Igneous Rock Identification (Basic)

My previous articles have covered the basics of how igneous rocks form as part of the rock cycle, how their emplacement creates different igneous structures, and how we can use Bowen’s Reaction Series as a guide for understanding the evolution of magma into rock. But how useful is this information when you are standing at an outcrop and staring at a pinkish-grey rock and trying to tell whether it’s a granite or a diorite, or something else entirely?

Basic igneous rock identification begins with understanding and identifying two things about an igneous rock. First, what size are the crystals, and second what are the identifiable minerals present in the rock. You will generally need a hand-lens when in the field to help in identifying the minerals and determining the crystal size. (See my article on field equipment for more information on what a geologist needs to carry.)

The classification chart will be our map for the basic identification of igneous rocks. First, notice that all the minerals listed on the chart are minerals that were also part of Bowen’s Reaction Series. These are the primary minerals that make up igneous rocks and are commonly found in most igneous rocks. The relative percentage of the minerals found in the rock is listed on the left side of the chart. The gradation of these minerals is listed at the bottom, with the elements silica, potassium and sodium increasing as we go from right to left. The elements calcium, iron and magnesium increase as we go from left to right. Also notice on the bottom the breakdown of the various columns into felsic (or light colored) rocks on the left, followed by intermediate, mafic (or basic) and ultramafic (or ultrabasic) rocks on the right. Finally at the top are the rock names themselves. These are grouped according to their grain size. Coarse-grained rocks (those with crystals that can be see with the naked eye) are at the top and are usually associated with intrusive rocks. Fine-grained rocks are those where a hand-lens or a microscope is needed to see the grain size. These are generally associated with extrusive igneous rocks.

So we have our road map and are ready to start identifying igneous rocks. But how do we know which mineral is which? Identification of the minerals is a key step in the identification of an igneous rock, so let’s take a quick look at the different minerals and how to identify them in a rock sample. (Refer to my earlier article on mineral identification for the terminology and methods used to identify minerals.)

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