5.3: Igneous Rock Classification

Igneous rocks are simply classified on two characteristics: composition and texture. The composition of igneous rocks is based on the minerals present viewed either with the naked eye or microscopically. For the classification based on composition, we return to Bowen’s Reaction Series. As we learned previously, the crystallization sequence of silicate minerals forming from a cooling magma follows the pattern outlined in Bowen’s Reaction Series.

Figure \(\PageIndex{1}\): Bowen’s Reaction Series. Developed through laboratory experimentation by Norman L. Bowen in the early 1900’s. It establishes the order of crystallization of minerals from a silicate magma. (Creative Commons Attribution 4.0 International License; Modified after an original by Steven Earle)

Igneous Rock Composition

Igneous rock composition is divided into four categories: ultramafic, mafic, intermediate and felsic. These categories refer to differing amounts of silica, potassium, sodium, calcium, iron, and magnesium found in the minerals that make up the rocks. Mafic refers to the overall composition being high in concentrations of magnesium (Mg) and iron (Fe). The minerals that compose mafic rocks are referred to as the ferromagnesian minerals, meaning, enriched in Fe and Mg. Felsic refers to the overall composition being enriched in the feldspar minerals of sodium-rich plagioclase and potassium feldspar plus silica (\(\ce{SiO2}\)) in the form of quartz.

It is important to realize these groups do not have sharp boundaries in nature, but rather lie on a continuous spectrum with many transitional compositions and names that refer to specific quantities of minerals. On the Bowen’s Reaction Series diagram, we read these igneous rock compositions as horizontal strips that run from right to left. For example, mafic igneous rock will typically contain calcium-rich plagioclase plus varying amounts of the ferromagnesian minerals: olivine, pyroxene (augite) and amphibole (hornblende) minerals. Felsic igneous rock will typically contain sodium-rich plagioclase, biotite, potassium feldspar, muscovite and quartz.

Using the Bowen’s Reaction Series diagram alone does not allow us to classify igneous rocks beyond their general composition. To determine their specific classification, we use an enhanced diagram where composition is quantitatively determined by the percentage of each of Bowen’s minerals that occur in any given sample. Examine the top portion of the diagram below. In essence, this is Bowen’s Reaction Series Diagram rotated 90\(^{\circ}\) to the right. Instead of ultramafic composition appearing along the top, as it does in Bowen’s Reaction Series, it exists along the right side of the diagram. The same minerals that we saw in Bowen’s Reaction Series that correspond to a rock of ultramafic composition, now exist in the far right column; they include a small percentage of calcium-rich plagioclase, and larger percentages of both pyroxene (augite), and olivine. On this diagram, Bowen’s Reaction Series sequence of mineral formation reads from right to left. This diagram helps us understand that rocks of any compositional category (ultramafic, mafic, intermediate and felsic) occur with varying percentages of Bowen’s Reaction Series minerals and though the compositional categories are separated by dashed lines, we can see that there are no hard and fast boundaries between each category.

Figure \(\PageIndex{2}\): Igneous Rock Classification Diagram. The diagram is read in columns from the composition at the top to the example textural types at the bottom.

Ultramafic refers to the extremely mafic rocks composed of mostly olivine with lesser percentages of pyroxene and calcium-rich plagioclase. Rock of this composition is rarely found on Earth’s surface; Where outcrops do occur, they represent rock derived from the upper mantle. This rock is very high in iron and magnesium and poor in silica, in the 40% or less range.

Mafic refers to igneous compositions with an abundance of dark ferromagnesian minerals of olivine and augite plus gray calcium-rich plagioclase feldspar. The oceanic crust is primarily mafic rock. Mafic rock is low in silica, in the 45-50% range.

Intermediate is a composition between felsic and mafic. This rock type contains roughly equal amounts of light and dark minerals, including light grains of plagioclase feldspar and dark minerals like hornblende. Rock of this composition is commonly associated with convergent plate boundary volcanoes. The silica content in rock of intermediate composition is in the 55-60% range.

Felsic refers to a composition with a majority of the light-colored minerals feldspar and quartz. Minor amounts of dark-colored (ferromagnesian) minerals like amphibole and biotite mica may be present as well. Rock of this composition dominates the continental crust. Felsic igneous rock is rich in silica (in the 65-75% range) and poor in iron and magnesium.

Igneous Rock Texture

If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving intrusive igneous rock a coarse-grained or phaneritic texture. The individual mineral crystals are readily visible to the unaided eye.

