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tholeiite

[tōlə-īt′]
  1. 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.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.