Classification
The most common characteristics used to classify igneous rocks are crystal size and color. The crystal size tells us how fast the rock cooled. Large crystals (coarse texture) found in rocks means that they cooled slowly. These rocks are usually formed deep below the surface and are called intrusive igneous rocks. Granite is an example of this type of igneous rock. Small crystals (fine texture) or no crystals (glassy texture) means that the rock cooled fast. These rocks usually form during a volcanic eruption and are called extrusive igneous rocks. Basalt is an example of this type of igneous rock. Just remember that the faster magma or lava cools the smaller the crystals will be.

Look at the diagram below, which rock cooled the fastest? slowest?

Rock 1 cooled the fastest because it has the smallest crystals and rock 4 cooled the slowest because it has the largest crystals. Look at the arrangement of the crystals. This is what intergrown crystals will look like in a diagram. Don't forget it.

The overall color of the rock gives us an indication of the minerals present in the rock. Rocks such as granite and rhyolite have a relatively light color and basalt and gabbro have a relatively dark color. Remember that the only igneous rocks that you have to know are found in the Scheme for Igneous Rock Identification chart on page 6 in your reference tables. This chart should be used for any questions related to igneous rocks.

Using the Scheme for Igneous Rock Identification Chart.
If you don't have it out now, please go get it.
The chart is divided into two sections, the top describes the characteristics of the rock and the bottom has the mineral composition of the rocks. In the top of the chart you use texture, color, density and composition to identify the rock. For example, you pick up an igneous rock and notice that it has a coarse texture and a dark color. This rock could be gabbro. Remember that color and composition go together. Light-colored rocks have a felsic composition. This means that they are usually rich in feldspar and silica. Dark-colored rocks have a mafic composition. These rocks are usually rich in magnesium and iron.

If you forget this relationship look at the characteristics section of the chart. Light, low and felsic are on the left side of the chart. Rocks such as granite, rhyolite, pumice and obsidian have these characteristics. While the opposite is true for rocks on the right side of the chart.

To use the mineral composition part of the chart, you have to know the percents of the minerals that make up your igneous rock. For example, if the rock is composed of 20% potassium feldspar, 40% quartz, 20% plagioclase feldspar, 10% biotite, 10% hornblende and has a coarse texture, what is the name of this rock?

Before we go any farther, look at the chart. See how it is divided into five columns. The rock names listed in the top of the column could have the range of mineral percents in the bottom half of the column. If you were to locate the percents of each of the minerals on the Mineral Composition chart, you would find that they are all in the left hand column. That means the rock could be granite, rhyolite, pumice or obsidian, it all depends on the texture of the rock. The example told us that the rock has a coarse texture. Therefore the rock is a granite.