Theory of Plate Tectonics Rocks that form the continents are different from the rocks that form the ocean floors. Scientists have noticed patterns in structures such as mountains and in events like earthquakes. They have seen ridges and trenches.

Theory of Continental Drift Alfred Wegener proposed the Theory of Continental Drift in 1912. He suggested that the continents drifted (moved) to their current location from a single super continent Pangaea. Coasts of South America and Africa seem to fit together. These continents share similar fossils and rock formation. Wegener could not explain why the drift happened though.

New Evidence Seismologists have since noticed: Most earthquakes and volcanoes are concentrated along narrow strips of land. There are very large areas of land with few earthquakes and no volcanoes. Additional research points out that more volcanoes are developing along the ridges in the ocean floor. We have also learned the ocean floor is moving from ridges toward deep trenches.

Theory of Plate Tectonics: Earth’s lithosphere (upper mantle and crust) is broken into large plates. Earthquakes and volcanoes tend to occur where the plates meet. Plates usually include crust from both continents and oceans. All of these plates are moving slowly over Earth. Evidence for plate tectonics: Matching patterns on both sides of a ridge- specifically magnetic patterns in the rocks. Rocks of the same age around the world have magnetic fields that point different directions but if you rearrange them to form Pangaea the magnetism lines up. Age of rocks on islands- older islands are never found near mid ocean ridges.

What Happens when Plates Meet?

Divergent Plates Diverging boundary= plates moving apart. When they create openings, lava flows creating volcanoes.

Converging boundaries= plates come together. Can form trench when heavier, thinner oceanic crust is pushed down under continental crust. Earthquakes rumble and mountains form on the continental crust. The oceanic crust moves lower into earth, melts and molten rock can rise in some plates to form volcanoes. Ex) Two continental plates converge to form huge mountain ranges. Ex)

Transform boundary Transform boundary= plates slide against each other; since this doesn’t happen smoothly, earthquakes can happen.

Most plate boundaries are under water but some ex) San Andreas Fault can be seen on land.