5.1: Igneous Rocks
- Page ID
- 33342
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Igneous rocks are those that form as molten material cools. When the molten material is found below the surface, we call it magma. The temperature of magma can range between about 800\(^{\circ}\) and 1600 \(^{\circ}\)C depending on where within the mantle the magma forms. Magma cools slowly within the crust (over centuries to millions of years)—forming intrusive igneous rock (also called plutonic rock), and crystallizes at depths of hundreds of meters to tens of kilometers below the surface. When molten material makes it to the surface, we call it lava. It cools quickly (within seconds to years)—forming extrusive igneous rock (also called volcanic rock).
To summarize:
- Molten material below the surface is magma. It cools on the "interior" of the Earth it forms intrusive or plutonic rocks.
- Molten material on the surface is lava. It cools on the "exterior" of the Earth it forms extrusive or volcanic rocks.
Classification of Igneous Rocks
Igneous rocks are classified on two characteristics: composition and texture. The composition of igneous rocks is based on the minerals present. The texture refers to the size of the mineral grains.
Igneous Rock Composition
For the classification based on composition, we use Bowen’s Reaction Series which outlines the crystallization sequence of silicate minerals forming from a cooling magma.
As the magma cools from 1300\(^{\circ}\) to 750\(^{\circ}\)C (as noted on the left side of the diagram), minerals form in the order indicated from top to bottom. Looking at the left branch, olivine forms first, then pyroxene (augite), then amphibole, and so on and so forth until quartz forms. This is called the discontinuous branch because the minerals that form are unique silicate minerals. Looking at the right branch, note it is called the continuous branch. Here, different versions of one silicate (plagioclase) forms. At higher temperatures, the version of plagioclase with calcium crystallizes (anorthite). At lower temperatures, the version with sodium crystallizes (albite). In the middle of that series about half the spots are taken up with calcium, and half with sodium. As you move along the branch there are an infinite number of possible versions formed as the amount of calcium decreases, and sodium increases.
Igneous rock composition is divided into four categories: ultramafic, mafic, intermediate and felsic. You can see these labeled on the right side of Bowen's Reaction Series such that we read these igneous rock compositions as horizontal strips that run from right to left. 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) and typically contain calcium-rich plagioclase plus varying amounts of olivine, pyroxene (augite) and amphibole (hornblende). 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 and typically contain sodium-rich plagioclase, biotite, potassium feldspar, muscovite and quartz.
Reading Bowen's Reaction Series above, we see that:
- 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 compositions with an abundance of dark iron- and magnesium-rich minerals of olivine and augite plus gray calcium-rich plagioclase feldspar. The rocks making up the oceanic crust are primarily mafic. 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 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.
Looking at the diagram below, you see the felsic rocks (granite and rhyolite) are light in color. As you move to the right toward ultramafic, the rocks get darker as the amount of iron and magnesium increases.
To summarize:
Compositionally, igneous rocks are categorized as follows:
- Ultramafic rocks are very low in silica and very high in iron and magnesium. They are very dark in color due to very high iron/magnesium.
- Mafic rocks are low in silica and high in iron and magnesium. They are dark in color due to high iron/magnesium.
- Intermediate rocks are medium in silica and medium in iron and magnesium. They are medium in color due to high iron/magnesium.
- Felsic rocks are high in silica and low in iron and magnesium. They are light in color due to low iron/magnesium.
As rocks contain more iron and magnesium, they get darker.
Igneous Rock Texture
If magma cools slowly, deep within the crust on the "interior" of the Earth, the 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 visible to the unaided eye.
If lava cools quickly, on the surface or on the "exterior" of the Earth, the rock is called extrusive or volcanic. The quick cooling process does not allow crystals to grow large, giving extrusive igneous rocks a fine-grained or aphanitic texture. The individual mineral crystals are too small to see with the unaided eye. These tiny crystals can be viewed using a special 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.
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.)
Naming Igneous Rocks
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 is either granite or rhyolite. If it is an intrusive (plutonic) rock, it will have coarse, visible grains and it is given the name granite. If it is an extrusive (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 and komatiite are the intrusive and extrusive names for ultramafic igneous rocks. Komatiite has been a rare rock because for most of Earth history, volcanic material that comes directly from the mantle is not common.
- andesite - an aphanitic (intrusive), intermediate igneous rock
- basalt - an aphanitic (intrusive), mafic igneous rock
- diorite - a phaneritic (intrusive), intermediate igneous rock
- extrusive - igneous rock formed as molten material cools on the surface
- felsic - a composition in which there are high (~70%) concentrations of silica; resultant rocks are typically light in color
- gabbro - a phaneritic (intrusive), mafic igneous rock
- granite - a phaneritic (intrusive), felsic igneous rock
- igneous rock - a rock that forms as minerals crystallize when molten material cools
- intermediate - a composition in which there are moderate (~60%) concentrations of silica; resultant rocks are typically medium in color
- intrusive - igneous rock formed as molten material cools below the surface
- komatiite - an aphanitic (intrusive), ultramafic igneous roc
- lava - molten material on the surface
- mafic - a composition in which there are low (~50%) concentrations of silica; resultant rocks are typically dark in color
- magma - molten material below the surface
- peridotite - a phaneritic (intrusive), ultramafic igneous rock
- plutonic - igneous rock formed as molten material cools below the surface
- rhyolite - an aphanitic (intrusive), felsic igneous rock
- volcanic - igneous rock formed as molten material cools on the surface
- ultramafic - a composition in which there are very low (~40%) concentrations of silica; resultant rocks are typically very dark in color