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tholeiite An extraordinarily abundant, fine-grained, igneous rock consisting of essential calcium-plagioclase, subcalcic augite, and pigeonite, with interstitial glass or fine quartz-feldspar intergrowths. Tholeiite is a type of basalt, oversaturated with silica; it occurs as plateau lavas on the continental crust and as the main extrusive component of the ocean floor. Compare MID-OCEAN-RIDGE BASALT.