CHAPTER 6 SECTION 2 Rocks Igneous Rock KEY IDEAS As you read this section, keep these questions in mind: What three factors affect whether rock melts? How does the cooling rate of magma or lava affect the texture of igneous rocks? What are the possible textures and compositions of igneous rock? What structures can igneous rock form? How Does Magma Form? Remember that igneous rock forms when melted rock (magma or lava) cools and hardens. Magma forms when rock is heated. The heat makes the minerals in the rock melt. However, not all rocks melt at the same temperature. There are three main factors that affect whether a rock will melt at a certain temperature. Those factors are the chemical composition of the rock the pressure on the rock whether there are any fluids in the rock Imagine mixing sugar crystals and ice together in a pan on a hot stove. The ice would melt at a much lower temperature than the sugar would. Just as ice and sugar melt at different temperatures, different minerals melt at different temperatures. The chemical composition of a mineral affects the temperature at which it melts. Therefore, the minerals that make up a rock affect how hot the rock has to become before it will melt. The pressure on a rock also affects whether it will melt. In general, the lower the pressure on a rock is, the lower the temperature the rock will melt at. Therefore, lowering the pressure on a hot rock can make it melt. A third factor that affects whether a rock melts is whether there are any fluids in the rock. Fluids cause a rock to melt at a lower temperature. Therefore, adding fluid to a rock can make the rock melt. Also, rocks that contain fluids tend to melt at lower temperatures than rocks that do not contain fluids. READING TOOLBOX Identify Main Ideas As you read this section, underline the main idea in each paragraph. When you finish reading, write a summary of the section using the underlined ideas. 1. List What are the three factors that affect whether a rock will melt at a certain temperature? 2. Describe What may happen to a hot rock if the pressure on it decreases? Holt McDougal Earth Science 73 Rocks
PARTIAL MELTING Different minerals melt at different temperatures. As the rock heats up, the minerals with the lowest melting temperatures will melt first. The higher the temperature becomes, the more minerals will melt. As each mineral in the rock melts, the elements in it enter the magma. As a result, the composition of the magma changes. The process in which different minerals in a rock melt at different temperatures is called partial melting. LOOKING CLOSER 3. Explain What causes the composition of the magma to change? Mineral Magma A solid rock is made up of several different minerals. In this picture, each mineral is a different color. When the rock begins to heat up, some of the minerals melt. As the temperature rises, more minerals melt. As the minerals melt, the composition of the magma changes. Critical Thinking 4. Infer A scientist is studying the rock that formed when a magma chamber cooled completely. Where in the rock will the scientist probably find minerals that form early in Bowen s reaction series? FRACTIONAL CRYSTALLIZATION Remember that Bowen s reaction series describes the order in which minerals form from magma. As magma cools, different minerals crystallize. The minerals that were the last to melt generally crystallize first. The minerals that melted first crystallize last. The formation of different minerals as magma cools is called fractional crystallization. When magma cools, it does not always form the same minerals that originally melted to form the magma. The elements in the magma can combine in new ways to form new minerals. Most magma is found underground in large holes called magma chambers. The crystals that form at the beginning of fractional crystallization can sink to the bottom of the magma chamber. They can also stick to the sides or top of the magma chamber. Crystals take time to grow. The slower the magma in the chamber cools, the larger the crystals that grow from it are. Because the composition of the magma changes as it cools, the composition of the crystal might change as it grows. A crystal that forms this way will have a different composition on its outside than on its inside. Holt McDougal Earth Science 74 Rocks
What Affects the Texture of an Igneous Rock? Many igneous rocks contain mineral crystals. The texture of an igneous rock is a description of the size of the crystals in it. The main factor that affects the texture of an igneous rock is how quickly it forms. Remember that melted rock below Earth s surface is called magma, and melted rock above Earth s surface is called lava. Igneous rock can form from either magma or lava. If an igneous rock forms from magma, which cools underground, it is an intrusive igneous rock. If an igneous rock forms from lava, which cools on Earth s surface, it is an extrusive igneous rock. Intrusive and extrusive igneous rocks generally look very different. Because magma cools slowly, large crystals can form. Therefore, intrusive igneous rocks generally contain large mineral crystals. Igneous rocks that contain large crystals have a coarse-grained texture. Granite and gabbro are examples of coarse-grained igneous rocks. In contrast, lava generally cools very quickly. Only small crystals can form. Therefore, extrusive igneous rocks generally contain only very small mineral crystals. Igneous rocks that contain small crystals have a fine-grained texture. Rhyolite and basalt are examples of fine-grained igneous rocks. Extrusive igneous rocks may also have porphyritic, glassy, or vesicular textures. The table below describes each of these kinds of textures. 5. Identify What is the main factor that affects the texture of an igneous rock? 6. Explain Why do intrusive igneous rocks generally contain large mineral crystals? Texture Description How It Forms Example Porphyritic Glassy Vesicular some large mineral crystals mixed with many small crystals looks like glass; contains no crystals fine-grained or glassy rock with tiny holes in it Magma begins to cool slowly, and some large crystals form. The magma erupts and finishes cooling quickly, producing small crystals. Lava cools so quickly that crystals cannot form. Gas in lava bubbles out as it cools, like the bubbles in a soda. As the lava hardens, the bubbles are frozen in place. some kinds of granite and basalt obsidian pumice Critical Thinking 7. Apply Concepts Do the minerals that form the large crystals in porphyritic rocks probably form at high temperatures or low temperatures? Holt McDougal Earth Science 75 Rocks
