1. Summary

The earth’s crust was originally thought of as static. It took the combined efforts of Alfred Wegener, Harry Hess, and Arthur Holmes to prove this theory wrong. Wegener’s theory of continental drift, Holmes’ presentation of thermal convection, and Hess’ discovery of rock history found on a deep seafloor ridge finally proved that the continents constantly move over the earth’s crust.

2. Why Watch This Video?

  • Have you ever wondered why the earth’s continents look like pieces of the same puzzle?
  • Would you like to know how the theory of earth’s plate tectonics was discovered and proved?
  • Have you ever been confused by the same animal or plant fossils appearing on different continents?

3. Key Terms

  • Continental drift – Alfred Wegener’s theory that states earth was made up of a single continent that gradually moved across the earth’s surface over time.
  • Plate Tectonics – The widely accepted theory that the earth’s surface is made up of multiple broken plates or slabs that are constantly in motion and interact with other plates.
  • Thermal Convection – When fluids such as gas or liquid are heated and than forced to move away from the heat source, thus carrying the thermal energy along with it.

4. Loose Ends

Loose end #1: What is Pangaea?

Pangaea is briefly mentioned at the beginning of the video. Alfred Wegener studied Pangaea before coming up with the theory of continental drift. Pangaea was a supercontinent that began to assemble during the Paleozoic era and was completed in the early Mesozoic era. When Pangea broke apart, its pieces formed today’s continent configuration.

Loose end #2: Why do plates move?

This video explains how the theory of continental drift was proved, but it only lists a few reasons why the plates move. Plate movement does occur by a push from an elevated mid-ocean ridge and also by denser oceanic crust moving underneath the less dense crust, and sinking under its own weight. But plates also move by suction by a sinking portion of a plate that has broken off from the end of a subducting plate. For a long time it was also thought that drag from convection in the asthenosphere moved plates, but this cannot be the main mechanism because of the weak connection between the asthenosphere and the overlying plates. It is exactly that weak connection that allows the plates to move.

Loose end #3: Why are there so many volcanoes along the Ring of Fire?

It was not explained why there is such a large percentage of earth’s volcanoes and earthquakes along the “Ring of Fire.” Earthquakes occur along or within a descending slab, as the slab descends to where it is cooler and denser, causing an earthquake when it suddenly shifts downward. Volcanoes arise when the descending slab releases water that lowers the melting temperature of the above mantle material. This portion of the mantle then melts, releasing magma in intervals from deep in the earth’s mantle to either solidify into a volcano or releases in volcanic eruptions.

5. Self-Test Questions

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Item Credit: It's Okay to Be Smart, PBS Digital Studios

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