CONTINENTAL DRIFT In 1812, Alfred Wegener presented his scientific theory called “Continental Drift”. It was based on his belief that the continents had drifted apart from one another.

Pangaea Wegener ‘s hypothesis was that all the continents had once been joined together in a single landmass and have since drifted apart. He believed Pangaea meaning “all lands” existed about 300 million years ago. He proposed that Pangaea began to break apart. The pieces of Pangaea slowly moved toward their present-day locations, becoming the continents as they are today.

Pangaea Show what Earth looked like in the distant past animation

Evidence for continental drift

Other evidence In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa.

Scientists Reject Theory In the early 1900’s many geologists thought that Earth was slowly cooling and shrinking. Mountains formed when the crust wrinkled like the dried-up skin of an apple. Wegener not only proposed that continents drifted, but he believed mountains formed when drifting continents collided, causing their edges to crumple and fold. Scientists rejected Wegener’s theory because he could not identify a force that could push or pull continents.

Possible Proof In 1960 Harry Hess, an American geologist proposed a wild idea. After examining maps of the mid-ocean ridge, he concluded that maybe Alfred Wegener was right! Perhaps the continents do really move.

Sea-Floor Spreading Hess suggested that the ocean floors move like conveyor belts, carrying the continents along with them. The movement begins at the mid-ocean ridge where molten material spreads out, pushing older rock to both sides of the ridge. Hess called the process that continually adds new material to the ocean floor sea-floor spreading. The sea floor spreads apart along both sides of the mid-ocean ridge as new crust is added. View the sea-floor spreading animation

Evidence for Sea-floor Spreading Molten material – Scientists in a submersible found pillow-shaped rocks near the ridge. These can only form when molten material cools rapidly. Magnetic stripes – Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized stripes. These record a record of reversals in Earth’s magnetic field. Drilling samples – Scientists gathered rock samples from the sea floor through pipes. They then determined the rocks ages. The youngest ones were closer to the ridge and the oldest the farthest away.

Subduction Subduction is the process where the ocean floor sinks beneath a deep-ocean trench and back into the mantle. The new crust that forms at the ridge is hot. As it cools and becomes more dense, gravity pulls it down beneath the trench and back into the mantle. This process takes tens of millions of years and allows the Earth to keep recycling. Show the demo of the washcloth in water and the subduction animation.

Look at Earth’s internal convection animation Plate Tectonics This theory was proposed by Tuzo Wilson, a Canadian scientist in 1965. He proposed that the lithosphere of the Earth is broken into sections called plates. They fit closely together along cracks in the lithosphere. He combined the theories of continental drift, sea-floor spreading and subduction into one theory. This theory states that the plates are in constant, slow motion, driven by convection currents in the mantle. Look at Earth’s internal convection animation

Tectonic Plates