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Characteristics of Igneous Rocks Dana Desonie, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2016 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: January 31, 2016 AUTHOR Dana Desonie, Ph.D. www.ck12.org C HAPTER Chapter 1. Characteristics of Igneous Rocks 1 Characteristics of Igneous Rocks • Compare and contrast intrusive and extrusive igneous rock. Are these both igneous rocks? These rocks don’t even look like they’re the same type! They are, at least in the same way that fish and mice are both vertebrates. They both cooled from magma, but the similarities end there. Can you tell what’s different? TABLE 1.1: Igneous Rock References 1: 2: 3: 4: Basalt Gabbro Granite Obsidian 5: 6: 7: 8: Pegmatite Pumice Rhyolite Scoria Magma & Igneous Rocks Molten rock inside Earth is called magma. There are large pools of magma inside Earth’s upper mantle and lower crust. Sometimes, magma rises through the cracks in rocks into the upper part of Earth’s crust. here, the temperature is cooler than the temperatures inside the mantle. As magma rises through the cracks in Earth’s crust, it cools and crystalizes forming intrusive igneous rocks. Intrusive igneous rocks have large crystals because they cool slowly under Earth’s surface. Granite is the most common intrusive igneous rock. Which of your rocks are intrusive igneous rocks, showing large crystals? Lava & Igneous Rocks When magma rises through cracks in rocks and reaches Earth’s surface it is called lava. Lava cools upon contact with air or water. Cooling makes lava harden into extrusive igneous rocks. Extrusive igneous rocks have small crystals or no crystals at all due to the quickly cooling lava. 1 www.ck12.org FIGURE 1.1 Pegmatite is an intrusive igneous rock that has quartz, mica and feldspar crystals that cooled slowly (large) then quickly (small). (A) Lava cools to form extrusive igneous rock. The rocks here are basalts. (B) The strange rock formations of Chiricahua National Monument in Arizona are formed of the extrusive igneous rock rhyolite. Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the ocean floor. Shown below are three types of extrusive igneous rocks ( Figure 1.2). Which of your rocks are extrusive igneous rocks and have small crystals or no crystals? FIGURE 1.2 Different cooling rate and gas content resulted in these different textures. Igneous Rock Classification The first step in classifying an igneous rock is to know its composition or what type of minerals it contains. Igneous rocks are made up of many different kinds of minerals. However, there are only six minerals that are commonly found in igneous rocks: quartz, feldspar, mica, olivine, amphibole, pyroxene. Look at a piece of granite. Can you identify the three main minerals that compose this rock? The composition of an igneous rock can be classified as felsic or mafic. The composition/color depends on the minerals the rock includes. A felsic rock will contain felsic minerals. Felsic rocks are mostly light in color. Mafic rocks are mostly dark in color. Try classifying your igneous rocks by their composition. The mineral compositions are listed below ( Table 1.2). The table includes examples of minerals. TABLE 1.2: Properties of Igneous Rock Compositions Composition Felsic 2 Color Light Density Low Minerals Quartz, feldspar orthoclase www.ck12.org Chapter 1. Characteristics of Igneous Rocks TABLE 1.2: (continued) Composition Mafic Color Dark Density High Minerals Olivine, pyroxene The second step in classifying igneous rocks is to identify the texture. Texture indicates how the magma cooled. Lava that cools rapidly has small or no visible crystals. Magma that cools slowly has large crystals. You can tell if a rock is granite or rhyolite by looking at its texture. Both rocks have the same composition (minerals), but cooled in different ways changing the texture of the minerals. Granite has large crystals that you can feel and see. Rhyolite has very small crystals that are hard to see. Extrusive igneous rocks will have a fine texture. Intrusive igneous rocks will have a coarse texture. Igneous rocks that do have have crystals, like obsidian, have a glassy texture. FIGURE 1.3 FIGURE 1.4 Pictured above are two pairs of igneous rocks ( Figures above and above). The first pair is two rocks that formed from a felsic magma. The rhyolite cooled rapidly, and the granite cooled slowly. The second pair is two rocks that formed from mafic magma. The basalt cooled rapidly, and the gabbro cooled slowly. The color of the rocks and sizes of the crystals are your clues. Try sorting your rocks by texture. Can you correctly identify the extrusive and intrusive rocks? Igneous Rocks in Missouri Giant elephant-shaped granite boulders are the star at Elephant Rocks State Park. These rocks are coarsely crystalline red granite. Granite and rhyolite can be found in the St. Francios Mountains in southeast Missouri. What can you infer about Missouri’s geologic past? Vocabulary • extrusive: Igneous rocks that form at Earth’s surface from rapidly cooling lava. 3 www.ck12.org FIGURE 1.5 Elephant Rocks State Park in southeast Missouri • intrusive: Igneous rocks that form inside the Earth from slowly cooling magma. • felsic: light in color • mafic: dark in color Summary • • • • Intrusive igneous rocks cool from magma slowly in the crust. They have large crystals. Extrusive igneous rocks cool from lava rapidly at the surface. They have small crystals. Texture reflects how an igneous rock formed. Igneous rocks are classified first by composition. Categories go from from felsic to mafic. Color can indicate composition. • Texture is second when classifying igneous rocks. Texture indicates how a rock cooled. • Igneous rocks are categorized in pairs. The two rocks in a pair have the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite. Practice Use the resource below to answer the questions that follow. • Igneous Rocks at http://www.youtube.com/watch?v=deC5af9AW6w (5:06) 1. 2. 3. 4. 5. 6. 7. 8. 9. How are intrusive rocks formed? What size are the crystals in very coarse rocks? Why are they that size? What are the most common coarse rocks? How are extrusive rocks formed? List the three textures for extrusive rocks. Describe rhyolite. Describe pumice. Explain why obsidian appears black. Review 1. 2. 3. 4. 5. 6. 4 How do intrusive igneous rocks form? How do extrusive igneous rocks form? How can you use texture to determine whether an igneous rock is intrusive or extrusive? How does the composition of a rock affect its color? What are mafic rocks? What is their color? www.ck12.org Chapter 1. Characteristics of Igneous Rocks Missouri Standards • 5.1.A.d • 5.2.C.b • 5.2.C.c References 1. (A) Courtesy of J.D. Griggs, US Geological Survey; (B) Flickr:SonoranDesertNPS. Lava cools to form extrusive igneous rock, and rhyolite formations in Chiricahua National Monument. 2. (a) Kevin Walsh (Flickr: kevinzim); (b) User:deltalimatrieste/Wikimedia Commons; (c) Courtesy of the National Oceanic and Atmospheric Administration. Obsidian, pumice, and basalt are extrusive igneous rocks that cool at different rates. 3. . Comparison between rhyolite and granite. 4. . Comparison between rhyolite and granite. 5. . Comparison between basalt and gabbro. 6. . Comparison between basalt and gabbro. 5