Geology 200

Study Guide

Unit 3:
Igneous Rocks

Overview

In this unit we discuss igneous rocks: rocks formed through the solidification of magma. The unit is divided into five sections: Section 1 introduces the concept of volcanism, discusses some of the ways in which igneous rocks are classified, and considers human interactions with volcanoes. Section 2 focuses on the types of volcanoes and the characteristic lavas that form from each type. Sections 3 and 4 distinguish between intrusive and extrusive igneous rocks, and Section 5 relates volcanism to plate tectonic theory.

Objectives

After completing this unit, you should be able to

  1. discuss the aspects of volcanism that are favourable to human activity and those that are destructive for humans.
  2. explain the classification of igneous rocks, and list and describe the major types of igneous rocks and their compositions.
  3. explain the difference between an intrusive and an extrusive igneous rock, describe the texture of each, and give examples of each.
  4. explain why felsic lavas erupt more violently than mafic lavas.
  5. define the terms vent, crater, and caldera, and describe the differences between shield volcanoes, cinder cones, composite volcanoes, and volcanic domes.
  6. describe how columnar jointing occurs and how “pillow” basalts form.
  7. define country rock, xenoliths, and chill zone.
  8. distinguish among dike, sill, and laccolith; and among pluton, stock, and batholiths, using carefully labelled diagrams.
  9. list the components of an ultramafic rock, and explain how and where such rocks form.
  10. describe the origin of pegmatites.
  11. explain how pressure, water vapour, and mixtures of minerals can affect the melting temperature of minerals.
  12. describe the differences between the continuous and discontinuous branches of Bowen’s reaction series.
  13. use the concepts of Bowen’s reaction series and crystal settling to explain the process of magmatic differentiation.
  14. list and explain six possible mechanisms by which granitic and andesitic magmas could form at convergent plate boundaries.
  15. recognize the major types of igneous rocks, and explain the “genetic” significance of their texture and composition.

Section 1: Volcanism and Human Interaction

The rocks that form from magmas may be classified according to their chemical compositions. Magmas always contain liquid and gaseous components; they may contain solids as well. Ninety per cent of the gaseous components of a magma consist of CO2 and H2O; the liquid portion is a molten silicate material; when present, the solid portion consists of early formed silicate crystals. The properties of a magma (e.g., its viscosity) are largely determined by both its silica content and its water content, but magmas are classified by their silica content. Igneous rocks that form from magma with a relatively low silica content are called mafic rocks; those that form from magma with a relatively high silica content are called felsic rocks.

Igneous rocks may be further differentiated according to where they have solidified. Thus, igneous rocks formed of magma that has solidified on or above the Earth’s surface are extrusive rocks; those that form from magma cooled beneath the Earth’s surface are intrusive rocks. Extrusive rocks can generally be distinguished from intrusive rocks by their much finer grain size.

The readings for this section discuss the effects of volcanism on human activity, the relationship between the names of volcanic rocks and their chemical composition, and the factors that determine how violent a volcanic eruption will be.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • Introduction to Chapter 4: Volcanism and Extrusive Rocks (pp. 77‑78).
  • “What are Volcanoes and Why Should We Study Them?” (pp. 78‑80).
  • “Eruptive Violence and Physical Characteristics of Lava” (pp. 80‑83).
  • “Living with Volcanoes” (pp. 95-102).

Study Questions

  1. Describe three aspects of volcanism that are favourable to humans and three that are destructive to humans.
  2. What is magma?
  3. Define and describe mafic, felsic, and intermediate volcanic rocks, and give an example of each.
  4. Why do felsic lavas erupt more violently than mafic lavas?

PDF icon Answer Key

Section 2: Types of Volcanoes and Eruptions

There are three major types of volcanoes: shield volcanoes, cinder cones, and composite volcanoes. Note the contrasting shapes of these volcanoes and the characteristic types of lavas that come from each. The readings also discuss volcanic domes and volcanic eruptions that are not associated with volcanoes: lava floods and submarine eruptions.

A section of the reading below (”Extraterrestrial Volcanic Activity, p. 102) describes volcanic features on the Moon, Venus, Mars, Jupiter, and Neptune. We encourage you to read this material for interest; you will not be tested on it.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Types of Volcanoes” (pp. 90-95).
  • “The Eruptive Products of Volcanoes” (pp. 83-90).

Study Questions

  1. Define the terms vent, crater, and caldera.
  2. Identify and describe (briefly) the three major types of volcanoes.
  3. Describe a spatter cone, and explain how such a structure forms.
  4. Differentiate between pahoehoe and aa lava flows.
  5. How does a volcanic dome form? Describe the composition of its lava.
  6. Why doesn’t a cone build up around the vents through which magma that forms plateau basalt flows?
  7. How does columnar jointing occur?
  8. What do pillow structures in basalt indicate?

PDF icon Answer Key

Section 3: Extrusive Rocks

The reading in this section describes how extrusive rocks are named and identified and describes the textures by which volcanic rocks are classified. Igneous rocks that have grains too small to be seen without a microscope are called aphanitic, while those that have grains large enough to be seen by the naked eye are phaneritic.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Igneous Rocks” (pp. 53-61)
  • “Eruptive Violence and Physical Characteristics of Lava” (pp. 80-83).

