Igneous rocks can be classified by their texture and composition. Learn how texture is influenced by the cooling rate of magma and how different mineral compositions lead to different igneous rock classifications in this video lesson.
Classification of Igneous Rocks
If you and your friends were igneous rocks, one thing we would know about you guys is that in your younger years you were very hot fluids – and I mean this literally, not as a statement of appearance. In fact, the basic definition of an igneous rock is a rock made from the cooling and solidifying of magma, which is the hot fluid we associate with volcanoes. So if you and your friends all came from the same thing, then why do you all look so different? In this lesson, we will answer this question as we look into the different classification of igneous rocks.
Texture of Igneous Rocks
Now, one of the most important reasons that you and your igneous rock friends look so different from one another is because of the rate at which you cooled. We’re reminded that magma forms deep within the Earth and as it rises toward the surface it cools down, which allows it to solidify or crystallize and form rock. This is somewhat like how warm water can be placed in a freezer and cooled enough to solidify into an ice cube.
The cooling rate of magma gives us one of the most distinguishing characterizes that help us classify igneous rocks, which is the texture. This is because the rate at which magma cools determines the crystals that will form. For example, if the magma cools while it is still underground, it will cool slowly, giving the crystals time to form and grow large. If, however, the magma reaches the surface, it will cool fast and the crystals will be small simply because they did not have time to grow.Therefore, rocks that form from magma underground contain large enough crystals that they can be seen with the naked eye. These are considered coarse-grained rocks, or phaneritic rocks. It may help you to recall this term if you remember that the root of this word, ‘phaneros,’ is Greek for ‘visible,’ so the Greeks must have been ‘fans’ of rocks with big crystals to name them phaneritic.
In contrast, rocks formed near the surface or aboveground, which could include rocks that form after a volcanic eruption, are aphanitic rocks because they are fine-grained rocks or may even appear glassy. You may recall that the prefix ‘a’ means ‘without,’ so aphanitic rocks are rocks without visible crystals.
So we see that texture of igneous rocks is one way to classify them, but they are also classified by the minerals they contain. Magma can be composed of different minerals and therefore solidify into rocks with different mineral compositions.
Let’s start with felsic rocks, which are light-colored rocks that contain a lot of feldspar and quartz.Quartz is a hard mineral composed of large amounts of a compound called silica, so these rocks get their name from the combination of ‘fel’ plus ‘sic,’ which is basically an abbreviated form of ‘feldspar’ plus ‘silica.’ Felsic rocks are not very dense and are light in color. Granite is the most common example of a felsic rock, and you are probably familiar with granite because it can be used for gravestones or polished to make a very attractive countertop.
We also have mafic rocks, which are dark-colored rocks that contain a lot of magnesium and iron. The symbol for iron is Fe and if something is ‘ferric’ it means it contains iron. So these rocks get their name from the combination of ‘ma’ plus ‘ferric,’ which is basically an abbreviated form of ‘magnesium’ plus iron, or ‘ferric.
‘Mafic rocks are denser and darker in color, so at least as far as their chemical composition, mafic rocks and felsic rocks can be thought to be quite different from each other. In other words, they are both born from magma, but after they grow up, they don’t have a lot in common. Basalt is a common example of a mafic rock and it makes up much of the bedrock of the ocean floor.
Of course, we do not have just light and dark igneous rocks; some are in between. Rocks that are between light- and dark-colored and share minerals of both felsic and mafic rocks are called, appropriately, intermediate rocks.
Andesite is an example of an intermediate rock that is not too light or too dark. It is formed in the Andes Mountains, which is how they get their name. The Andes is known for its long mountain range that travels along the west coast of South America, but it is also known for its volcanoes, which is where we get these intermediate rocks.
There are also ultramafic rocks, which are rocks that contain mainly magnesium and iron and very little silica.
We recall that magnesium and iron are mafic minerals, so ultramafic igneous rocks are like super-charged mafic rocks, kind of like Ultra-Frisbee is super-charged Frisbee. And because ultramafic rocks contain very little silica, they are pretty different from felsic rocks, which we recall contain a lot of silica or quartz. Peridotite is an example of an ultramafic rock and it is a common rock that makes up the Earth’s mantle.
Let’s review. Igneous rocks can be classified by both their texture and their composition. Texture is largely determined by the rate at which the magma cools.
If it cools slowly, as magma deeper underground tends to do, it will have time to form larger crystals. This gives us coarse-grained rocks, known as phaneritic rocks, with crystals large enough to be visible to the naked eye. If, however, the magma cools quickly, as magma near the Earth’s surface tends to do, then it will cool too fast for crystals to grow. This gives us fine-grained rocks, known as aphanitic rocks. The crystals in these rocks are so small that they are not visible to the naked eye and they may even appear glassy.Composition is another way to classify igneous rocks. The composition of the rocks is determined by the minerals that make up the magma from which they form.
Felsic rocks are light-colored rocks that contain a lot of feldspar and quartz. The name is a combination of ‘fel’ for ‘feldspar’ and ‘sic’ for ‘silica,’ or ‘quartz.’ Mafic rocks are dark-colored rocks that contain a lot of magnesium and iron. These rocks get their name from the combination of ‘ma’ plus ‘ferric,’ or ‘iron.’ Intermediate rocks are appropriately named because they are rocks that are between light- and dark-colored and share minerals with both felsic and mafic rocks.
There are also ultramafic rocks, which are rocks that contain mainly magnesium and iron and very little silica. Because they have mostly mafic minerals they are like super-charged mafic rocks – in other words, ultramafic.
After completion of this lesson, you should be able to:
- Understand differences in igneous rocks
- Identify phaneritic rocks having large crystals
- Recognize aphanitic as fine grained
- Remember composition of igneous as felsic, mafic and ultramafic
- Describe the intermediate igneous rocks