1. Continental Drift

2. In 1915 – Alfred Wegener published a book that challenged geologists’ assumption that the continents remain fixed in position through all of Earth’s history.

3. Evidence of Continental Drift Activity!

4. Wegener suggested that a vast supercontinent, Pangaea, existed until near the end of the Mesozoic Era (from 251 to 65 million years ago).
Pangaea would later fragment into separate continents that drifted apart, moving slowly to their present positions.

5. This model came to be known as continental drift.
Wegener’s Evidence for Continental Drift:
1. The Fit of the Continents!

6. 2. Locations of Past Glaciations
2. By studying the age of the glacial till (left over sediment from glaciers)
3. In the belt (equator) of Pangaea Wegener found late Paleozoic sedimentary rock layers of coal (old swamp and jungle) and relicts of reefs
3. The Distribution of Climatic Belts

7. 4. The Distribution of Fossils
5. Matching Geologic Units

8. Present day (20th century) proof of Continental Drift:
1. Paleomagnetism: the record of ancient magnetism preserved in rock!

9. By studying different ages of rocks in the same location, scientists discovered that each continent moves relative to the Earth’s magnetic poles!
And that each continent has a different apparent polar-wander path!

10. 2. Sea-floor Spreading
Sea Floor Maps constructed during the s revealed several important features:
The floor beneath all major oceans includes abyssal plains – broad, relatively flat regions of ocean at about 4-5 km below sea level
And mid-ocean ridges - mountain ranges whose peaks lie only about km below sea level.

12. Deep-ocean trenches: Along much of the perimeter of the Pacific Ocean, the ocean floor reaches depths of 8 to 12 km!
Trenches border volcanic arcs, curving chains of active volcanoes.

13. During the 1960s, sea-floor spreading was proven:
Molten rock (magma) rises upward beneath mid-ocean ridges and solidifies to form oceanic crust! The new sea floor then moved away from the ridge!
Old ocean floor is consumed in the ocean trenches - into the mantle by subduction!