Question

Consider the igneous rocks granite, rhyolite, and basalt. Discuss the environment of formation for each of these three rocks, specifically addressing where they formed and what types of minerals must have been in the material from which they formed.

Answer 1

Granite & rhyolite have the same or very similar chemistry and so form the same minerals. The difference between these two rocks is the environment in which they cool; granite cools slowly developing as a large crystalled rock in sub-terranean batholiths while rhyolite cools rapidly at or near the surface & therefore has small crystals. Because granite & rhyolite are HIGHLY FRACTIONATED, from an original basaltic melt deep in the Earth's lower crust, these rocks are FELSIC, being high in silica, in aluminia, sodium & potassium and are formed of the minerals quartz, orthoclase feldspar, sodic plagioclase feldspar and biotite (or muscovite) mica.

Answer 2

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Answer 3

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