CHAPTER 4: FORMATION OF MAGMA AND IGNEOUS ROCKS
Expanded Pathway to Learning • Authors’ Intent • Learning Outcomes • Animations for You and Your Students • Active Learning Ideas • End-of-Chapter Questions and Answers
The chapter on the “Formation of Magma and Igneous Rocks” includes concepts that spread across two chapters in those physical geology textbooks that treat volcanism separately from other aspects of igneous-process geology. We feel that this division creates an unnatural break in subject matter when learning about igneous rocks, so we integrate knowledge of magma generation and the formation of plutonic and volcanic rocks throughout a single chapter. A single chapter also helps maintain student interest by incorporating aspects of volcanoes and volcanic eruptions, a common topic of intrinsic interest, throughout the learner’s study of igneous rocks and the processes that form them.
The general outline follows an inductive process of first describing the phenomena that require explanation and then seeking the explanations. We do this partly in order to give a feeling for the steps in understanding natural processes—the essence of science—and partly because research shows that most students learn better inductively than deductively.
We feel that it is very important to develop an understanding of how magma forms. Many students hold a misconception that much of Earth’s interior is molten. The fact that the mantle and crust are almost entirely solid means that special circumstances are necessary to explain how melting can take place and where volcanoes can form. Exploring these special circumstances permits a linkage to a developing understanding of plate tectonics and also utilizes concepts of pressure and temperature in Earth’s interior that will be repeatedly built upon in subsequent chapters.
Chapter 4 also provides insights into how experiments are essential to understand the processes that take place out of sight below Earth’s surface. This theme emerges again in several succeeding chapters. The “How Do We Know?” section highlights experimentation by leading the reader through an understanding of the classic Tuttle and Bowen experiments on granite melting (O.F. Tuttle and N.L. Bowen, 1958, Origin of granite in light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O: Geological Society of America Memoir 74, 153 pp.). The section also shows a linear use of a scientific method where a problem is recognized, an experiment is designed to address understanding of the problem, and the results offer solutions to the problem along with additional insights. The role of water in magma behavior that is learned in this section is subsequently utilized to evaluate explosiveness of volcanic eruptions, the formation of ore deposits, and the solidification of plutons during magma ascent through the crust.
Bowen’s reaction series receives varied treatment in current physical geology texts and we find most of these treatments unappealing. Bowen developed the concept to explain the limited range of minerals typically found in particular igneous rocks and why some mineral combinations (magnesian olivine and quartz, for example) rarely if ever occur. Although his demonstration of reaction relationships between minerals demonstrated that there were no thermodynamic barriers that prohibit deriving felsic melts from mafic ones, the reaction-series concept was not intended to explain the diversity of igneous-rock compositions. Many texts implicitly or explicitly confuse this concept with an expression of fractional crystallization, which is not a logical interpretation of the reaction series. Treatments of this type leave students with the erroneous conception that all felsic magmas begin as mafic ones that march down the reaction-series diagram. We feel that Bowen’s reaction series is unnecessary in the general-education geology class and geoscience students will encounter it in a more rigorous context in their mineralogy or petrology course. For the instructor who finds Bowen’s contribution important to teach, we provide Extension Module 4.1, which develops the reaction principle through illustration of the observations made in rocks that contributed to development of the reaction series.
Discussing volcanic eruptions and volcanic hazards is perhaps the most exciting topic for physical-geology instructors to present in their classes. We weave together the various eruption phenomena, volcano types, and volcanic hazards throughout the chapter to show the relationship of these topics to a more general understanding of magma generation and the diversity of magma compositions. If the instructor desires to go deeper into volcanic hazards and how geologists collect data to mitigate those hazards, then we encourage assignment of Extension Module 4.2. This module provides content that could complement a class session of lecture, discussion, or exercises related to volcanic hazards, and may be timely if an erupting volcano has recently been in the newspaper headlines.
