Igneous Rocks

In this section, we focus on igneous rocks that form in different igneous environments giving them unique composition and textures. Magma and lava in volcanic zones solidify to form igneous rocks. Igneous rocks are classified by composition into groups based on their silica content. Igneous rocks with high silica content will contain different minerals than those with low silica content, which will cause the rocks to be different colors. With some exceptions, we can usually tell the composition of a rock by its color. Below we will examine different types of igneous rocks and learn about their composition and cooling history.

Composition of Igneous Rocks

Here we focus on the types of minerals that are found in igneous rocks and how these compositions can be identified using color.

• Felsic rocks contain high silica and are made of minerals that are light or pink colored.
• Mafic rocks contain low silica and are made of high magnesium and iron minerals that are dark or green colored.
• Intermediate rocks fall between Felsic and Mafic rocks in terms of composition and color.

The texture of igneous rocks

Here we will focus on two main textures found in igneous rocks – coarse-grained (phaneritic) and fine-grained (aphanitic).

• Phaneritic textured rocks have large crystals (the rock is coarse-grained). These rocks formed from slow-cooling magma underground, allowing time for crystals to grow large.
• Aphanitic textured rocks have small crystals (the rock is fine-grained) and these rocks appear in one color overall. These rocks formed from faster-cooling lava at Earth’s surface, so there was no time for the crystals to grow.