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This article explores Alfred Wegener's theory of continental drift and its evolution into plate tectonics. It discusses the evidence for continental drift, the discovery of sea floor spreading, and the development of the plate tectonics theory. The article also explains the different types of crust and plate movements, as well as their geological effects such as earthquakes and volcanoes.
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In 1912 Alfred Wegener (1880-1930) noticed that the continents fit together like the pieces of a puzzle. He proposed that the continents were once compressed into a single continent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current distribution. He believed that Pangaea was intact until about 300 million years ago, when it began to break up and drift apart.
Wegener had four main pieces of evidence. 1. He noted the jigsaw fit of South America and Africa, especially, but also elsewhere.
2. Wegner also noted that fossils from South America and Africa came from the same extinct animal. Both continents back then had the same climate and vegetation, today that is not the case.
3. He found that on both sides of the Atlantic, mountains were the same; both in terms of age and structure.
4. He found that ice sheets covered parts of Africa, India, Australia and South America 250 million years ago. How could this happen in places that are so warm today?
As technology progressed, one other evidence was added to Wegner’s Theory: Sea Floor Spreading which will be explained in detail in the next few slides.
Sea Floor Spreading: “Advances in sonic depth recording during World War II (SONAR) led to a detailed mapping of the ocean floor. The Ocean Floor in the Mid-Atlantic was found to be spreading apart. Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches and island arcs” http://www.ucmp.berkeley.edu/geology/tecmech.html
The crust near the continental margins (the shoreline of the continents today) is about 200 million years old. It gets progressively younger toward the mid-Atlantic ridge, where oceanic crust is forming today (red).
14_09.JPG Scientists learned that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. http://www.ucmp.berkeley.edu/geology/tecmech.html
Wegener's hypothesis of continental drift lacked a geological mechanism to explain how the continents could drift across the earth's surface. It wasn’t until the 1960s that the theory of Plate Tectonics was advanced to explain how the continents could separate. A Canadian by the name of Tuzo Wilson played an important part in the development of this theory.
What Tuzo Wilson did was change the way scientists viewed the internal structure of the earth.
Old http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html
In order for the theory of plate tectonics to be possible. The crust of the earth called the Lithosphere was subdivided. The upper portion of the Lithosphere was called the Earth’s crust.
The bottom portion of the Lithosphere was renamed and called the Asthenosphere and it has plastic characteristics, thus allowing the plates of the earth to float on top and move!
The 4 main features of plate tectonics are: • The Earth's crust is broken into a series of plates (crustal plates) or pieces. • These plates are continually, moving, spreading from the center, sinking at the edges, and being regenerated. • Convection currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is radioactivity deep in the Earth's mantle.
As mentioned before there are actually two types of crust: • Oceanic crust, which is thin and of course found at the bottom of the oceans. It is a compact, thin and heavy crust. • Continental crust, since it has been exposed to the atmosphere is bulkier (air) and lighter than Oceanic crust.
http://geog.ouc.bc.ca/physgeog/contents/10i.html Convection Currents power the plate movements. Convection currents rise up from the radioactive core, carrying heat to the thin crust of the earth.
The currents in the asthenosphere are generated by heat rising to the earth’s surface from the hot radioactive core • At their boundaries, the plates spread apart, converge, and slide past one another • This makes these areas the most geologically active: earthquakes and volcanoes and mountains
http://www.ngdc.noaa.gov/mgg/image/mggd.gif The Surface of the Earth without water
There are 3 basic Plate movements or boundaries: 1. Divergent: This is where the plates PULL apart, new magma wells up to the surface, forming new crust. The Mid-Atlantic ridge is a prime example. New land is created. 2. Convergent: Two plates PUSH together. Usually one of the plates subducts (goes under) the other plate, creating a Subduction zone. The crust at the leading edge of the subducting plate melts back to magma. The Pacific Rim of Fire is a good example. Land is destroyed – balance.
3. Transform Boundaries: This occurs when two plates rub against each other. This creates tremendous friction which is eventually released in the form of earthquakes. The San Andreas Fault is a Transform boundary.
14_10b.JPG Another look at the famous ‘Ring of Fire’
Click here to see an excellent ‘Shockwave’ image of the movements of the plates.
Transform Zone Result - Earthquakes
http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm Finally Canada’s role: The oceanic Juan de Fuca plate is moving beneath the continental North America plate at a rate of about 4 cm/year. Earthquakes occur along part of the boundary between the two plates and Volcanoes occur as well. Mt. St. Helens is a result.