Use this information as a guide to decide what you want your students to focus on when making assignments in this chapter.
Stated student learning outcome (page 56)
|Chapter sections that help build mastery of the outcome||End-of-the-chapter questions that assess the outcome|
|Explain the conditions that cause rocks to partly melt into magma and how these conditions relate to plate tectonics||4.4
|Confirm Your Knowledge: 2, 9, 10, 12
Confirm Your Understanding: 4
|Describe the processes that produce magmas of different compositions and show how differences in magma composition relate to the many types of volcanic eruptions and shapes of volcanoes||4.2 (background knowledge)
4.3 (background knowledge)
Extension Module 4.1
Extension Module 4.2
|Confirm Your Knowledge: 3, 8, 13, 14, 17
Confirm Your Understanding: 5, 6, 7
|Apply your comprehension of igneous processes and products to understand how some important economic resources form and where they are found||4.5
|Confirm Your Knowledge: 16|
This chapter permits opportunities to develop course-scale outcomes mastery:
|Course-level outcome||Relevant examples in the chapter||
|Understanding how geoscientists collect data||There are many examples in this chapter. Observation of grain sizes and colors to infer composition are used to classify rocks (Fig. 4.3). Rocks are melted in the laboratory to determine melting temperature of rocks (Figs. 4.15 and 4.16). Experiments conducted by Tuttle and Bowen also show the conditions of melting granite as well as the effects of water in magma (Fig. 4.18).||Consider asking clicker questions with photos of rock textures or rocks of various colors to infer composition.
Consider assigning the Active Art animations and asking students to work with them in class to draw new interpretations, or ask them clicker questions that demonstrate mastery of how geoscientists gather information.
Questions to assign/discuss: Confirm Your Knowledge 2, 4, 18
|Interpreting graphical displays of quantitative data||There are several graphs in this chapter. Of particular relevance to learning, the pressure-temperature graphs that illustrate conditions for rock melting and magma crystallization are accompanied by Active Art tutorials (for students to access at the companion web site) based on animations that you can use in class—Using Graphs to Understand Mantle Melting, and Understanding Tuttle and Bowen’s Data.||Consider assigning the Active Art animations and asking students to work with them in class to draw new interpretations, or ask them clicker questions that demonstrate mastery of graphical interpretation.
Questions to assign/discuss: Confirm Your Knowledge 9, 10, 18; Confirm Your Understanding 4.
See Active Learning Ideas for this chapter.
|Using evidence to support a scientific conclusion||The How Do We Know… section models all aspects of scientific problem solving, from posing the question, to experimental design, to interpretation of results.||Ask students to explicitly explain the role of evidence in supporting the interpretations in the How Do We Know… section.
Questions to assign/discuss: Confirm Your Knowledge 7; Confirm Your Understanding 3, 4.
See Active Learning Ideas for this chapter.
|Applying conceptual understanding to relevant or interesting problems||The chapter shows how to use understanding of magma formation, magma properties, and changes in magma composition through time to explain volcanic hazards and the formation of some important ore deposits.||Have students apply their knowledge at the end of the chapter to explain the different types of volcanic eruptions “witnessed” while in the field.
Questions to assign/discuss: Confirm Your Knowledge 14, 16, 17; Confirm Your Understanding 5, 17.
See Active Learning Ideas for this chapter.
|Synthesizing information to develop new understanding||Descriptions of volcano types and varieties of volcanic deposits are linked with magma properties to explain volcanic landscape features and volcanic hazards.
Experimentally determined pressure-temperature relationships for rock melting are linked to plate tectonics to explain the location of volcanoes.
|Questions to assign/discuss: Confirm Your Knowledge 16; Confirm Your Understanding 5, 7.
See Active Learning Ideas for this chapter.
Relating content and outcomes in this chapter to the rest of How Does Earth Work?
