Color index (geology)
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Color index, as a geological term, is a measure of the ratio between generally dark mafic minerals and generally light felsic minerals in an igneous rock. The color index of an igneous rock is the vol
Gneiss, displaying characteristic darker (melanocratic) and lighter (leucocratic) bands
Basalt
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Basalt; US: buh-SALT is an aphanitic extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon
Color index (geology)
…be entirely composed of a mafic mineral. Some examples of felsic rocks include granite and rhyolite, while examples of mafic rocks include gabbro and basalt. According to the Encyclopædia Britannica, color indices, 0–50 are felsic, 50–90 are mafic, and 90–100 are ultramafic. An online geology textbook…
Basalt 12
QAPF diagram with basalt/andesite field highlighted in yellow. Basalt is distinguished from andesite by SiO2 < 52%.
Vesicular basalt at Sunset Crater, Arizona. US quarter (24mm) for scale.
Columnar basalt flows in Yellowstone National Park, US