Spreading

Spreading - Plate Tectonics II: Seafloor Spreading 1. Plate...

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Plate Tectonics II: Seafloor Spreading 1. Plate tectonic theory holds that the lithosphere is divided into rigid plates that are all in constant relative motion. There are 12 major plates and at least 40 smaller ones, plus ambiguous “orogens”. 2. The boundaries between plates can be of 3 types: divergent (“spreading,” “ridge”), area-preserving (“transform”) or convergent (“subduction zone”). 3. At spreading ridges, submarine volcanism creates new crust of basalt, while cooling turns asthenosphere into lithosphere below. Sea-floor geologic features include fault-bounded axial valleys (at low spreading rates), axial volcanoes, pillow basalts, open cracks, hydrothermal vents, and very little sediment. 4. Farther from the ridge, the lithosphere is older and cooler and denser and thicker, so (because of isostasy) it floats lower, and the ocean is deeper. This leaves the “ridge” standing higher.
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1 c:\work\classes\ess1\spreadin.doc PLATE TECTONICS II: SEAFLOOR SPREADING 1. Plate tectonic theory holds that the lithosphere is divided into rigid plates that are all in constant relative motion. There are 12 major plates and at least 40 smaller ones, plus ambiguous “orogens”. The 12 largest plates have been recognized since ~1965: Pacific, Africa, Antarctica, North America, Eurasia, Australia, South America, Nazca, India, Cocos, Caribbean, Arabia. (Note that many plates named for continents also include adjacent regions of sea-floor. Pacific, Nazca, and Cocos plates are entirely seafloor.) A recent compilation (PB2002 model by Bird [2003]) recognizes another 40 small plates; this list is probably incomplete and subject to change. About 13 “orogens” ( = “mountain-forming” regions, such as: Alps, Persia-
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This note was uploaded on 10/14/2010 for the course ESS ESS1 taught by Professor Bird during the Spring '10 term at UCLA.

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Spreading - Plate Tectonics II: Seafloor Spreading 1. Plate...

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