- Igneous rock/magma compositional terms are used extensively through the remainder of the book, especially in Chapters 5–13. Students should be encouraged to use Figure 4.3 as a reference resource throughout the course.
- The relationship of magma-generation processes to plate-boundary processes (section 4.6) is an essential learning competency for Chapter 12 (Global Tectonics: Plates and Plumes).
- Experimental petrology (section 4.3) is also explored in the How Do We Know… section for Chapter 6 (Formation of Metamorphic Rocks) and provides for a potential comparison/synthesis opportunity in Chapter 6.
- The roles of partial melting and fractional crystallization to determine the composition of magma, and resulting igneous rocks, is applied in Chapter 9 (Making Earth) to explain the origin of the crust.
Animations for You and Your Students
There are eight animations in the Geoscience Animation Library that link to the content of this chapter. They are available as Flash® animations within PowerPoint slides, ready to use in your classroom presentation, and as Active Art tutorials at www.mygeoscienceplace.com, to enhance your students’ self-paced visual learning.
Forming Igneous Features and Landforms. See how intrusions form and how they produce unique landforms when exposed by erosion. The animation illustrates the intrusion of magma to form batholiths, dikes, sills, and necks and demonstrates how these intrusive features are exposed in the landscape after erosion.
Forming Volcanoes. See how different types of volcanoes form. The animation consists of four parts that separately illustrate the formation of cinder cones, shield volcanoes, composite volcanoes, and dome complexes.
How Calderas Form. See how explosive eruptions form calderas. The animation shows the formation of a “Crater Lake-type” caldera in a composite volcano, contemporaneous with the eruption of large volumes of pyroclastic debris.
Using Graphs to Understand Mantle Melting. See how animated and annotated graphs explain how magma forms in the mantle. Separate animations show how decompression and wet melting can account for magma formation by “moving” mantle rock in pressure-temperature space in a graph; an excellent companion to Plate Tectonics and Magma Generation to compare graphical and diagrammatic illustrations of the same concepts.
Understanding Tuttle and Bowen’s Data. See how animated and annotated graphs explain Tuttle and Bowen’s data. The animation plots Tuttle and Bowen’s data points in pressure-temperature space, draws curves to separate the experimental results into different fields, and interprets the results.
Plate Tectonics and Magma Generation. See how magmas form at plate boundaries and hot spots. The animation consists of separate parts that schematically illustrate decompression and wet-melting in two-dimensional cross sections; an excellent companion to Using Graphs to Understand Mantle Melting to compare graphical and diagrammatic illustrations of the same concepts.
Fractional Crystallization. See how magma composition changes during crystallization. The animation shows the simple case of the crystallization of a single mineral phase that incorporates two elements from magma. The animation tracks the changing composition of the remaining melt through both schematic illustrations of crystallization and pie charts.
Density and Magma Movement. See how magma rises or stalls because of density contrast between magma and rock. The animation schematically illustrates the fate of magma (intrusion or volcanic eruption) based on the density of adjacent rock.
Active Learning Ideas
Clicker Questions – ConcepTests
Do you use a classroom response system in your class? There are 21 pre-prepared PowerPoint slides with clicker questions available for you to use at the instructor resources web page.
Additional questions are available on the ConcepTest page at the Science Education Resource Center (SERC): http://serc.carleton.edu/introgeo/interactive/ctestexm.html. Useful search phrases include: “ConcepTest volcano” (21 questions), “ConcepTest igneous” (46 questions).
For more information about teaching and learning with classroom response systems, see Chapter 3 in Part I of this instructors’ resource guide and SERC. (http://serc.carleton.edu/introgeo/interactive/conctest.html)
For more information about the implementation and purpose of think/write-pair-share questions, refer to Chapter 3 in Part I of this instructors’ resource guide.
What are the two characteristics of igneous rocks that are used to classify and name these rocks?
Volcanic explosions commonly produce pyroclastic-fall and pyroclastic-flow deposits. Which deposit is associated with the more hazardous process? Why is one process more hazardous than the other?
