Chapter 4: Igneous Rocks and Plutons

Transcription

1 Chapter 4: Igneous Rocks and Plutons Chapter Outline 4.1 Introduction 4.2 The Properties and Behavior of Magma and Lava 4.3 How Does Magma Originate and Change? 4.4 Igneous Rocks Their Characteristics and Classification 4.5 Intrusive Igneous Bodies Plutons GEO-FOCUS 4.1: Granite Common, Attractive, and Useful 4.6 The Origin of Batholiths Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. With few exceptions magma is composed of silicon and oxygen with lesser amounts of several other chemical elements. Temperature and composition are the most important controls on the mobility of magma and lava. Most magma originates within Earth s upper mantle or lower crust at or near divergent or convergent plate boundaries. Several processes bring about chemical changes in magma, so magma may evolve from one kind into another. All igneous rocks form when magma or lava cools and crystallizes, or by the consolidation of pyroclastic materials ejected during explosive eruptions. Geologists use texture and composition to classify igneous rocks. Intrusive igneous bodies called plutons form when magma cools below Earth s surface. The origin of the largest plutons is not fully understood. Chapter Summary Magma is the term for a molten rock below Earth's surface, whereas the same material at the surface is called lava. Silica content distinguishes among ultramafic (<45% silica), mafic (45-52 % silica), intermediate (53-65 % silica), and felsic (>65 % silica) magmas. Magma and lava viscosity depend mostly on temperature and composition: The higher the temperature, the lower the viscosity; the more silica, the greater the viscosity Minerals crystallize from magma and lava when small crystal nuclei form and grow. Rapid cooling accounts for the aphanitic textures of volcanic rocks, whereas comparatively slow cooling yields the phaneritic textures of plutonic rocks. Igneous rocks with markedly different-sized minerals are porphyritic.

2 Igneous rock composition is determined mostly by the composition of the parent magma, but magma composition can change so that the same magma may yield more than one type of igneous rock. According to Bowen's Reaction Series, cooling maffic magma yields a sequence of minerals, each of which is stable within specific temperature ranges. Only ferromagnesian silicates are found in the discontinuous branch of Bowen's Reaction Series. The continuous branch yields only plagioclase feldspars that become increasingly enriched with sodium as cooling occurs. A chemical change in magma may take place as early ferromagnesian silicates form and, because of their density, settle in the magma. Compositional changes also take place in magma when it assimilates country rock or one magma mixes with another. Geologists recognize two broad categories of igneous rocks: volcanic or extrusive and plutonic or intrusive. Texture and composition are the criteria used to classify igneous rocks, although a few are defined mostly by texture. Crystallization from water-rich magma results in very large minerals that form rocks known as pegmatite. Most pegmatite has an overall composition similar to granite. Intrusive igneous bodies known as plutons vary in their geometry and their relationship to country rock: Some are concordant, whereas others are discordant. The largest plutons, known as batholiths, consist of multiple intrusions of magma during long periods of time. Most plutons, including batholiths, are found at or near divergent or convergent plate boundaries. Enrichment Topics Topic 1. Igneous Rock Classification. There are lots of ways to classify igneous rocks. Grab a handful of igneous rocks, preferably with their chemical analyses, and explore some of those ways color-texture, modal, normative. Topic 2. Devil's Tower. Geologists do not agree on whether Devil s Tower is the neck of a long-dead volcano or an igneous intrusion. How could geologists tell the difference? Topic 3. Peridotite and Global Warming. Although little peridotite is found at Earth s surface, there are some large exposures in Oman, Papua New Guinea, and in a few locations in Greece, Yugoslavia and the western U.S. Geologists have found that peridotite reacts with carbon dioxide to form a solid carbonate. They have proposed to inject heated water containing pressurized CO 2 into peridotite. The pressure would crack large amounts of rock, which would allow the solution to penetrate further into the rock. The Earth s heat at depth would speed up the reaction. The reaction would take little energy once it started. The Oman ophiolites could absorb more than 10% of the CO2 released by human activities into the atmosphere.

3 Common Misconceptions Misconception 1: Once they accept the fact that magmas can exist within Earth's crust, many students believe that these bodies of liquid are squeezed into surrounding rocks, or squirted out at the surface by some unknown forces acting in a manner like the squeezing of toothpaste from a tube. Fact: Magma bodies rise and intrude in response to the simple pull of gravity. Liquid rock magma has a lower specific gravity than solid rock. This causes the magma to rise and perhaps, over time, even to deform other rocks. As the magma approaches the surface and the pressure decreases, there is the additional push provided by the expansion of the dissolved gases coming out of solution. Misconception 2: Rock needs a heat source to melt. Fact: Decompression melting is common in Earth, especially at mid-ocean ridges. When rock at a high temperature rises in the mantle so that pressure is lowered, its melting temperature lowers as well. The rock can then melt. Lecture Suggestions 1. Bowen's reaction series can be compared to changing two brick walls into two new walls. In one case the discontinuous series the bricks are individually removed, and a new wall is simultaneously built from the bricks but in a new pattern. The constituents of olivine, for example, are thus recreated into the new structure of pyroxene. In the second case the continuous series bricks are individually removed from the wall and replaced by different bricks having a different color but retaining the original pattern. For plagioclase, an original constituent (calcium) is replaced by a new constituent (sodium) but the structure is never torn down. 2. Bowen's reaction series is a very powerful tool for explaining more than one facet of rock formation. The series helps us understand why some minerals are typically found together in igneous rocks (e.g., olivine and pyroxene with calcium plagioclase; or quartz, biotite, and orthoclase with sodium plagioclase), but others are not (e.g., olivine and pyroxene with sodium plagioclase and orthoclase). Bowen's series also explains temperature relations (why mafic magmas are hotter than felsic ones). It can also be seen why a mafic magma, by giving up heat, can partially melt material to form felsic magma, but the reverse isn't possible. Later, when weathering is discussed, Bowen's series will help understand the greater stability at Earth's surface of the lower temperature minerals (e.g., quartz) and lesser stability of the higher temperature ones (e.g., olivine). 3. Crystal settling may be explained by an analogy of a series of salts that precipitate as saltwater evaporates while the remaining water becomes increasingly saturated in the remaining salts. Precipitation from a solution and crystallization of a magma are not the same thing. 4. Dust off the lava lamp you relegated to the attic and bring it to class. You can use it to illustrate points about how densities vary with temperature, how gravity is the driving force in a rising magma, and what shapes are assumed by the rising bodies as they deform and shoulder aside the other material. You may want to note that unlike the lava

