How to Name Igneous Rocks

Figure 28

• Pyroclastic – this is when explosive eruptions happen to blast lava into the air, giving rise to something called volcanic bombs.

• Porphyritic textures – this is when the cooling rate of magma varies over time. Large crystals are formed with slow cooling before sudden rapid cooling will contribute to much smaller crystals. The large crystals are called phenocrysts.

HOW TO NAME IGNEOUS ROCKS

You already know how to do this for most rocks. You need to understand more clearly why there are pink streaks in some igneous rock and not in others. There are different naming properties depending on how much ferromagnetic silica products are in the rock. High quantities are in amphibole, biotite, pyroxene, and olivine. These will be very dark in general.

You should remember that the felsic rocks will often be high in quartz, potassium feldspar, and plagioclase feldspar. These will be pink or white. Those that are extrusive are called rhyolite or andesite. The intrusive ones are granite and diorite.

The mafic and ultramafic rocks are going to be varying dark shades. You'd get gabbro and peridotite, which are intrusive. Extrusive rocks of this type include basalt and komatiite. Figure 29 is a good cheat sheet on naming igneous rocks:

Figure 29.