What evidence do we have that over time continents have moved?

Evidence from land features

Evidence from fossils

Evidence from land features Evidence from fossils Evidence from climate

Evidence from land features -Mountain ranges on Africa and South America line up -European coal fields match up with North American coal fields

Evidence from fossils -Fossils of freshwater reptiles found in places now separated by oceans -Fossils of a fern-like plant found on Africa, South America, Australia, India, and Antarctica

Evidence from climate -Fossils of tropical plants found on an island in the Arctic Ocean -Deep scratches in rock showed that continental glaciers once covered South Africa

Wegener’s hypothesis that the continents slowly move over Earth’s surface

Wegener could not explain how the continents moved His idea was not accepted by scientists at the time

Using sonar, scientists have been able to map the ocean floor. Sonar—device that bounces sound waves off underwater objects and then records the echoes of those sound waves.

Using sonar, we discovered mid- ocean ridges and deep ocean trenches Mid-ocean ridge—underwater mountain chain Deep ocean trench—underwater canyon where the oceanic crust bends downward

Idea proposed by Harry Hess, an American geologist In sea-floor spreading, the sea floor spreads apart along both sides of a mid-ocean ridge as new crust is added. As a result, the ocean floors move like conveyor belts, carrying the continents with them.

Molten material Magnetic stripes Drilling samples

Molten material—small submarine Alvin found rocks formed only when molten material hardens quickly after erupting underwater. These rocks were found again and again along the mid-ocean ridge

Magnetic stripes—rocks that make up the ocean floor lie in a pattern of magnetized “stripes” forming a record of Earth’s magnetic pole reversals This pattern of stripes is the same on both sides of the mid- ocean ridge—one side is a mirror image of the other

Drilling samples—the Glomar Challenger collected rocks from drilling holes in the ocean floor and determined the age of the rocks The youngest rocks were always in the center of the mid-ocean ridge; the farther away from the ridge, the older the rocks were

If the sea floor is spreading apart, then how can the ocean floor keep getting wider and wider?

At the deep-ocean trenches, subduction occurs.

Subduction—the process by which ocean floor sinks beneath a deep-ocean trench. In other words, the older, denser oceanic crust sinks back into the mantle. New ocean crust is added at the mid-ocean ridge.

Sea-floor spreading and subduction work together. One of the oceans is shrinking and one is expanding. Which one is which?

The Pacific Ocean is shrinking. The Atlantic Ocean is expanding.

Canadian scientist, J. Tuzo Wilson proposed the idea of plate tectonics. He discovered cracks in the continents similar to those on the ocean floor.

Plates—separate sections of the lithosphere which fit together along cracks Wilson combined the ideas of sea-floor spreading, Earth’s plates, and continental drift.

The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.

The edges of Earth’s plates meet at plate boundaries. Faults form along these boundaries. Faults—breaks in Earth’s crust where rocks have slipped past each other

There are three types of plate boundaries: Divergent Convergent Transform A different type of plate movement occurs along each type of boundary.