and the rise and fall of the Tethys OceanTethys Ocean six million years ago are merely brief stages of the cycle of the birth and death of oceans. Together these lines of evidence unify ocean creation and destruction into a single mechanism—plate tectonicsPlate tectonics.