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tholeiite in Science
Any of a series of igneous rocks that are similar in composition to basalt, but are richer in silica and iron and poorer in aluminum than basalt is. Tholeiites form especially at mid-ocean ridges and in continental rift areas.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Article for tholeiite

fine-grained extrusive igneous rock, a basalt that contains plagioclase feldspar (labradorite), clinopyroxene (augite with pigeonite), and iron ore (magnetite and ilmenite). Tholeiitic lavas often contain glass, but little or no olivine. Tholeiite occurs as extensive plateaus (volumes on the order of 200,000 to 800,000 cubic km [50,000 to 200,000 cubic miles]) in the Pacific Northwest and the Deccan, western India, as well as in the lavas of the Lake Superior region, South Africa, Western Australia, and South America.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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