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Intrusive and Extrusive 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-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2014 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/terms. Printed: December 8, 2014 AUTHOR Dana Desonie, Ph.D. www.ck12.org C HAPTER Chapter 1. Intrusive and Extrusive Igneous Rocks 1 Intrusive and Extrusive 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? Cooling Igneous rocks cool from magma. The appearance of the rock is created by the composition of the magma. It is also determined by the rate that the magma cools. If the magma cools deep underground, it cools slowly. If the magma cools at or very near the surface, it cools quickly. This results in two different rock types. The rock types can be told apart by the size of their crystals. The size of the crystals creates the texture of the rock. Intrusive Igneous Rocks Intrusive igneous rocks cool underground. Deep in the crust, magma cools slowly. Slow cooling gives crystals a chance to grow. Intrusive igneous rocks have relatively large crystals that are easy to see. Intrusive igneous rocks are also called plutonic. A pluton is an igneous rock body that forms within the crust. Granite is the most common intrusive igneous rock. Pictured below are four types of intrusive rocks ( Figure 1.1). Geological processes have brought some igneous rocks to the surface. Pictured below is a landscape in California’s Sierra Nevada Mountains made of granite that has been raised to create mountains ( Figure 1.2). Extrusive Igneous Rocks Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface ( Figure 1.3). Extrusive igneous rocks cool much more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form. So igneous extrusive rocks have smaller crystals than igneous intrusive rocks. Extrusive igneous rocks are also called volcanic rocks. 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 1 www.ck12.org FIGURE 1.1 (A) This granite has more plagioclase feldspar than many granites. (B) Dior- ite has more dark-colored minerals than granite. (C) Gabbro. (D) Peridotite contains olivine and other mafic minerals. FIGURE 1.2 California’s Sierra Nevada Mountains are intrusive igneous rock exposed at Earth’s surface. FIGURE 1.3 (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. 2 www.ck12.org Chapter 1. Intrusive and Extrusive Igneous Rocks 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 1.4). FIGURE 1.4 Different cooling rate and gas content resulted in these different textures. Vocabulary • • • • extrusive: Igneous rocks that form at Earth’s surface from rapidly cooling lava. intrusive: Igneous rocks that form inside the Earth from slowly cooling magma. pluton: Igneous intrusive rock body that has cooled in the crust. volcanic rock: Rock that originates in a volcano or volcanic feature. 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. Explore More Use the resource below to answer the questions that follow. • Igneous Rocks at http://www.youtube.com/watch?v=deC5af9AW6w (5:06) MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/58884 3 www.ck12.org 1. 2. 3. 4. 5. 6. 7. 8. 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. How do intrusive igneous rocks form? What is another name for these rocks? 2. How do extrusive igneous rocks form? What is another name for these rocks? 3. How can you use texture to determine whether an igneous rock is intrusive or extrusive? References 1. (A) Image copyright MARGRIT HIRSCH, 2013; (B) Image copyright Tyler Boyes, 2013; (C) Mark A. Wilson (Department of Geology, The College of Wooster); (D) Image copyright Marcin Sylwia Ciesielski, 2013. Granite, diorite, gabbro, and peridotite are all intrusive igneous rocks. (A, B, D) Used under licenses from Shutterstock.com; (C) Public domain 2. User:Dcrjsr/Wikimedia Commons. The Sierra Nevada Mountains are made of intrusive igneous rock. CC BY 3.0 3. (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. (A) Public Domain; (B) CC BY 2.0 4. (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. (a) CC BY 2.0; (b) Public Domain; (c) Public Domain 4