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Images of Plutonic Igneous Rocks |
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Igneous plutonic (or intrusive) rocks are those which have
formed deep underground as magma cooled slowly over
thousands of years following their emplacement. Such rocks
have visible crystals (phaneritic texture). The rocks are
classified on the basis of the proportions of the feldspar
family of minerals, quartz, and the various dark (iron-rich)
minerals like biotite, hornblende, augite and olivine. The
felsic plutonic rocks usually have quartz, large proportions
of potassium feldspar (orthoclase), and lesser amounts of
plagioclase and mafic minerals. Intermediate and mafic rocks
have increasingly larger proportions of dark iron-rich
minerals. Ultramafic minerals have large significant amounts
of the most iron-rich rock-forming mineral olivine. |
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