1. In Wegener ’ s day, little was known about the sea floor. Continental Drift was revised as the Theory of Plate Tectonics only after sonar mapping revealed strange features on the sea floor The Mid-Atlantic Ridge was discovered in the 1950 ’ s. It is part of the largest known geographic feature on Earth. By the early 1960 ’ s scientists knew that the Mid-Atlantic Ridge was only a part of a worldwide mid-ocean ridge system that was about 80,000 km long. they wrap around entire Earth like seams on a giant baseball

2. Sea-Floor Spreading as the sea floor diverges at mid-ocean ridges; lava erupts to fill widening rift cycle of rift widening and lava eruptions forms new sea floor proposed in 1962 by Harry Hess as the sea-floor plates pull apart, the rift valley widens and cracks open up magma rises up from the mantle to fill the crack this process produces new sea floor sections of mid-ocean ridges are connected by transform faults mid-ocean ridges have deep rift valleys down the middle all rifts have cracks (from splitting) and recent pillow lava (formed only when lava cools under deep water)

3. Age Pattern of the Sea Floor Basalt age of sea floor basalt increases away from the ridge no ocean floor is nearly as old as any continent Sea-Floor Sediments sediments near ridges look like this: (see notes) sediment layers are discontinuous because sea-floor spreading splits them the younger the sediments, the closer they are to the ridge older sediments have been carried away from the ridge as the plates move no sediment layers are older than 200 million years in any ocean If the sea floor had never spread, the pattern would look like this: (see notes) each layer would cover entire sea floor, with lowest layers billions of years old

4. Hot Spots hot spot volcanoes form when a plate moves over a mantle plume about 100 hot spots are known examples: Hawaiian Islands Iceland Yellowstone near hot spot volcanoes, there are a string of older volcanoes, and the ages of those volcanoes increases as you move away from the hot spot