Plate Tectonics Continental Drift and Seafloor Spreading 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. 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. 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. Evidence: Puzzle Like Fit Looking at the edges of some of the continents it is easy to see how they could have fit together. 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. 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.. 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. 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. 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… 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. 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. 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. 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. 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. 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. 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. 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) 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.