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Basaltic andesite from Parícutin volcano in Mexico
TAS diagram for classifying volcanic rock, with the basaltic andesite field (O1) highlighted
Basaltic andesite is a fine-grained (aphanitic) igneous rock that is moderately low in silica and low in alkali metal oxides. It is not separately defined in the QAPF classification, which is based on the relative percentages of quartz, alkali feldspar, plagioclase feldspar, and feldspathoids, but would fall in the basalt-andesite field. This corresponds to rock in which feldspathoid makes up less than 10% and quartz less than 20% of the total QAPF fraction, and in which at least 65% of the feldspar is plagioclase. Basaltic andesite would be further distinguished from basalt and andesite by a silica content between 52% and 57%.
Although classification by mineral content is preferred by the IUGS, this is impractical for glassy or very fine-grained volcanic rock, and then the chemical TAS classification is used. Basaltic andesite is then defined as volcanic rock with a silica content from 52% to 57% and a total alkali content (K2O plus Na2O) of less than about 6%, corresponding to the O1 field in the TAS diagram.
A basaltic andesite (or andesite) with more than 8% MgO and less than 0.5% TiO2 is termed a boninite.
Basaltic andesite is also common where the Earth's crust is undergoing extension. For example, mid-Cenozoic volcanic sequences in western Mexico, southwestern New Mexico, and southeastern Arizona are capped by basaltic andesite of the Southern Cordilleran Basaltic Andesite (SCORBA) suite, which may be the most extensive Cenozoic mafic suite in North America.Mt. Mazama, a large composite volcano in south-central Oregon, consists of several overlapping basaltic andesite shields.
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