Evidence for Continental Drift Early maps of the world caused Alfred Wegener to propose the continental drift theory (early 20 th c.). The continents looked as though they fit together like puzzle pieces. The fit including continental shelves is even better! Wegener named the original supercontinent ‘Pangaea’ Other evidence to support his theory:  evidence of different climates (ie. glaciers) on continents that are now have warm climates  matching geological features (ie. mountain ranges) & rocks (age & type) on different continents  matching fossils (ie. Mesosaurus) on different continents

Mesosaurus! sharp-toothed, freshwater reptile fossils found in SE South America & SW Africa Rowr!

The ‘Jigsaw Puzzle’ Fit Like pieces of a jigsaw puzzle, the continents fit together into one, large whole. Pangaea – millions of years ago!

Paleoglaciation …refers to the extent of ancient glaciers, and the rock markings they have left behind. 1. scientists have discovered evidence of glaciers in areas that are now tropical  millions of years ago, much of Pangaea’s land mass sat toward the South Pole 2. coal deposits in Antarctica – coal is made of decomposed remains of once-living things – usually tropical swamp material

Paleoglaciation

How Can Continents Move? Wegener’s theory of continental drift didn’t explain how entire continents moved. In Wegener’s time, it was not known that Earth’s crust consists of tectonic plates.  tectonic plates slide over a layer of partially molten rock  a map of volcanoes & EQ zones aligns with the outline of tectonic plate boundaries

How Can Continents Move? Mapping of the ocean floor (1872) revealed the Mid-Atlantic Ridge, a long mountain range running N-S along the floor of the Atlantic Ocean. The youngest rocks were found closest to the Mid-Atlantic Ridge…older ones farther away. The layer of ocean sediment became thicker further away from the ridge.

Magnetic Reversals over 1000s of years, the direction of Earth’s magnetic field can reverse completely! magnetic reversal is caused by changes in the motion of the liquid iron in Earth’s interior paleomagnetism shows that iron- based rocks along the ridges are striped with reversing magnetic fields (1950s) scientists using magnetometers detected a pattern of stripes in the direction that iron- containing minerals pointed on the sea floor  ‘magnetic striping’

Sea Floor Spreading: An Explanation Hess (1960) gave an explanation that tied together data on the age of ocean rocks, sediment thickness & magnetic striping.  magma rises because it is less dense than the material surrounding it  when magma breaks through Earth’s surface at a spreading ridge, it cools & hardens  like a “new sea floor” conveyer belt, pushing older rock aside = sea floor spreading  Magnetic striping of basalt rock shows long stripes of new rock moving away from ocean ridges, and also reveals the direction of Earth’s magnetic field at that time.  farther from the ridge, ocean sediments are thicker  rock is older & has had more time to accumulate sediment

Sea Floor Spreading: An Explanation (mid-1960s) J. Tuzo Wilson (a Canadian!) unified the ideas of sea floor spreading & paleomagnetism to explain continental drift  Plate Tectonic Theory Explained:  formation of mountain ranges & ocean basins  cause of EQs & volcanic eruptions  movement of tectonic plates  transformation of rocks from one type to another in the rock cycle  formation of Hawaiian Islands occurred when a tectonic plate passed over a stationary hot spot  geologic hot spots are anywhere magma rises to Earth’s surface