Chapter Overview

Chapters 3 and 4 are about igneous rocks and igneous processes. (Either chapter may be read first). Chapter 4 focuses on volcanoes and igneous activity that takes place at Earth's surface. Chapter 3 describes igneous processes that take place underground. However, you will learn early in this chapter how volcanic as well as intrusive rocks are classified based on their grain size and mineral content.
We begin the chapter by introducing the rock cycle. This is a conceptual device that shows the interrelationship between igneous, sedimentary, and metamorphic rocks. We then begin focusing on igneous rocks. After the section on igneous rocks classification, we describe structural relationships between bodies of intrusive rock and other rocks in the earth's crust. This is followed by a discussion of how magmas form and are altered. We conclude by discussing various hypotheses that relate igneous activity to plate tectonic activity.

Learning Objectives

1. The rock cycle relates the three major rock types, igneous, sedimentary and metamorphic, and the processes by which they are formed. These processes reflect a lack of equilibrium caused by external forces (weathering and erosion) or internal forces (tectonism). The major rock types are also related to one another at convergent plate boundaries.

2. Intrusive rocks crystallize from magmas emplaced into country rock. They possess mineralogies identical to volcanic rocks, but coarse-grained (= slow cooling) textures. Intrusions exhibit both "baked" and chill zone contacts, and they may contain xenoliths.

3. Names of plutonic rocks are the counterparts of extrusive rocks, sharing their mineralogy, but distinguished by their coarse-grained textures. Mineralogically equivalent granite-rhyolite, diorite-andesite, gabbro-basalt. The gabbro-basalt pair is dominated by ferromagnesian minerals and plagioclase feldspar. The granite-rhyolite pair is dominated by feldspars and quartz. The diorite-andesite pair is composed of feldspars and significant ferromagnesian minerals (30%-50%).

4. Classification systems are arbitrary and there is considerable variation in the composition of granite and rhyolite.

5. Silica content varies significantly among rock types and influences the minerals comprising various rock types. Mafic rocks contain 50% or less silica by weight. They are silica-deficient and have high magnesium, iron, and calcium content. Silicic (Felsic) rocks are silica-rich (greater than 65%), and have significant content of aluminium, sodium, and potassium. Intermediate rocks fall between mafic and silicic (felsic), and Ultramafic rocks, of which peridotite is the most abundant, are composed of pyroxene and olivine and have less than 45% silica. They have no fine-grained counterparts.

6. Intrusive bodies are defined by size, shape and relationship to country rock. Volcanic necks are the solidified throats of volcanoes, dikes are discordant, tabular intrusions, while sills, are concordant, tabular intrusions. Plutons crystallize at great depth, and most are granite. Batholiths are large and discordant, while stocks are small and discordant. Detached bodies of magma that moved to shallow depths are called diapirs.

7. Granite comprises the bulk of continents. Basalt and to a lesser extent gabbro underlay the oceans, while andesite forms most volcanoes along continental margins. Ultramafic rocks are thought to form the mantle.

8. Magmas are melted by a combination of the effects of the geothermal gradient, mantle plumes, water under pressure, pressure release, and mixed mineralogies.

9. Bowen's Reaction Series (Fig. 3.18) explains the variation in rock composition that can be produced from a single magma. Crystallization proceeds simultaneously along two branches: a discontinuous branch for ferromagnesian minerals that remain reactive with the magma, and a continuous branch for plagioclase feldspars that exhibit zoning from changes in calcium and sodium content. These minerals are formed by silicon-oxygen tetrahedra that control their silica content. Any magma left after the discontinuous and continuous branches are complete is enriched in silica, and the last minerals to form are potassium feldspar, muscovite and quartz. Differentiation, crystal-settling, partial melting, assimilation, and magma mixing also account for compositional differences in magmas.

10. Basaltic magmas are produced at diverging plate boundaries from partial melting of the asthenosphere and build oceanic crust. Mantle plumes produce intraplate volcanism that is basaltic under oceanic crust, and rhyolitic under continental crust. Converging plate boundaries produce andesite by partial melting, and magmatic underplating that promotes melting of the lower continental crust for granite production.

Related Readings

Barker, D. S. 1983. Igneous Rocks. Englewood Cliffs, NJ: Prentice-Hall.

Blatt, H., and R. C. Tracy. 1996. Petrology: Igneous, Sedimentary, and Metamorphic. 2d ed. New York: W. H. Freeman.

Hibbard, M. J. 1995. Petrography to Petrogenesis. Englewood Cliffs, NJ: Prentice-Hall.

MacKenzie, W. S., C. H. Donaldson, and C. Guilford. 1982. Atlas of Igneous Rocks and Their Textures. New York: Halsted Press.

Answer to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 3:

Boxed Readings