1. Plate Tectonics Continental Drift and Seafloor Spreading

2. What is the problem?  The idea of continental movement was not even proposed until the late 1800’s, why?  In the average human’s lifetime there are little or no changes seen in the Earth’s surface.  The processes that we can see in our lifetime are:  Earthquakes  Volcanic Eruptions  Landslides/ mud slides, etc.  However, if you measure time in millions of years, there is tremendous amounts of movement.

3. Early Ideas  Eduard Suess (pronounced Zys) was the first person to hypothesize that the southern continents were once joined.  Suess’s evidence was the presence of a fossilized land plant, the Glossopteris fern, which was found in South America, Africa and India.  Right!  He was concluded that the southern continents were connected, called the super-continent “Gondwanaland”  He proposed that the rest of the planet was a massive ocean he called the “Tethys Ocean” which covered most of the globe.  Wrong!  He thought the continents were connected by a series of land bridges that have since been covered by water.

4. Alfred Wegener  Eduard Suess only had one real piece of evidence and was not taken seriously.  In 1912 Alfred Wegener (vegener) presented his theory of continental movement and was finally taken seriously.  Theory of Continental Drift  All of Earth’s land masses were once joined, which he called, “Pangea”, which is Greek for “all the Earth.”  He believed that about 200 million years ago the continents began to move apart and have done so in a slow and constant fashion ever since.

5. Evidence: Puzzle Like Fit  Looking at the edges of some of the continents it is easy to see how they could have fit together.

6. Evidence: Split Geologic Structures  Wegener thought that when Pangea broke apart it would split large geologic structure. Ex. Mountain ranges, rock layers, etc.  Similar mountain range structures are found from Alaska to Russia.  The Appalachian Mt. range extends through Greenland and across to northern Europe.

7. Evidence: Land Fossils  Wegener hypothesized that since living things would have once lived all across Pangea that there would be fossil evidence on continents now separated by thousands of miles!  Wegener used the Glossopteris fern as evidence  The Lystosaurus, a small pig like reptile found in Africa, India and Antarctica that lived about 200 million years ago..

8. Evidence: Coal Deposits  Coal is formed when water logged plant material in tropical areas is buried and the rich organic material is compacted.  Wegener noticed that there are large deposits of coal near the north pole.  Meaning that those land masses had to have been closer to the equator when the coal was forming.

9. Evidence: Glacial Deposits  Massive glacial deposits located in southern Africa, India, Australia, and South America about 300 million years ago.  These continents had to have been close to the south pole.  Grooves from the growing glaciers further show how the continents fit together.

10. Five Pieces of Evidence  What were Wegener’s five piece of evidence to support Continental Drift?  1  2  3  4  5  Don’t say that he didn’t have any evidence!  BUT…

11. Rejected!  After decades of gathering impressive data, his theory was rejected.  He could not explain what force could move the massive continents.  He thought it was the rotation of the earth, if true all the continents would be piled up around the equator.  He could not explain how the continents moved without shattering.  He thought that the continents plowed through the ocean floor, but geophysicists proved him wrong.  Wegener froze to death in 1930 in Greenland on an expedition to collect more data to complete his theory.  With Wegener gone, his theory was completely dismissed.

12. Seafloor Spreading  During Wegener’s day, it was thought that the ocean floors were thick, flat and very old compared to the continents. They were WRONG!  As it turns out, all the evidence Wegener needed to complete his theory was at the bottom of the ocean.

13. Ocean Floor Topography  With the invention of sonar, scientists were able to map the depth of the ocean. Here is what they discovered.  Ridges – underwater mountain chains that were the longest on Earth.  Trenches – narrow, elongated depressions in the sea floor with steep sides. Mariana trench is over 6 miles deep.

14. Ocean Rocks and Sediments  Samples were taken of the rocks on the ocean floor and two patterns were discovered.  1 – The ages of the rocks made a pattern. The youngest rocks were located near the ocean ridges and the oldest rocks were located near the trenches.  The ocean floor was also very young, some of the oldest is around 180 million years old. (Bed rock in Nicholasville is about 270 million years old.  2 – Sediments on the ocean crust were much thinner than expected. Thickness increased as you got farther from the ridges.

15. Magnetism  Basalt – is an iron rich rock produced from lava.  The iron in the lava aligns to Earth’s magnetic field like a compass needle  When the lava cools the iron rich minerals (compass needles) are locked in place.  Earth’s magnetic poles flip about 4 or 5 times every million years.

16. Ocean Floor Magnetism  When scientist studied the magnetism of the ocean floor, another pattern emerged.  In places where the minerals were pointing the SAME direction as earth’s current magnetic field the reading was STRONGER than normal.  In places where the minerals were pointing the OPPOSITE direction as earth’s current magnetic field the reading was WEAKER than normal.

17. Ocean Floor Magnetism  The pattern of magnetism was perfectly mirrored on either side of mid ocean ridges.  We know that due to volcanism new rock is being made at the ridges.  This allows us to count the magnetic stripes like tree rings to determine rough relative age.

18. Putting It All Together  Harry Hess, a young American scientist came up with the theory of seafloor spreading.  New rock is created at the ridges, when it cools acts like a wedge, pushing the continents apart. (5% - 10% of motion)  Old rock is destroyed at the trenches, as it is forced under it pulls the continent towards the trench, called slab pull. (90% - 95% of motion)

19. Putting It All Together  Seafloor Spreading was the missing part to Wegener’s theory.  Continents move due to convection currents in the mantle.  Continents do not plow through the oceanic crust, but are passengers on it as it travels from ridge to trench.