Figure 4.14 in your textbook shows a photograph of Crater Lake, Oregon, and a series of diagrams that illustrate how a caldera, like that represented by Crater Lake, forms. No geologist witnessed the eruption that formed Crater Lake, 7600 years ago. What evidence would you look for in the field today that would support the hypothetical development of the caldera that is shown in the diagram?
Why are most of the world’s volcanoes located at divergent plate boundaries, close to convergent plate boundaries, or at hot spots? Why is it very rare to encounter volcanoes at other locations?
Why have all of the very deadly volcanic eruptions in history happened close to convergent plate boundaries?
You have been hired to evaluate the volcanic hazards associated with a dormant volcano. The volcano has no historically recorded eruptions, but is showing signs of stirring to life. Assuming that future volcanic activity would be similar to the prehistoric eruptions, what information would you seek out in the field as a part of your evaluation, and why would you collect this information?
A classmate states, “fractional crystallization is something that could only interest academic geologists.” How do you respond, arguing that fractional crystallization explains processes that are very relevant to nonscientists?
In-Class Exercises (Lecture Tutorials)
For more information about the implementation and purpose of in-class exercises, refer to Chapter 3 in Part I of this instructors’ resource guide.
Three examples are provided of exercises that author Aurora Pun has her physical geology students complete in pairs during class time. Answer keys follow each exercise.
In-Class Exercise: Rock Melting
Using the diagrams below, answer the following questions:
- Compare the two diagrams. Rock “X” and rock “Y” are located at exactly the same depth and experience the same temperature in both diagrams. Which rock is melting?
- Why is one rock melting, while the other is solid at the same depth and temperature? Please explain what causes one of the rocks to melt and not the other. [Hint: What does the title of one of the graphs suggest has been added to the system?]
- For the rock (X or Y) that is still completely solid, draw an arrow on the graph that would represent a process that would cause the rock to melt without changing the temperature of the rock. Describe below how changing pressure made the rock melt (did you increase pressure or decrease pressure?).
KEY: In-Class Exercise: Rock Melting
- Rock Y is melting because the temperature and pressure conditions that this rock is experiencing plots in the part of the graph where crystals and liquid both exist. Rock X is in the solid-rock part of the diagram.
- Rock Y is melting whereas rock X is not because melting temperature at a particular pressure is lower in the presence of water (wet melting) than in the absence of water (dry melting).
- The appropriately drawn arrow will be vertical, because the question specifies that temperature is not changing. An arrow drawn vertically upward will cross into the crystals and liquid part of the diagram, indicating the onset of melting. Decreasing pressure, as might happen when the rock physically moves closer to the surface, causes melting (decompression melting).
In-Class Exercise: Fractional Crystallization
Your instructor has a bag of 30 colored candies. For some reason there are only two colors of candy in the bag: red and yellow (15 of each). Your instructor eats some candy at one-minute intervals. She likes the yellow candies better than the red ones, so every time she eats 1 red candy she also eats 2 yellow candies.
- Finish the chart below to show how many of each color candy is left as each minute passes.
- Use the data in the table to complete the graph. Label each dot you plot with its “Time,” as shown with the points already plotted for you.
- Over time, as your instructor eats the candies, which of the following statements is (are) true? Circle all that apply.
- There are now more yellow than red candies left in the bag
- There are now more red than yellow candies left in the bag
- The red candies are depleted (eaten) twice as fast as the yellow candies
- The yellow candies are depleted (eaten) twice as fast as the red candies
- Let’s make an analogy between the candies left in the bag and a fractionally crystallizing magma. If a crystallizing magma progressively became more enriched in Si (silicon) and depleted in Mg (magnesium), which color in the candy bag would represent Si in the magma?
