Chapter 4: Up From the Inferno: Magma and Igneous Rocks
Study Plan
Lava moves across the ground as a pahoehoe flow, Kilauea Volcano, Hawaii
Credit: J. D. Griggs, USGS
Guide to Reading
Sometime in your previous school years you probably learned there are three basic kinds of rocks: igneous, sedimentary, and metamorphic. The classification is based on the origin of the rock. Igneous rocks, the fire-formed ones, are the logical group to start with for two reasons: (1) they make up the greatest part of planet Earth, and (2) they were the first rocks to exist on Earth.
The igneous category is subdivided into two main branches: intrusive igneous rocks and extrusive igneous rocks. In the first, molten rock below Earth's surface (magma) hardens and creates various igneous rocks (including granite, diorite, gabbro, and peridotite) and structures (irregular shapes called plutons and tabular intrusions). In the second, molten rock spills out on the surface as lava that produces rocks (including rhyolite, andesite, basalt, obsidian, pumice, and pyroclastic rocks), structures (volcanoes and lava flows), and some wild and exciting events, for "extrusive igneous activity" is synonymous with the word volcanics. This chapter concentrates on magmas and rocks and touches only briefly on volcanic activity. You'll have to wait until Chapter 7 for the wild and exciting events; that's the chapter devoted to volcanic eruptions and their importance to humans throughout history.
Any study of igneous activity begins with a discussion of magma. As you learned in Chapter 1, Earth's interior is not composed chiefly of molten rock. Instead there are relatively few interior areas that are liquid, and there has to be some special reason for the solid rock of these areas to have become molten rock, that is, magma. Your author organizes his discussion of magma formation as follows:
- the conditions that cause melting of mantle and crustal rocks (decreased pressure, addition of volatiles, and heat transfer)
- the chemical composition of magmas
- the four major types of magmas (felsic, intermediate, mafic, and ultramafic)
- movement of magma, why it goes where it does, and why it has different viscosities
Once formed, magma doesn't always stay molten. Why and how does it harden? Your author discusses:
- the sequence of hardening of a melt (fractional crystallization, Bowen's Reaction Series)
- factors that control the cooling rate
- structures that result when magma solidifies within the Earth (plutons, tabular intrusions, laccoliths, batholiths, xenoliths and the stoping process, sills, and dikes)
- classification of igneous rocks (based on chemical compositions and textures)
What determines where igneous activity occurs? Those same two words crop up again: plate tectonics. Once again you'll read about volcanic arcs, mid-ocean ridges, subducting plates, continental rifting, and hot spots.
A word of advice: A discussion of magma is not conceptually difficult, but it can be confusing. After all, the intrusive activity is happening in the unseen and unfamiliar world of Earth's interior, and extrusive lava is not something the average person has had experience dealing with. Keep in mind the following two sets of word associations to help you follow the discussions of solid rock melting and liquid rock freezing.
Set one: A chemical composition high in silica, low in iron and magnesium (felsic); light-colored, light-weight rocks; low-temperature but high-viscosity melts
Set two: A chemical composition high in iron and magnesium, low in silica (mafic); dark-colored, heavy rocks; high-temperature but low-viscosity melts
Earth is a dynamic place; given enough geologic time, nothing on it remains unchanged. Once igneous rocks exist, they become part of that rock cycle you no doubt learned about long ago, at the same time you learned to recite "igneous, sedimentary, and metamorphic." Chapters 5 and 6 continue the story, as natural forces change igneous rock into sedimentary and metamorphic rocks.
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