Study Questions

  1. How are extrusive rocks named and identified?
  2. Why are mafic extrusive rocks dark in colour? Why are felsic extrusive rocks light coloured?
  3. What is the mineral composition of a rhyolite? —of a basalt?
  4. Is quartz present in andesite? —in granite? —in basalt?
  5. What is a rock’s most important textural characteristic?
  6. What two factors determine the grain size of igneous rocks? Explain.
  7. What is obsidian?
  8. Draw and label a sketch to illustrate a porphyritic texture. How does such a texture develop?
  9. Describe the texture of a tuff and that of a volcanic breccia.
  10. What is a vessicular texture, and how does it form? What is the difference between scoria and pumice?

PDF icon Answer Key

Section 4: Intrusive Rocks

The reading for Section 4 describes a scheme for naming and classifying intrusive rock bodies. The size and shape of intrusive rock bodies and their relationship to surrounding rocks all give important information about their formation, their history, and the origin of the magma that produced them. Figure 3.1 below illustrates some of the common intrusive rock masses.

Figure 3.1. Some of the more common intrusive masses and their relationships to the surrounding country rock.

Figure 3.1. Some of the more common intrusive masses and their relationships to the surrounding country rock. (Modified after R. G. Schmidt and H. R. Shaw, U.S. Geological Survey). Intrusive igneous masses are named after their size, shape, and /or geometric relationships with the surrounding country rock. Masses with no known bottoms are called batholiths if their bedrock surface is more than 100 square kilometres, but are known as stocks if they measure less than 100 square kilometres. Floored, generally lenticular-shaped masses that are concordant (i.e., their contacts are parallel to the bedding of the country rock) are laccoliths. Tabular masses that are concordant are sills; tabular masses that are discordant (i.e., they cut across the bedding or foliation of the invaded country rocks) are dikes. Lopoliths may be described as generally concordant, basin-shaped masses that typically have their component rocks arranged in a stratified fashion. Several other kinds of igneous masses have been recognized and named. A historical review is given by R. A. Daly in his Igneous rocks and the depths of the earth.

Source: Dietrich, R. V., & Skinner, B. J. (1979). Rocks and rock minerals (p. 110). Toronto: Wiley.

Note: Rhyolite and granite have the same composition, as do basalt and gabbro. The extrusive equivalent of ultramafic rock is called komatiite. Ultramafic lavas were voluminous about 2.5 billion years ago. They erupt rarely from volcanoes today.

In Chapter 17 of the textbook, Plummer, Carlson, and Hammersley mention that all meteorites are thought to be about 4.5 billion years old. This age is similar to that of the oldest rocks on Earth, which suggests that meteorites and the Earth may have a similar origin.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Igneous Rocks” (pp. 53-61).
  • “Intrusive Bodies” (pp. 65-69).
  • “Abundance and Distribution of Plutonic Rocks” (pp. 69-70).

Study Questions

  1. What is an intrusive rock?
  2. Define country rock, xenoliths, and chill zone.
  3. Distinguish (using a diagram), between a dike, a sill, and a laccolith.
  4. What is a pluton? What is the difference between a stock and a batholith?
  5. Which intrusive igneous rock corresponds in composition to basalt? —to rhyolite? —to andesite?
  6. What minerals are present in a granite, but absent in a basalt or a gabbro?
  7. Describe the composition of an ultramafic rock. Where does such rock form?
  8. What is pegmatite? How does pegmatite form? What rare elements are found in pegmatite?
  9. What is the mineralogical difference between granite and granodiorite?

PDF icon Answer Key

Section 5: The Origin of Magma and Plate Tectonics Theory

In the last section of Chapter 3, the textbook introduces the source of lava, the cause of melting within the Earth’s crust and in the uppermost part of the mantle, and the factors that control the composition of magma.

It is important that you understand the process of magmatic differentiation. You must comprehend Bowen’s reaction series (Figure 3.15) to understand how felsic and mafic rocks can form from the same parent magma, and to understand the discussion of weathering in Unit 4. Pay close attention to these items in the textbook readings.

The readings in this section also address the origin of basaltic oceanic crust and the underlying ultramafic rock. Pay special attention to the relationship of these rocks to spreading continental margins (mid-oceanic ridges). Figure 3.26 provides a diagrammatic explanation. Other basaltic eruptions can also occur in intra-plate areas (e.g., Hawaii), and are thought to be the result of “hotspots” or mantle plumes. This subject is discussed in more detail in Unit 8.

Figure 3.28 shows the relationship of intermediate and felsic magmas (and the resulting intrusive and extrusive rocks) to converging plate boundaries. Be certain you understand this relationship.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “How Magma Forms” (pp. 61-62).
  • “How Magmas of Different Compositions Evolve” (pp. 62-65).
  • “Explaining Igneous Activity by Plate Tectonics” (pp. 70-74).

Study Questions

  1. What is partial melting?
  2. What two factors could cause melting in the asthenosphere?
  3. Describe how basaltic magma is formed at spreading centres. Sketch and label a diagram incorporating the features shown in Figure 3.26.
  4. What causes the eruptions in the Hawaiian archipelago?
  5. What kind of igneous rock would you expect to find under the oceans? —on continents? —on or near continental margins?
  6. What is the geothermal gradient?
  7. What is differentiation?
  8. Describe the differences between the continuous and discontinuous branches of Bowen’s reaction series.
  9. According to Bowen’s reaction series, which of each set of minerals below forms at a higher temperature?
    1. olivine or amphibole
    2. potassium feldspar or biotite
    3. sodium-rich plagioclase or calcium-rich plagioclase
    4. pyroxene or amphibole
  10. Explain the process of magmatic differentiation.
  11. List six possible mechanisms by which granitic and andesitic magma could form at convergent plate boundaries.

PDF icon Answer Key

Unit 3 Self Test

You have now finished Unit 3, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.

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