- Red represents Si
- Yellow represents Si
- Over a period of time, a slowly crystallizing magma liquid develops a higher silica content while magnesium abundance decreases through fractional crystallization. Do the minerals crystallizing from the magma contain more silica relative to the starting magma or do they contain more magnesium relative to the starting magma?
- The minerals contain more Si than the magma liquid
- The minerals contain more Mg than the magma liquid
- (a) Two minerals that crystallize from common magmas are olivine (Mg2SiO4) and quartz (SiO2). If a magma liquid is getting enriched in Si and depleted in Mg as it cools and starts to crystallize, which of these minerals crystallizes first? Hint: use the mineral formulas to help explain your answer.
- Olivine crystallizes first
- Quartz crystallizes first
(b) Explain your reasoning behind the answer you selected:
KEY: In-Class Exercise: Fractional Crystallization
- b) There are now more red than yellow candies left in the bag
- d) The yellow candies are depleted (eaten) twice as fast as the red candies
- Red represents Si
- The minerals contain more Mg than the magma liquid
- Olivine crystallizes first
6b. Because the silica content of the magma is increasing and the magnesium content is decreasing, the crystallizing mineral must contain more magnesium than the magma does. Quartz contains only silica, so if it crystallized, the silica content of the remaining liquid would decrease and the magnesium content would increase. Only removal of olivine, which contains magnesium, can cause the magnesium content of the remaining liquid to decrease.
In-Class Exercise: Magma Composition and Volcanic Eruptions
Volcanoes near a community in the western United States are made up of lava flows and pyroclastic deposits consisting of black, aphanitic rocks with about 50% SiO2.
- What was the composition of the magma that erupted to form the volcanoes?
- Based on the composition of the magma that erupted to form the volcanoes, which of the following statement(s) would most likely be true?
- The magma that erupted contained very little gas
- The magma that erupted contained a lot of gas
- The eruptions were very explosive
- The eruptions were non-explosive
- The erupting magma had a high viscosity
- The erupting magma had a low viscosity
- (a) The mayor of the community is concerned about possible future eruptions and what hazards might be anticipated during an eruption. Assume that a future eruption would most likely erupt the same magma composition as during past eruptions. A consultant warns that such a future eruption will be highly explosive and would threaten large areas of the city; the city would be buried by pumice and devastated by pyroclastic flows. You are asked to give a second opinion. Do you agree or disagree with the consultant?
- I agree with the consultant
- I disagree with the consultant
(b) Please explain the reasoning behind the answer you selected, citing supporting evidence from your textbook.
KEY: In-Class Exercise: Magma Composition and Volcanic Eruptions
- b) mafic
- a) The magma that erupted contained very little gas
- d) The eruptions were non-explosive
- f) The erupting magma had a low viscosity
- a) b) I disagree with the consultant
- b) The properties of eruptions of a mafic magma, summarized in the answer to question 2, are inconsistent with the consultant’s warning. Instead, a nonexplosive eruption is expected and the greatest hazard will be from far-traveled lava flows.
Online Teaching and Learning Resources
Encounter Earth – Interactive Geoscience Explorations:
These are interactive exercises using Google Earth, authored by Steve Kluge and available at www.mygeoscienceplace.com, to enhance your students’ self-paced learning. (If using any Encounter Earth exercises, you should be sure that students also complete Exercise 1, which explains how to download Google Earth and navigate the software.)
Exploration 13 – Volcanism – Global Distribution of Volcanoes
This exercise is a fly over in Central America and the “ring of fire” where students address questions related to various plate boundaries and the distribution of volcanoes. It also asks questions related to hot spot volcanism.
Exploration 14 – Volcanism – Lava Flows
This exercise visits Davis Lake, Oregon where students are asked questions regarding the relative ages and thicknesses of lava flows.
Exploration 15 – Volcanism – Calderas
This exercise visits Crater Lake, Oregon and Newberry Volcano, Oregon. Students are asked questions about features related to Crater Lake such as the diameter and surface area as well as questions related to recent volcanic activity. Students are asked to describe and name the volcanic landforms associated with Newberry Volcano and to compare this volcano to Crater Lake.