8. Identify What are the three groups of igneous rock compositions? Talk About It Make Connections With a partner, discuss the meaning of the word intermediate. If you wish, look it up in a dictionary. Together, talk about why some igneous rocks are said to have an intermediate composition. LOOKING CLOSER 9. Compare Which type of rock has the largest amount of silica: diorite, gabbro, or granite? What Compositions Can Igneous Rock Have? Different igneous rocks contain different minerals. The chemical composition of the magma or lava affects the minerals that form. Scientists have defined three main groups of igneous rock compositions: felsic, mafic, and intermediate. The main difference between the three types of rock is the amount of silica they contain. Silica refers to the ratio of silicon-oxygen tetrahedrons to other elements in a mineral. Minerals that are high in silica have large ratios of silicon-oxygen tetrahedrons to other elements. Minerals that are low in silica have low ratios of siliconoxygen tetrahedrons to other elements. Felsic igneous rocks contain large amounts of silica. Mafic igneous rocks contain smaller amounts of silica. They also contain large amounts of iron and magnesium. Intermediate igneous rocks contain less silica than felsic rocks, but more silica than mafic rocks. The table below summarizes the three main igneous rock compositions. Composition Description Common Minerals Felsic Mafic Intermediate high in silica; generally light-colored low in silica; high in iron and magnesium; generally dark-colored higher in silica than mafic rocks; lower in silica than felsic rocks; may be lightcolored or darkcolored quartz, some kinds of feldspar and mica olivine, some kinds of feldspar, amphibole some kinds of feldspar and mica, pyroxene, amphibole Examples granite, rhyolite, obsidian, pumice basalt, gabbro andesite, diorite What Structures Can Intrusive Rock Form? Remember that intrusive igneous rocks form when magma cools underground. Magma can move through cracks and gaps in rock underground. Therefore, intrusive igneous rocks can be found in many different places. A body of intrusive igneous rock is called an intrusion. Batholiths, stocks, laccoliths, sills, dikes, and volcanic necks are examples of intrusions. Holt McDougal Earth Science 76 Rocks
A dike forms when magma cuts across several rock layers. A laccolith forms when magma flows between two rocks layers and pushes one of them up to form a dome. Volcanic necks form when magma inside a volcano cools and hardens. Talk About It Use Logic The word root lith comes from a Greek word that means rock. With a partner, discuss the words you have learned so far that include the root lith. Talk about why that word root is appropriate for each of the words. A sill forms when magma flows between two rock layers. Dike A stock is simialar to a batholith, but has an area less than 100 km 2. A batholith is an intrusion that has an area greater than 100 km 2. LOOKING CLOSER 10. Describe What is the main difference between a stock and a batholith? What Structures Can Extrusive Rock Form? A body of extrusive igneous rock is called an extrusion. The most common kind of extrusion is a lava flow. A lava flow forms when lava moves out of a volcano, cools, and hardens. The lava flow forms flat masses of rock. In some areas, a volcano has erupted many times. The lava flows have built up to form a lava plateau. A lava plateau is made up of many thick layers of extrusive igneous rock. Some volcanoes do not give off large amounts of lava. Instead, they mainly give off volcanic ash and dust. The ash and dust can build up to form a type of rock called tuff. Because ash and dust are light, wind can carry them long distances. They can form thick layers of tuff that cover very large areas. These layers of tuff are called tuff deposits. 11. Describe How does a lava flow form? Holt McDougal Earth Science 77 Rocks
Section 2 Review SECTION VOCABULARY extrusive igneous rock rock that forms from the cooling and solidification of lava at Earth s surface felsic describes magma or igneous rock that is rich in feldspars and silica and that is generally light in color igneous rock rock that forms when magma cools and solidifies intrusive igneous rock rock formed from the cooling and solidification of magma beneath Earth s surface mafic describes magma or igneous rock that is rich in magnesium and iron and that is generally dark in color 1. Describe Complete the concept map below to describe how scientists classify igneous rocks. Use the terms composition, texture, fine grained, coarse grained, mafic, felsic, and intermediate. can be Igneous rocks classified by can be 2. Apply Concepts Which will probably melt at the lower temperature, a rock that contains fluids and is under low pressure or a rock that does not contain fluids and is under high pressure? Explain your answer. 3. Compare Describe two differences between intrusive igneous rocks and extrusive igneous rocks. 4. Infer If there were a laccolith below the ground in an area, what might the area look like? Explain your answer. Holt McDougal Earth Science 78 Rocks