4 lamp example, plutons, once they ascend, do not later descend back into the same rocks through which they rose. 5. Granite batholiths create some of the most amazing mountains in the world. Show some slides of the Sierra Nevada in California and especially the domes of Yosemite to interest students in this amazing rock type and its manifestations. Consider This 1. If small bodies such as dikes (e.g., basalt dikes) and sills cool rapidly and are therefore usually fine grained, how is it possible for pegmatites, having crystals several feet in length, to form in these small bodies? 2. Batholiths are believed to have been emplaced and solidified several kilometers underground, yet we can see them exposed in mountain ranges today. What has happened to all of the rocks that must have been above the batholiths? 3. How can a single volcano erupt distinctly felsic composition lavas in one event, and distinctly mafic lavas in another? Important Terms aphanitic texture assimilation batholith Bowen s reaction series concordant pluton country rock crystal settling dike discordant pluton felsic magma hot spot igneous rock intermediate magma laccolith lava lava flow mafic magma magma magma chamber magma mixing mantle plume phaneritic texture pluton plutonic (intrusive) rock porphyritic texture pyroclastic (fragmental) texture pyroclastic materials sill stock stoping ultramafic magma vesicle viscosity volcanic neck volcanic pipe volcanic (extrusive) rock

5 Internet Sites, Videos, Software, and Demonstration Aids Internet Sites 1. Atlas of Igneous Rocks, Minerals, and Textures This University of North Carolina website was developed to help undergraduate students understand igneous and metamorphic rocks. 2. Rob's Granite Page An introduction to granitic rocks from the University of Texas. Videos 1. Journeys from the Center of the Earth: Architecture. Insight Media (2004, 50 mins.) Rocks and how they determined the structures that could be built by ancient civilizations. 2. Intrusive Igneous Rocks. Insight Media (1999, 19 mins.) The formation of intrusive igneous rocks. 3. Rock Cycle. Insight Media (2003, 30 mins.) How minerals form rocks and rocks change into other types of rocks. 4. Elements of Earth Science: Rocks, Minerals, and Soils (2005, 30 mins.) The rock cycle including the three main types of rocks. 5. Earth Revealed. Annenberg Media (1992, 30 mins., free video): #14: Intrusive Igneous Rocks. The processes that create intrusive rocks and the types of intrusive rocks that are found on Earth. Slides and Demonstration Aids 1. Geology EOA: Rocks, Minerals and Resources (Mac/Windows CD-ROM) Mineral and rock types and resources discussed and identified. 2. Rock and Topography, 100 slides. Educational Images, Ltd. Slide set including many topics important to introductory geology courses. 3. Intrusive Rocks, digital images. GeoPhoto Publishing. Digital images of intrusive and metamorphic rocks from the continental core. 4. Science Stuff, has an assortment of rock and mineral collections, including Introductory Earth Science Rocks and Minerals Igneous Rock Collection Metamorphic Rock Collection Sedimentary rock Collection Advanced Rock and Minerals Collection

6 Answers to Figure-Related Critical Thinking Questions Critical Thinking Question Figure 4.3 Which one of these volcanoes would you expect to erupt explosively? Explain. Mauna Loa Volcano has more mafic lava flow. The Novarupta lava dome in Katmai National Park in Alaska is more viscous.in general, mafic magma/lava tends to be hotter and contain less silicon dioxide than felsic and intermediate magma/lava, so mafic eruptions tend to result in lava flows, but felsic and intermediate eruptions tend to be explosive and quite dangerous. Therefore the Novarupta lava dome in Katmai National Park in Alaska is more likely to erupt explosively. Critical Thinking Question Figure 4.11 Of the magmas that crystallized to form rhyolite, andesite, and basalt, which one would have been the most viscous? How do you know? Rapid cooling in lava flows results in many small minerals and an aphanitic (fine-grained) texture. Slower cooling yields a phaneritic (coarse-grained) texture. The magmas that produced basalt would have been the most viscous since basalt shows a uniform aphanitic texture while rhyolite and andesite show a more phaneritic texture.. Critical Thinking Question Figure 4.13 How can you tell from these images which specimens are volcanic and which are plutonic? Volcanic rocks have a more uniform (aphanitic) texture because cooling occurs very rapidly. The specimens with a coarser (phaneritic) texture are plutonic. The cooling in these rocks occurred beneath the earth s surface and thus at a slower rate, allowing the crystallization of minerals. Critical Thinking Question Figure 4.16 Why do pumice and scoria have so many vesicles but obsidian has none? Obsidian, pumice and scoria all lack crystals because they cooled too quickly for crystal to form. Scoria and pumice have vesicles of rapid cooling of gas entrained (frothy) felsic lava. Critical Thinking Question Figure 4.20 How can you account for the fact that the dike in the foreground stands above the surface like a wall? While Ship Rock itself is a result of a highly explosive volcano (volcanic neck), the dikes radiating outward from Ship Rock are believed to have been formed by the extrusion of magma along transform fault boundaries.