Resources at the SERC Website (Summary excerpts from SERC)
Igneous Rocks Model: While working in groups to facilitate peer tutoring, students use samples of four igneous rocks (gabbro, basalt, granite, and rhyolite) to observe differences in texture, color, and grain size and make inferences about the relative cooling histories and silica content associated with each magma type.
Crystallization from Melt Demonstration: This demonstration uses melted phenyl salicylate (C13H10O3—melting temperature 43°C) to show how crystals nucleate and grow as the temperature of the liquid melt decreases. To use the demonstration in class, you can reproduce the original experiment, or show video footage of the experiment that can be downloaded at the website.
USGS Volcano Hazards Program
The USGS Volcano Hazards Program home page (http://volcanoes.usgs.gov/) lists the current alerts and information updates on volcanoes in Hawaii, Alaska, and the western continental United States. The home page also includes links to real-time webcams filming more than a dozen U.S. volcanoes. This information can be brought into the classroom to make your students’ study current to that day. The USGS web site also has links to many fact sheets related to volcanic hazards and photo galleries of recent eruptions. Links to the USGS volcano observatories in Hawaii, Alaska, Cascades, Yellowstone, and Long Valley open doors to more data and photos.
The Volcano World web site (http://volcano.oregonstate.edu/) provides a wealth of information and educational activities related to volcanoes. The educational materials on this site are primarily designed for pre-college students. However, the “current activity” page includes information about volcanic activity around the world that may be useful in your class.
End-of-Chapter Questions and Answers (p. 83)
Confirm Your Knowledge
- How does magma differ from lava? How are magma and lava classified?
Magma is molten material below Earth’s surface whereas lava is molten material at Earth’s surface.
- Geologists gain understanding of the formation of magma and igneous rocks by making field observations, geochemical analyses, and laboratory experiments. Give an example of each.
This is an open-ended question where students may refer to field studies of modern or ancient volcanoes, volcanic rock, and ancient intrusive rock exposed to the surface by erosion; geochemical studies including the identification and measurement of the abundances of elements in igneous rocks; laboratory experiments including melting and/or crystallization studies that mimic the temperature and pressure conditions inside Earth.
- What are the four basic types of magma compositions, and how do they differ in their respective abundance of silica, iron, and magnesium?
Felsic: highest silica content, lowest Fe-Mg content
Intermediate; medium silica content; medium Fe-Mg content
Mafic: low silica content; high in Fe-Mg content
Ultramafic: lowest silica content; highest Fe-Mg content
- What aspect of igneous rock formation is best illustrated by crystal grain size? Explain the relationship between rock formation and grain size.
Magma cooling rate is most closely related to crystal size in igneous rocks. A slow cooling rate results in large crystals, whereas a fast cooling rate results in small crystals.
- How are pyroclastic deposits classified?
The size of the fragments is used to classify pyroclastic material.
- Which of the nine igneous rocks shown in Figure 4.3 were formed during last century’s eruptions of Mount St. Helens and Mount Pinatubo?
- 7. List the three pieces of evidence for how we know that there is a magma chamber and the formation of plutonic rocks below an active volcano?
There are earthquakes caused by the movement of magma beneath volcanoes.
Volcanoes erupt pieces of plutonic rock along with the magma.
Erosion of volcanic rock exposes plutonic rock below.
- What factors determine the size and shape of a volcano?
The size is determined by the volume of magma (or lava) erupted. The shape is determined by the type of erupted material (lava, loose pyroclastic material, or both). Viscosity of erupted lava determines the steepness of the volcano slope if it is composed mostly of lava flows.
- Explain how peridotite can melt at a divergent plate boundary without an increase in temperature.
