Chapter 6 Igneous Rocks
The topics covered in this chapter can be summarized as follows:
| Section | Summary |
|---|---|
| 6.1 The Basics of Igneous Rocks | The three types of rocks are igneous: formed from melts; Melts that remain within the Earth are called magma, and melts that erupt to the surface are called lava. Intrusive igneous rocks form from the solidification of magma, and typically cool slowly (over the course of 1000’s to millions of years). Extrusive igneous rocks form from the solidification of lava, and cool more quickly (over the course of seconds to years) |
| 6.2 Magma and Magma Formation | Magma is molten rock, and in most cases, it forms from partial melting of existing rock. The two main processes of magma formation are decompression melting and flux melting. Magmas range in composition from ultramafic to felsic. Mafic rocks are rich in iron, magnesium, and calcium, and have around 50% silica. Felsic rocks are rich in silica (~75%) and have lower levels of iron, magnesium, and calcium and higher levels of sodium and potassium than mafic rocks. Intermediate rocks have compositions between felsic and mafic. |
| 6.3 Crystallization of Magma | As a body of magma starts to cool, the first process to take place is the polymerization of silica tetrahedra into chains. This increases the magma’s viscosity (makes it thicker) and because felsic magmas have more silica than mafic magmas, they tend to be more viscous. The Bowen reaction series allows us to predict the order of crystallization of magma as it cools. Magma can be modified by fractional crystallization (separation of early-forming crystals) and by incorporation of material from the surrounding rocks by partial melting. |
| 6.4 Classification of Igneous Rock | Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% ferromagnesian silicates (amphibole and/or biotite) plus varying amounts of quartz and both potassium and plagioclase feldspars. Mafic igneous rocks have more than 50% ferromagnesian silicates (primarily pyroxene) plus plagioclase feldspar. Most intrusive igneous rocks are phaneritic (crystals are visible to the naked eye). If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals). If the cooling was extremely slow, or if water was present during cooling, the texture may be pegmatitic (very large crystals). |
| 6.5 Intrusive Igneous Bodies | Magma intrudes into country rock by pushing it aside or melting through it. Intrusive igneous bodies tend to be either irregular (stocks and batholiths), tabular (dykes and sills), or pipe-like. Batholiths have exposed areas of greater than 100 km2, while stocks are smaller. Sills are parallel to existing layering in the country rock, while dykes cut across layering. A pluton that intruded into cold rock it is likely to have a chilled margin. |
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