1. Development of Plate Tectonic Theory In the late 1940’s scientists discover that the ocean floor is not flat but contains vast mountain ranges that are longer that any on land. Mountain ranges in the middle of the ocean are mid- ocean ridges. In the 1960’s Harry Hess helps explain the movement of the continents due to sea floor spreading. Sea floor spreading is the process by which new oceanic crust forms along a mid ocean ridge. (Diverging plates). Older crust moves away from the ridge and newer crust is closer to the ridge as magma (lava) oozes up between the ridges. Magnetic reversal also supports sea floor spreading. Magnetic reversal is the process where Earth’s north and south poles switch places about every ½ million years. Continental Drift Theory, Sea Floor Spreading, and the discovery that earthquakes and volcanoes occur most frequently at mid ocean ridges and certain places on Earth led to the development of Plate Tectonics Theor y.

3. Mid-Ocean Ridge