Geology And Geophysics

A Guide to Plate Tectonics how Continents Move



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English philosopher Francis Bacon noted in 1620 that the shapes of the continents allowed them to be fit together. He came to this realization after viewing new maps that had been created due to heightened seafaring activity in the previous century. A less distant and more coherent origin of Plate Tectonics lies in the "Continental Drift" hypothesis created by Alfred Wegener, a meteorologist who proposed this idea in 1915. Though the seafloor was virtually unexplored by this time, Wegener had several other pieces of supporting evidence for his theory that for this reason differs drastically from the modern idea of plate tectonics. The shapes of the continents allowed them to easily fit together into a single continent he labelled "Pangaea", which was surrounded by a super ocean "PanThalassa." He supported this idea with fossil evidence (similar finds from places far apart), the distribution of modern plant and animal life, and geological evidence for past climates that differed greatly from the modern climates. Wegener's theory however stated that the continents plowed through the ocean floor, an aspect that makes his theory much different than the modern notion of plate tectonics.

In the 1950s exploration of the sea floor became possible after advances in technology made during World War II that allowed for echo sounding sonar. Subsequent expeditions revealed previously unknown geological features of the sea floor such as ridges and valleys. Harry Hess resurrected (albeit in a slightly different form) the ideas of Wegener in 1962 with his theory of "Sea Floor Spreading" and changed our understanding of Earth.

So how does Plate Tectonics work?

The Driving Force of Motion - Convection, or the flow of heated fluids due to differences in temperature is believed to cause the plates to move as molten rock rises from the Earth's mantle. These form cells in which hot liquid rises and cools as it moves along the mantle until it eventually descends again. Some theories suggest that the plates themselves contribute to their motion; a descending plate will have its lower part's composition changed into denser rock, possibly causing gravity to aid its descent. Another idea is that gravity also helps plates move away from an elevated divergent boundary.

Types of Boundaries -

Divergent - These types of boundaries form when two neighboring cells have heat rising at their boundary. This type of motion is known as sea floor spreading (as new sea floor is created and pushed away from the center on either side) and is characterized by volcanic activity that leads to the formation of undersea mountain ranges and ridges.

Convergent - This movement is caused when two convection cells have cooler liquids descending at their boundary. Typically, the denser plate will submerge. Oceanic plates are thicker than continental plates. When one of the plates can submerge the process is known as subduction. The heat generated from this boundary can be enough to form a chain of volcanos (an island arc) that can rise above sea level. When an oceanic plate sinks underneath a continental plate, the result is volcanos on land. When two continental plates collide, it is known as continental collision since neither can descend. This creates high mountain ranges along the boundary, such as the Himalayas in India.

Transform - This describes a boundary in which two plates slide past one another. Since neither plate is descending this type of boundary usually creates little volcanic activity, but it does produce powerful earthquakes. The plates don't slide past each other at a uniform speed; they grind and build up potential energy that is released when it becomes great enough to snap the plates past each othe. The San Andreas fault in California is an example of this type of motion.

Hot Spots - These are stationary plumes caused by upwellings of heat in the mantle. They are associated with volcanic and seismic activity. As plates move across these, old volcanos die out and become islands that form in a chain, such as Hawaii.

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