Igneous Rocks

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May 1, 2001

You have probably seen pictures on television or in a magazine, images of red-hot lava flowing from the crater of a volcano. Maybe you have been able to actually see lava flowing from a volcano. The site is often awe-inspiring and invokes images of creation and destruction. But consider for a moment what becomes of the lava after it has cooled. Lava is molten rock, and when it cools, new rock is formed. Rocks formed from the cooling of molten rock, magma, are called igneous rocks. Igneous rocks are our second stop on our study of the rock cycle.

When studying igneous rocks, the first question you must ask is how did this rock come to be where it is now? Igneous rocks have two different ways of being emplaced. The first is known as extrusive igneous rocks. These are formed either by the flow of lava or the deposition of ash over preexisting surfaces. The second are known as intrusive igneous rocks. These are formed from magma that has solidified in preexisting rock beneath the surface of the earth. Each type of emplacement creates unique types of igneous rocks.

There are a couple of ways to classify igneous rocks, other than simply noting if it’s extrusive or intrusive. The first is by texture. If you have ever looked at a piece of granite you will notice that it has many sizes of crystals, from very small, fine crystals, to larger coarse crystals. Knowing the size of the crystals tells us how quickly the rock cooled as it was being emplaced. Small crystal size denotes very rapid cooling and is called aphanitic texture. The rock cooled so quickly that large crystals could not form. If you can see large crystals in the rock, it cooled slowly, and called phaneritic texture. Sometimes, you will see large crystals in a very fine matrix. This type of texture is called porphyritic and shows that the rock started to cool slowly, forming the large crystals, then was rapidly cooled, forming the fine matrix.

The second method of describing igneous rocks is by knowing the rocks chemical composition, or mineralogy. I will cover minerals and how to identify mineral in a later article. To identify the more common igneous rocks, only a simple understanding of mineralogy is needed. One of the first things to determine is how much silica is in the rock. Silica is common in all igneous rocks, but occurs in varying amounts. Rocks with a high silica content are called acidic. These rocks generally have more than 66 percent silica in them. Intermediate rocks have between 52 to 66 percent silica in them. A rock that contains 45 to 52 percent silica is considered basic. Finally, rocks that have less than 45 percent silica in them are considered ultrabasic. These terms are from an old idea that igneous rocks formed from silicic acids. We know a lot more about how igneous rocks are formed, but this method of classifying igneous rocks based on silica content is still used.