Plate Tectonics
Brief HistoryIn 1858, Antonio Snider-Pellegrino and others observed a jigsaw fit between the continents (most notably between South America and Africa).
In 1912, Alfred Wegener developed the Theory of Continental Drift which states that the continents themselves had shifted away from a single ancient super-continent called Pangaea. He used fossil evidence from South America and Africa to support his theory. The problem was that Wegener could not explain how the continents moved - what force was behind this motion. In 1928, Arthur Holmes proposed that the earth's mantle contained convection cells that moved the earth's continental plates. In 1960, Harry Hess discovered that the floor of the Atlantic Ocean was widening along the mid-Atlantic Ridge; a concept called sea-floor spreading. Plate TectonicsPlate tectonics helps to explain most geologic processes. The theory states that Earth's outer shell is made up of about 20 plates. Most of these plates are made up of both oceanic and continental crust. They are floating on a layer of hot rock, several hundred kilometers below Earth's surface, which flows like a slow-moving plastic. It is likely that convection currents caused by the equal distribution of heat within Earth's core cause the plates to move (Clark et al 2006).
Types of Plate BoundariesConvergent:
Where two plates, moving toward one another, converge. If the two plates are both continental crust, and of roughly equal density, they will crumble upward and form a mountain. If one of the plates is oceanic crust, which is denser, it will subduct below the other plate and be recycled back into the mantle. Divergent: Where two plates, moving away from another, are pulling apart. Divergent boundaries create rift valleys. The mid-Atlantic Ridge is an example of a divergent plate boundary. Transform: Where two plate slide past one another. The San Andreas Fault is an example of a transform boundary. Earthquakes & VolcanoesEarthquakes and volcanoes occur at plate boundaries. Earthquakes occur when there is movement between two or more plates. Pressure builds up and leads to a release of energy (rock movement) that sends seismic waves through the ground.
Volcanoes are formed when an opening in the crust allows mantle material to escape. Definitions
Cenozoic Era: most recent era of geologic time, which began about 66 million years ago.
Continental Drift: theory by German scientist Alfred Wegener stating that 300 million years ago all of Earth's land masses, which were in constant motion, collided to form one supercontinent called Pangaea. About 200 million years ago Pangaea broke apart and the continents have drifted apart to their present locations. According to his theory, only continents drifted. Continental Shelf: gently sloping outer edge of a continent that extends below the surface of the ocean to a maximum depth of about 200 meters. Convection Current: circular movement in a gas or liquid created by uneven heating. Eras: major divisions of geologic time (for example, the Paleozoic Era). |
Chapters
11 Canada's Geologic History (p. 110-124)
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