Ch9_Circulation

Ch9_Circulation - Establishing Establishing the Circulation...

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Establishing the Circulation 1 CHAPTER 9 ESTABLISHING THE CIRCULATION SUMMARY: Planetary differentiation created the large scale layers of the terrestrial planets billions of years ago, and these layers still exist today. Core, mantle, crust, and volatile rich exterior were a common heritage for Earth, Venus and Mars. Our conception of these layers as F xed and static is deeply ingrained in our daily experience. Rocks are solid, and while they can be broken, they do not fl ow. Missouri is far from the ocean and Manhattan is an island. Such facts are both concrete observation and common sense. Therefore when Alfred Wegener proposed early in the twentieth century that Africa and South America were once joined together and that continents move across the surface of the Earth, the idea was met with skepticism, harsh criticism, and even derision by the geological community. After World War II, a new generation of earth scientists began to explore the oceans. The gradual acquisition of new data revealed a remarkable symmetry to the Atlantic Ocean. Down the center ran the mid-Atlantic Ridge; depth of the ocean and thickness of sediment increased regularly from the ridge towards the continents. The symmetry extended to the pattern of magnetic anomalies, which soon became related to the periodic reversals in the Earth’s magnetic F eld that had been discovered on land. All these data could be explained by sea fl oor spreading, where new ocean crust was created at the ridge, and aged progressively as it spread away to both sides. Sampling conF rmed this idea by showing that young volcanic rocks were recovered at the ridge axis and the oldest sediments were near the margins of continents. Global seismology then showed the complement to sea fl oor creation at ocean ridges—the ocean crust is recycled into the mantle at ocean trenches, and its return to the mantle can be mapped precisely by the distribution of earthquakes in locations such as Japan. The new theory of plate tectonics accounted for these observations by proposing that the Earth’s surface is made up of F xed plates created at ridges, destroyed at subduction zones, continually in motion. The plates consist of brittle lithosphere, and they fl oat on top of the mobile asthenosphere fl owing underneath. Continents drift not by plowing through the oceans as Wegener implausibly suggested, but as light rafts fl oating on top of the plates that are created and destroyed in the surrounding oceans. The continents are too light to be recycled, and hence continents preserve a much longer record of earth history than the continually recycled ocean fl oor. Mountain belts occur where plates collide. Earthquakes and volcanoes occur at the margins of plates where they spread apart, converge towards each other, or slide by one another. In a period of a few years during the mid 1960’s, our view of earth changed from a static surface with F xed and isolated continents and oceans to a surface in constant
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This note was uploaded on 02/08/2011 for the course EAS 1601 taught by Professor Lynch during the Spring '08 term at Georgia Tech.

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Ch9_Circulation - Establishing Establishing the Circulation...

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