When lava is extruded onto the surface, or intruded into shallow fissures near the surface and cools, the resulting igneous rock is called extrusive or volcanic. Extrusive igneous rocks have a fine-grained or aphanitic texture, in which the grains are too small to see with the unaided eye. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals. These tiny crystals can be viewed using a special petrographic microscope used for viewing and magnifying very thin slices of rock (see photos below). In some cases, extrusive lava cools so rapidly it does not develop crystals at all. This non-crystalline material is volcanic glass which is a common component of volcanic ash and rocks like obsidian.

Figure \(\PageIndex{3}\): Hand sample of basalt. Note the aphanitic texture with no visible mineral grains. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

This is a magnified view of a 30 micron thick slice of a sample of basalt rock using a petrographic microscope. All of the light gray, elongate crystals are plagioclase. The very bright blues and greens are olivine. The golden brown crystals are pyroxene (augite). (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Once composition and texture are determined, the classification diagram can be used to give an igneous rock a name. The Igneous Rock Classification diagram (above) is read in columns. Let’s take a look at the “FELSIC” column on the left side of the diagram. As we move down the column we can see that felsic igneous rock will contain the following minerals in varying percentages: potassium feldspar, quartz, sodium-rich plagioclase, biotite and amphibole. The silica content (\(\ce{SiO2}\) %) will range between 65-75%. If it is an intrusive rock (plutonic) rock, it will have coarse, visible grains and it is given the name granite. If it is an extrusive rock (volcanic) rock with no visible (or few, very small) mineral grains, it is given the name rhyolite. Diorite and andesite likewise refer to intrusive and extrusive intermediate rock. Gabbro and basalt are the intrusive and extrusive names for mafic igneous rock. Peridotite is ultramafic and intrusive and komatiite as the fine-grained extrusive equivalent. Komatiite has been a rare rock because for most of Earth history, volcanic material that comes directly from the mantle is not common.

Figure \(\PageIndex{4}\): Left: Granite characteristics: Felsic composition, phaneritic texture, intrusive, plutonic. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Rhyolite. Felsic composition, aphanitic texture, extrusive, volcanic. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{5}\): Left: Diorite. Intermediate composition, phaneritic texture, intrusive, plutonic. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Andesite. Intermediate composition, aphanitic texture, extrusive, volcanic. Special note: This texture is more correctly “porphyritic.” The large crystals formed early in the magma chamber prior to the volcanic eruption. The remaining rock cooled very quickly, preserving the early formed plagioclase crystals. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{6}\): Left: Gabbro. Mafic composition, phaneritic texture, intrusive, plutonic. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Basalt. Mafic composition, aphanitic texture, extrusive, volcanic. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Figure \(\PageIndex{7}\): Left: Peridotite. Ultramafic composition, aphanitic texture, intrusive, plutonic. Special note: This sample is called a xenolith. It is a piece of the mantle that was ripped away by magma making its way to the surface through a volcanic eruption. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Komatiite. Ultramafic composition, aphanitic texture, extrusive, volcanic. (CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Did I Get It?

Exercise \(\PageIndex{1}\)

Which statement is true for igneous rocks?

a. intrusive = volcanic

b. extrusive = intrusive

c. extrusive = volcanic

d. volcanic = plutonic

Answer

c. extrusive = volcanic

Exercise \(\PageIndex{2}\)

Igneous rock is classified based on which two characteristics?

a. color and luster

b. hardness and density

c. composition and texture

Answer

c. composition and texture

Exercise \(\PageIndex{3}\)

Which of the igneous rock compositions would contain the most silica (silicon and oxygen)?

a. Intermediate

b. Ultramafic

c. Mafic

d. Felsic

Answer

d. Felsic

Exercise \(\PageIndex{4}\)

Which of the two samples of igneous rock in the photo would be classified as felsic in composition?

a. Neither sample is felsic in composition.

b. Both samples are felsic in composition.

c. The sample on the right is felsic in composition.

d. The sample on the left is felsic in composition.

Answer

c. The sample on the right is felsic in composition.

Exercise \(\PageIndex{5}\)

Which of the two samples of igneous rock in the photo would have formed extrusively?

a. Neither sample formed extrusively.

b. Both samples formed extrusively.

c. The sample on the right formed extrusively.

d. The sample on the left formed extrusively.

Answer

d. The sample on the left formed extrusively.