How Volcanoes Work

HISTORICAL NOTE ON ROCK TERMINOLOGY (basic/acidic vs. mafic/felsic)

The terms mafic and felsic were originally defined to describe a "color index" for rocks, varying from dark to light, respectively. In contrast, the terms basic and acidic originated in the late 19th century as a chemical classification for rocks, based on the idea that high-silica magmas were acidic and that low-silica magmas were basic. Althought we now know that this idea is erroneous, the use of these terms has persisted as a broad and indeed useful classification scheme for both rocks and magmas. Unfortunately, this scheme leaves students with the misconception that felsic rocks are derived from acidic magmas. Thus, although these original definitions are still valid, most American geologists have prefered to expand the original definitions of mafic and felsic, so that they are now used interchangeably with the terms basic and acidic to describe both color-index and chemical classifications

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