Tuesday, January 15, 2008

Tholeiitic basalt is an igneous rock, a type of basalt. Like all basalt, the rock type is dominated by clinopyroxene plus plagioclase, with minor iron-titanium oxides. [1] Orthopyroxene or pigeonite may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes. Tridymite or quartz may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent. Tholeiitic rocks may have a fine, glassy groundmass(AGI, 41), as may other types of basalt.
Chemically, these rocks have been described as subalkaline basalts, that is, they contain less (Na2O plus K2O) at similar SiO2 than alkali basalt; the field for basalts plotted with these chemical variables is shown in the TAS classification, and the distinction between basalt types is discussed by Le Maitre and others (2002).
The International Union of Geological Sciences recommends that "tholeiitic basalt" be used in preference to the term "tholeiite" (Le Maitre and others, 2002).
Basalt magmas are partial melts of peridotite produced by decompression melting in the Earth's mantle, a process described for igneous rocks. Tholeiitic basalts are the most common volcanic rocks on Earth, as they are produced by submarine volcanism at mid-ocean ridges and make up much of the ocean crust. MORB, the acronym for typical mid-ocean-ridge basalt, is a type of tholeiitic basalt particularly low in incompatible elements. In contrast, alkali basalt is not typical at ocean ridges, but is erupted on some oceanic islands and on continents, as also is tholeiitic basalt. (AGI, 41), [2]

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