Chapter 7.2 Notes: Restless Continents 1910- Alfred Wegener developed the Continental Drift hypothesis *Hypothesis states that 245 million years ago the continents once formed a super continent (Pangaea) when reptiles, winged insects, and great tropical forests which formed coal deposits, covered large parts of the Earth’s surface.
Wegener published his evidence to support the Continental Drift 1 Wegener published his evidence to support the Continental Drift 1. Evidence from Landforms: -piece together Africa & S. America & mountain ranges line up where the continents join. -European coal fields match up with coal fields in N. America. 2. Evidence from Fossils: -fossils of Mesosaurus & Lystrosaurus have been found in places now separated by oceans. -Glossopteris (fern) lived 250 mill. yrs. ago and appears on separated landmasses of Africa, S. Amer., Australia, India, & Antarctica. 3. Evidence from climate: -island of Spitsbergen in Arctic Ocean is ice-covered & harsh polar climate, but evidence of tropical plants from a warm climate 300 mill. years ago found there.
Wegener’s hypothesis was not accepted b/c he couldn’t explain the force that moves the continents. Many geologists (from 1920’s to 1960’s) believed Earth was slowly cooling & shrinking & mountains formed when the crust wrinkled like the skin of a dried-up apple. Wegener disputed this by stating that if this were true, then mountains would be found everywhere instead of the edges of continents. Also, they knew that by the strength of rock it wasn’t possible for the crust to move.
Scientists theorize that supercontinents occur in cycles 500 mill. yrs. before Pangaea, a supercontinent (Rodinia) existed 245 mill. yrs. ago Pangaea existed 135 mill. yrs. ago Pangaea split up into 2 huge continents (Laurasia & Gonwana) 65 mill. yrs. ago, split again to form present day continents
In 1960, Harry Hess, supported Wegener’s hypothesis Hess proposed sea-floor spreading (that at the mid-ocean ridge, sea floors moved like conveyor belts carrying the continents with them) -Mid-ocean ridges--underwater mountain chains that run thru center of Atl. Ocean-cracks in ocean crust allow molten material to spread out, pushing older rock to both sides of the ridge
Evidence of sea-floor spreading: 1. Evidence from molten material-scientists dived to mid-ocean ridge in Alvin (submersible) & found pillow rocks formed when molten material hardens quickly after erupting under water. 2. Evidence from magnetic pole reversal: throughout history, north & south magnetic poles have reversed themselves several times. Last happened 600,000 years ago. Scientists found that the ocean floor rock with iron bits in them lined up in the direction of Earth’s magnetic poles & lie in a pattern of magnetized stripes which hold a record of reversals.