Melting occurs because the melting temperature decreases with decreasing pressure. Peridotite in the mantle rises to a lower depth at divergent boundaries with little or no decrease in temperature. The lower depth corresponds to lower pressure. The lower pressure results in a lower melting point and the peridotite partly melts.
- How do magmas form at convergent plate boundaries?
The melts that form at a convergent plate boundary mostly originate in the asthenosphere above the subducting plate, because the melting temperature of the asthenosphere peridotite is reduced by injection of water released from metamorphism of the subducted crust.
- Explain how calderas form.
Removal of magma from shallow magma chambers causes caldera formation when the unsupported roof of the magma chamber collapses.
- Define partial melting and explain how it affects the magma composition.
Partial melting occurs when a rock does not melt completely to form magma. Only some minerals in the rock completely melt, so the resulting magma does not have the same composition as the original rock. Partial melting typically creates magma that is relatively richer in silica than the original rock. For example, partial melting of an ultramafic rock (peridotite) forms a mafic (basaltic) magma; partial melting of a mafic rock (basalt) forms to intermediate or felsic magmas.
- What processes can form intermediate and felsic magmas from a melt that is originally mafic?
Fractional crystallization: Removal of early-formed, relatively low-silica minerals leaves a melt that is more silica-rich.
Magma assimilation: Mafic magma may melt more silica-rich surrounding rock; the resulting silica-rich melt blends with the originally mafic magma to produce an intermediate or felsic magma.
Magma mixing: A mafic magma may mix with a felsic magma to produce an intermediate magma.
- How does the type of volcanic eruption relate to gas content and magma viscosity?
Gas content and viscosity are both greater in higher-silica magma than in lower-silica magma. Release of dissolved gas from rising magma under decreasing pressure causes expansion of bubbles in the melt that explode to cause explosive eruptions. High viscosity inhibits the ability of bubbles to rise through the magma so that gas pressures tend to be greater in intermediate and felsic magmas than in less viscous (and less gas-rich) mafic magmas. High viscosity also favors the formation of thick but not far traveled lava flows and lava domes, whereas low viscosity favors the formation of thin and extensive lava flows. These roles of gas content and viscosity to determine eruption characteristics explain why intermediate and felsic magmas generally feed relatively explosive eruptions that produce abundant pyroclastic deposits and steep-sided volcanoes, such as composite volcanoes and dome complexes. Mafic magmas, in contrast, tend to erupt less explosively and form low-sloping shield volcanoes with widespread lava flows and very little pyroclastic material.
- Obsidian is a felsic rock, yet has a black color. Explain.
The unexpectedly dark color of this felsic rock results from the presence of tiny crystals of black magnetite scattered throughout transparent glass. Thin slivers of obsidian are colorless, rather than black, which shows that even a low concentration of magnetite crystals strongly affects the color of thick samples.
- How does fractional crystallization contribute to the formation of economically important metal ores?
Most economically important metal atoms do not fit easily into the silicate minerals that form most of the crystals in solidifying magma. As a result, fractional crystallization concentrates originally trace quantities of the metal atoms so that they have relatively high concentrations in the last remaining melt. At these high concentrations, the metal atoms typically join with sulfur atoms to make sulfide ore minerals.
- 17. Explain why some lava flows cover large areas and others do not.
One obvious reason is that the more lava that is erupted, the larger the area that it will cover. We also know that the area covered by lava relates to the viscosity of the lava flows. Low-viscosity lava flows (mafic) are able to flow more readily and cover large areas, whereas high-viscosity (felsic) lava flows are unable to move beyond the immediate vicinity of the volcanic crater.
- Identify the following igneous rocks based on their texture and mineral content.
