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Rocks Igneous Rocks Key Concepts What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind. Before Statement • How do igneous rocks form? • What are the common types of igneous rocks? After 3. Large crystals form when lava cools quickly on Earth’s surface. 4. Igneous rocks form when cooling magma crystallizes. 3TUDY#OACH Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Igneous Rock Formation Recall that lava is magma that has reached Earth’s surface during a volcanic eruption. What do you think of when you hear the word lava? Perhaps you picture hot, gooey liquid that flows easily. When lava cools and crystallizes, it becomes igneous rock. Lava cools quickly when it comes in contact with the cooler air. Make Flash Cards For each head in this lesson, write a question on one side of a flash card and the answer on the other side. Quiz yourself until you know all of the answers. Not all magma makes it to Earth’s surface. Large amounts of magma cool and crystallize beneath Earth’s surface. No cool air comes in contact with the magma. As a result, the process of cooling and crystallizing takes much longer. Igneous rock that formed beneath Earth’s surface looks much different from rock that formed from lava cooling on Earth’s surface. Over time, wind, rain, and other factors wear down rocks on Earth’s surface. Igneous rock that was once underground can become exposed on Earth’s surface. Extrusive Rocks There are two types of igneous rocks. When volcanic material erupts and cools and crystallizes on Earth’s surface, it forms a type of igneous rock called extrusive rock. Extrusive igneous rocks form as lava and ash become solid. Reading Essentials Key Concept Check 1. Explain How do igneous rocks form? Rocks 51 Fine-Grained Textures Lava that erupts onto Earth’s surface Use a sheet of paper to make a horizontal two-tab book. Collect information on extrusive and intrusive igneous rocks. cools rapidly. Rapid cooling keeps large crystals from growing. Some extrusive igneous rocks have fine-grained textures. Volcanic glass is rock that forms when lava cools too quickly to form crystals. Obsidian is an example of volcanic glass. It has a shiny, glasslike surface with small, fine grains. Holes in Extrusive Rocks Magma stored underground can Extrusive Intrusive Rocks Rocks Reading Check 2. Relate Why are there holes in some igneous rocks? Reading Check 3. Locate Where do Intrusive Rocks Igneous rocks that form as magma cools underground are called intrusive rocks. Magma cools slowly underground. As a result, large crystals form. The crystals are arranged randomly in intrusive rocks. These large crystals lock together like pieces of a jigsaw puzzle. The random arrangement and large size of the crystals are features of intrusive igneous rocks. Igneous Rock Identification As you learned, geologists use texture and composition to help identify all rocks. Geologists identify igneous rocks by the arrangement and size of the mineral crystals in the rock. Chemical composition can also help to identify igneous rocks. Texture Geologists decide whether an igneous rock is extrusive or intrusive by studying the rock’s texture. If the crystals are small or impossible to see with the unaided eye, the rock is extrusive. If all the crystals are large enough to see and have a texture that locks together, the rock is intrusive. Composition Geologists also study the mineral composition of igneous rocks. Igneous rocks are classified, in part, based on their silica content. Light-colored minerals contain greater amounts of silica. Dark-colored minerals contain less silica and greater amounts of elements such as magnesium and iron. You can sometimes estimate a rock’s mineral composition by its color. Lighter-colored rocks are similar to granite in mineral composition. Darker-colored rocks are similar to basalt in mineral content. 52 Rocks Reading Essentials Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. intrusive rocks form? contain dissolved gases. As magma moves slowly toward the surface, pressure decreases and the gases separate from the molten mixture. When gas-rich lava erupts from a volcano, the gases escape. This process is similar to carbon dioxide escaping when a carbonated beverage is opened. The rocks that form have holes left by the escaping gases. Pumice (PUH mus) is one of the extrusive igneous rocks that forms in this way. Types of Igneous Rocks The type of igneous rock that forms is determined by • magma composition, • the environment in which the lava or magma cools and crystallizes, and • the cooling rate. Granite is an intrusive igneous rock. It has a high silica content and cools slowly beneath Earth’s surface. Basalt is an extrusive igneous rock that has a low silica content. It forms as lava cools quickly on Earth’s surface. Common Igneous Rocks The table below lists common igneous rocks by their texture and composition. Notice that an extrusive igneous rock can have the same mineral composition as an intrusive igneous rock. Their compositions might be the same, but their textures are different. Key Concept Check 4. Contrast How are extrusive and intrusive rocks different? Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Common Igneous Rocks Main Rock-Forming Minerals Present Intrusive Texture (all crystals can be seen with the unaided eye) Extrusive Texture (some or no crystals can be seen with the unaided eye) quartz, feldspar, mica, amphibole granite rhyolite pyroxene, feldspar, mica, amphibole, some quartz diorite andesite olivine, pyroxene, feldspar, mica, amphibole, little or no quartz gabbro basalt Visual Check 5. Identify Which minerals are common to these igneous rocks? Reading Essentials Rocks 53 Mini Glossary extrusive rock: a type of igneous rock that forms when volcanic glass: an igneous rock that forms when lava cools material that erupts from a volcano cools and crystallizes on Earth’s surface too quickly to form crystals intrusive rock: an igneous rock that forms as magma cools underground 1. Review the terms and their definitions in the Mini Glossary. Using the terms, explain in one or two sentences why the crystals are larger in intrusive rocks than in extrusive rocks. 2. Classify each description as a feature of either extrusive rocks or intrusive rocks. Place a check mark in the correct column in the chart. Description Extrusive Rocks Intrusive Rocks (a) form underground Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. (b) coarse grains (c) form above ground (d) cool quickly (e) form from lava (f ) volcanic glass 3. Describe the texture of an intrusive rock. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? 54 Rocks Connect ED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources. END OF LESSON Reading Essentials