Components of Igneous Rocks

When molten rock erupts onto the Earth's surface, it cools quickly, freezing the growth of existing minerals and preventing the development of new minerals. Such rapid cooling will typically produce lava rocks with a few small minerals suspended in a groundmass of volcanic glass. Molten rock that remains below the ground, however, cools very slowly so that existing minerals continue to grow and many new minerals develop. A slow rate of cooling will produce a coarse-grained plutonic rock that consists entirely of large crystals. Different names are given to such slow-cooling plutonic rocks on the basis of chemical composition and mineral proportions (for example, plutonic rocks of basaltic composition are called gabbro).
This graph shows the volume percent of minerals present in a plutonic rock that consists entirely of crystals. For example, a granite with 70% SiO2 might have 22% quartz, 38% alkali feldspar, 28% plagioclase feldspar, and 12% biotite.
The volume percent of minerals present in volcanic rocks typically varies 0-50%. To calculate the relative amounts of the crystals likely to be present, multiply the volume percent in the graph by the actual volume percent in the rock. For example, a rhyolite volcanic rock with 10% crystals, is likely to 2.2% quartz, 3.8% alkali feldspar, 2.8% plagioclase, and 1.2% biotite.