SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 4: Continental Drift & Seafloor Spreading

2 Lecture 4: Continental Drift & Seafloor Spreading

3 Plate Tectonics Plate tectonics is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s. The outermost part of the Earth's interior is made up of two layers: the lithosphere, & asthenosphere. Although solid, the asthenosphere has relatively low viscosity and shear strength and can flow on geological time scales. The deeper mantle below the asthenosphere is more rigid again. The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.66 to 8.50 cm per year.

4 Continental Drift Plate tectonic theory arose out of the hypothesis of continental drift first proposed by Alfred Wegener in 1912 that said in part that all the continents once been together in a single land mass that had drifted apart. Many geologists had noted earlier that the shapes of continents on either side of the Atlantic Ocean (most notably, Africa and South America) seem to fit together. The similarity of southern continent fossil and some geological formations had led a small number of geologists as early as 1900, believe that all the continents had once been joined into a supercontinent (now known as Pangaea). Taylor suggested that the continents were dragged towards the equator by increased lunar gravity during the Cretaceous, thus forming the Himalaya and Alps on the southern faces. Alfred Wegener was unable to provide a convincing explanation for the physical processes which might have caused this drift. His suggestion that the continents had been pulled apart by the centrifugal force of the Earth's rotation was considered unrealistic by the scientific community. The idea of continental drift did not become widely accepted even as theory until the late 1950s and the 1960s. The hypothesis of continental drift became part of the larger theory of plate tectonics.

5 Continental Drift The hypothesis of continental drift introduced by German meteorologist Alfred Wegener suggested a supercontinent, called Pangaea (meaning "all land"), that broke up millions of years ago, slowly moved to their current positions, and continue to move today.

6 Continental Drift When Wegener first noticed the similarities in the shoreline of continent on both sides of the Atlantic Ocean, he then began fitting them together. He particularly liked the way in which South America and Africa fit. It was not until the 1960's that scientists proposed a new map using the edges of the continental shelfs as the true boundaries of the continents.

7 Continental Drift Wegener used documented cases of fossil organisms that had been found on different landmass that could not have crossed the current oceans. Fossils of Mesosarus have been found only in South America and Africa. Remains of other organisms appear to link these two continents during the late Paleozoic and early Mesozoic eras.

8 Continental Drift This picture shows matching mountain ranges on each side of the Atlantic Ocean. The Appalachian Mountains that disappear off the coast of Newfoundland match mountains in the British Isles and Scandinavia which are comparable in age and structure.

9 Continental Drift The last form of evidence that Wegener use to support his continental drift hypothesis was ancient climates. When the continents are put together to form Pangaea the remains of glacial materials fit together to form a pattern like the large ice sheets that cover our poles today.

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11 Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is well-accepted now, and the phenomenon is known to be caused by convection currents in the plastic and weak asthenosphere. Seafloor Spreading

12 In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. The process starts with heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to more dense objects, the area being heated becomes a broad dome. As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. Seafloor Spreading

13 If spreading continues past the initial stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is thined as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. Seafloor Spreading

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