Plate Tectonics

Continental DriftProposed by Alfred Wegner

Continental Drift - theory which states that parts of Earth's crust slowly drift atop a liquid core

Continents were once joined together as a gigantic supercontinent (Pangaea) 200 mya

Broke up and drifted to present position

Used matching coastlines of continents, rock formations, fossils, and ancient climates to support his theory

Evidence From RocksAreas of similar rock types on opposite sides of Atlantic Ocean

Ex: rock layers in Appalachain Mountains in US were identical to those found in mountains of Greenland and Europe

Evidence From FossilsFossils of several different animals and plants that once lived on or near land were found on widely separated continents

Ages of fossils also supported his timeline of breakup of Pangaea

Evidence From ClimateSeed ferns (Glossopteris) grow in temperate climates, so the places where these fossils had been found were once closer to equator

Coal deposit found in Antarctica indicate that this frozen land once had a tropical climate - located closer to equator

Glacial deposits found in regions where it is too warm for them to develop indicates these landmasses had once been closer to the south pole

A Rejected NotionContinental drift was not initially accepted for two reasons:

No explanation for how continents moved

No explanation for what caused their motion

Ocean Floor TopographySonar technology - using sound waves to measure water depth and map topography of ocean floor

Revealed ocean ridges and deep-sea trenches

Seafloor SpreadingProposed by Harry Hess

Seafloor Spreading - process by which new oceanic crust forms at mid-ocean ridges by upwelling of magma due to mantle convection

Mantle convection - slow, churning motion of Earth’s mantle

As crust moves away from ridge it gets cooler, denser, and thicker

Crust eventually subducted and recycled at deep-sea trenches

Evidence in Rocks and Sediments

Age of crust increases with distance from ridge

Thickness of sediments increases with distance from ridge

Evidence in Paleomagnetism

Earth's magnetic field experiences periodic reversals in polarity

Ocean floor has corresponding patterns of normal and reversed polarity in “magnetic stripes” on each side of rift