The topics covered in this chapter can be summarized as follows:
7.1 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.
7.2 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.
7.3 Classification of Igneous Rocks
Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% of dark minerals (ferromagnesian silicates, amphibole and/or biotite) plus varying amounts of quartz, and both potassium and plagioclase feldspars. Mafic igneous rocks have more than 50% dark minerals (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).
7.4 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 (dikes 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.
Questions for Review
1. What is the significance of the term reaction in the name of the Bowen reaction series?
2. Why is it common for plagioclase crystals to be zoned from relatively calcium-rich in the middle to more sodium-rich on the outside?
3. What must happen within a magma chamber for fractional crystallization to take place?
4. Explain the difference between aphanitic and phaneritic textures.
5. Explain the difference between porphyritic and pegmatitic textures.
6. Name the following rocks:
(a) An extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene
(b) An intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene
(c) An intrusive rock with 25% quartz, 20% orthoclase, 50% feldspar, and minor amounts of biotite
7. With respect to tabular intrusive bodies, what is the difference between a concordant body and a discordant body?
8. Why does a dike commonly have a fine-grained margin?
9. What is the difference between a batholith and a stock?
10. Describe two ways in which batholiths intrude into existing rock.
11. Why is compositional layering a common feature of mafic plutons but not of felsic plutons?