In the previous tutorial we learned about how rocks melt, differentiate, and crystallize.
Now we will discuss the different types of igneous rocks and how they are classified.
Say you are given an igneous rock and asked what it is. Where would you begin? Your first step
might be determining whether it was extrusive or intrusive. Intrusive is a word describing
an igneous rock that cooled slowly at depth, and extrusive is a word describing a rock that
cooled quickly at the surface, usually during a volcanic eruption. Intrusive rocks are also called
plutonic, and extrusive rocks may be described as volcanic. It is usually pretty easy to distinguish
a plutonic rock from a volcanic rock simply by analyzing the size of the mineral grains. Plutonic
rocks, which cool slowly, have ample time to grow large crystals, while volcanic rocks, which
crystallize rapidly, are composed of tiny grains that usually can’t be seen by the naked eye.
Let’s first discuss volcanic rocks, since they are simply named after the
magma from which they are derived. Both are classified based on their silica content,
and are as follows: with rhyolite, silica content is greater than 69%, dacite, 62-69%,
andesite, 52-62%, basalt, 45-52%, and picrite, which contains less than 45% silica. The term
ultramafic is commonly used to describe picritic rocks, mafic for basaltic rocks, intermediate for
andesitic rocks, and felsic for dacitic and rhyolitic rocks. Rhyolite and basalt are very
different rocks, and it is typical for them not to have any common minerals. For example, rhyolite is
rich in alkali feldspar, quartz, and sodium-rich plagioclase with minor amounts of amphibole,
while basalt is mainly composed of pyroxene, olivine, and calcium-rich plagioclase with minor
amounts of iron and titanium oxides. Andesites and dacites can have both felsic and mafic
minerals but are typically mostly plagioclase with lesser amounts of pyroxene, amphibole, and quartz.
Plutonic igneous rocks have an entirely different naming convention than volcanic rocks, which are
classified based on their composition; instead being named based on their three most abundant
minerals. This is because the minerals in plutonic rocks can be easily identified in hand sample,
whereas volcanic minerals are typically too small. For felsic plutonic rocks,
names are based on the relative amounts of quartz, plagioclase, and alkali feldspar,
which are placed at the corners of a triangular, or ternary diagram, called a QAP diagram.
Plagioclase-rich rocks are called diorite, quartz-rich rocks are called granitoid,
and alkali feldspar-rich rocks are called syenite. Rocks of intermediate composition are called
granite if they are quartz-rich and monzonite if they are feldspar-rich. On the other hand, mafic
plutonic rocks are named based on their relative abundances of olivine, pyroxene, and plagioclase,
and again are plotted on ternary diagrams. Gabbro, which is the plutonic version of basalt, contains
roughly equal parts plagioclase and clinopyroxene. Likewise, ultramafic rocks can be classified
using the minerals olivine, clinopyroxene, and orthopyroxene. These rocks, like harzburgite and
lherzolite, are representative of the upper mantle and are rarely found at the surface.
So, what types of igneous rocks are found at the surface? That depends on where you look.
For instance, the lava that erupts from midocean ridges is exclusively basaltic, with gabbro
forming deeper beneath the surface, which is basically just basalt with big crystals. Hotspots
are also capable of producing massive amounts of basaltic lava, especially when they are beneath
oceanic crust. Subduction zones tend to produce andesitic magma, but can be much more variable,
also being influenced by crustal thickness. Island arcs, which are subduction zones where the
overriding plate is oceanic crust, tend to produce basaltic and andesitic magma, while continental
arcs, like the volcanoes of the Pacific Northwest, tend to erupt rhyolite and dacite. With igneous
rocks now covered, let’s move forward and check out some other types of rocks and minerals.