Condensed lecture notes 18-26

Condensed lecture notes 18-26 - LECTURE 18: GEOLOGY OF THE...

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LECTURE 18: GEOLOGY OF THE OCEAN FLOOR The character of the mid-ocean ridge system may be seen in Ohio-size Iceland, an island that sits astride the Mid-Atlantic Ridge. Iceland is constructed of basalt volcanoes and flood basalt flows, and it is subject to numerous but not especially destructive shallow earthquakes . As we follow the Mid- Atlantic Ridge to the south, we see that it maintains a midway position between Europe and Africa on the east, and South and North America on the west. It is severed in many places where the ridge crest is offset laterally by fracture zones . The ridge system, which occupies approximately the central one- third of the Atlantic Ocean basin, has a symmetrical profile that features a sharp medial valley running along the crest of the ridge. Parallelism of the ocean margins and the central ridge makes the Atlantic appear somewhat like a great winding river. The ridge system runs below Africa, continuing toward the central Indian Ocean where it forks. One branch leads northward, thence through the Gulf of Aden, which separates the Arabian Peninsula from the Horn of Africa. There it branches again, one arm leading up the axis of the long, narrow Red Sea, thence up the Gulf of Aqaba and through the Dead Sea Rift along the Jordan-Israel border. Another branch leads down through Ethiopia and African countries farther south where it is manifested as the East African Rift Valleys . These fault-bounded valleys contain active volcanoes and a series of lakes, some of them with floors well below sea level. Streams, which cannot erode significantly below sea level, could not have carved these depressions; rather, the valleys were down-dropped along big faults. In places, the ocean ridge system runs onto a continent, East Africa being an example. In size and shape, the profile of the continental rift valley system (for example, in Kenya) closely resembles the profile of the submerged ridge (as in the mid-Atlantic). Resuming back at the mid-Indian branching point, we follow the ridge system as it maintains a midway position between Australia and Antarctica, thence into the Pacific Ocean. Here the ridge is not symmetrically positioned, but trends way toward the eastern side where it is known as the East Pacific Rise . The crest of the Rise trends up the axis of the Gulf of California, which separates mainland Mexico from a long prong of Mexican territory called Baja California. The ridge system runs aground up through the western side of the State of California where it is known locally as the San Andreas Fault System , thence out into the Pacific just north of San Francisco, and offshore up to its termination in the Gulf of Alaska. This great underwater mountain range, 75 thousand kilometers long, which occupies all of the world’s major ocean basins, was not even known until serious exploration of the ocean floor commenced in the mid-1900s. Just as the ridge system is a topographic high in the ocean floor, its opposite is the
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This note was uploaded on 03/20/2012 for the course GEO 303 taught by Professor Leon during the Fall '08 term at University of Texas at Austin.

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Condensed lecture notes 18-26 - LECTURE 18: GEOLOGY OF THE...

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