- Fine-grained rock with 0% quartz, 0% potassium feldspar, 61% plagioclase feldspar, 13% biotite, 17% amphibole, 9% pyroxene, 0% olivine
- Fine-grained rock with 0% quartz, 0% potassium feldspar, 67% calcium-rich plagioclase feldspar, 0% biotite, 0% amphibole, 25% pyroxene, 8% olivine
- Coarse-grained rock with 0% quartz, 0% potassium feldspar, 52% calcium-rich plagioclase feldspar, 0% biotite, 0% amphibole, 32% pyroxene, 16% olivine
- Coarse-grained rock with 0% quartz, 0% potassium feldspar, 0% plagioclase feldspar, 0% biotite, 0% muscovite, 0% amphibole, 56% pyroxene, 44% olivine
- Fine-grained rock with 32% quartz, 26% potassium feldspar, 26% sodium-rich plagioclase feldspar, 5% biotite, 11% amphibole, 0% pyroxene, 0% olivine
By reference to text Figure 4.3: Andesite, Basalt, Gabbro, Peridotite, Rhyolite
Confirm Your Understanding
- Write an answer for each question in the section headings.
Open-ended questions. Authors suggest that the instructor develop a rubric that matches outcomes and expectations for the students. In particular, expect students to provide examples and evidence to support their answers, and expect more than a repetition of the Putting It Together text provided at the end of each section.
- What is a geothermal gradient? Given an average geothermal gradient for continents and a surface temperature of 15°C, what is the temperature at a depth of 10 kilometers?
The geothermal gradient is the increase in temperature that occurs with the increase in depth below Earth’s surface. Using an average of 25˚C/km, then at 10 km depth the temperature would be 265˚C (15º at the surface + (10 km)(25º C/km)).
- What evidence is necessary to prove that batholiths form over a long period of time by multiple intrusions of differing composition?
Field relationships would show multiple intrusions. The earliest intruded magma solidifies into plutonic rock. Subsequent magma intrusions form a cross-cutting network of igneous intrusions that comprise the batholith. These different plutonic bodies would have different compositions as illustrated by different rock types.
- Mafic magma at a temperature of 1200°C intrudes into felsic continental-crust rocks 15 km below the surface. Will the granite melt? If so, what will happen to the resulting magma? Use information presented in the chapter to support your answer.
Figure 4.18 is useful for answering these questions. At 15 km below the surface, granite completely melts into granitic magma at about 705oC, which is much less than the temperature of the intruding mafic magma. However, this rising magma may not make it to the surface because water will escape as gas as the pressure decreases and this causes an increase in the melting temperature.
- What type of volcanic products (lava flows, lava domes, or pyroclastic material) and types of volcanoes would you expect from an eruption of basalt? of andesite? of dacite? of rhyolite? If there is more than one type of volcanic product, rank them from most abundant to least abundant.
Products: (most abundant) lava flows, pyroclastic material, lava domes (least abundant).
Volcano types: shield volcanoes, cinder cones
Products: (most abundant) pyroclastic material, lava flows, lava domes (least abundant).
Volcano types: cinder cones, composite volcanoes
Products: (most abundant) pyroclastic material, lava domes, lava flows (least abundant).
Volcano types: composite volcanoes, dome complexes
Products: (most abundant) pyroclastic material, lava domes, lava flows (least abundant).
Volcano type: dome complexes
- Write a paragraph explaining the processes that create the various igneous rock compositions on Earth.
Key concepts: Partial melting of the mantle produces mafic magma that crystallizes to form basalt or gabbro. Fractional crystallization of mafic magma can produce intermediate or felsic magma. Partial melting of different rock types produces different magma compositions. These magmas of different starting composition may mix with one another to produce still different compositions.
- You are planning to move to a volcanic island and you want to choose the safest location. You have to decide between an island with a composite volcano consisting of dacite and andesite or an island with a shield volcano consisting of basaltic lava flows. Which island do you choose? Why?
You should choose the island with the shield volcano because lava flows usually advance slowly enough so that people can get out of the way. The pyroclastic flows and lahars associated with composite volcanoes move so rapidly that people in their path have little chance of survival.