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8 Igneous Rocks Karen Tefend 8.1 Introduction All rocks found on the Earth are classified into one of three groups: igneous, sedimentary, or...
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Sir the PDF here has the info with the reference for the rocks on

page 184.  Let me know if you have any questions.  Thank you

Page | 177 8.1 INTRODUCTION All rocks found on the Earth are classiFed into one of three groups: igneous, sed - imentary, or metamorphic. This rock classiFcation is based on the origin of each of these rock types, or if you prefer, based on the rock-forming process that formed the rock. The focus of this chapter will be on igneous rocks, which are the only rocks that form from what was once a molten or liquid state. Therefore, based on their mode of origin, igneous rocks are deFned as those rock types that form by the cooling of magma or lava. You would be right in thinking that there is more to the classiFcation of igneous rocks than stated in the previous sentence, as there are dozens of di±erent igneous rocks that are considered commonplace, and dozens of more types that are less common, and also quite a few igneous rock types that are quite scarce, yet each igneous rock has a name that distinguishes it from all the rest of the igneous group of rocks. So, if they all start out as molten material (magma or lava), which must harden to form a rock, then it is logical to assume that these igneous rocks di±er from one an - other primarily due to: 1) the original composition of the molten material from which the rock is derived, and 2) the cooling process of the molten material that ended up forming the rock. These two parameters deFne the classiFcation of igneous rocks, which are simpliFed into the two terms: composition and texture. Igneous rock com - position refers to what is in the rock (the chemical composition or the minerals that are present), and the word texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock. 8.1.1 Learning Outcomes After completing this chapter, you should be able to: • Classify igneous rock types based on color, texture, and maFc color index Identify, when possible, the minerals present in an igneous rock Determine the cooling history of the igneous rock 8 Igneous Rocks Karen Tefend
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Page | 178 INTRODUCTORY GEOLOGY IGNEOUS ROCKS 8.1.2 Key Terms 8.2 IGNEOUS ROCK ORIGIN 8.2.1 Magma Composition It seems like a bad joke, but before any igneous rock can form, there must be molten material known as magma produced, which means that you must frst have a rock to melt to make magma in order for it to cool and become an igneous rock. Which brings more questions: what rock melted to Form the magma? Was there more than one rock type that melted to form that magma? Did the rocks complete- ly melt, or did only certain minerals inside oF those rocks melt (a process known as partial melting)? Once that melted material formed, what happened to it next? Did some other process occur to change the composition of that magma, before ending up as the igneous rock that we are studying? These are just a few of the questions that a person should consider when studying the origin of igneous rocks. Most rocks (there are very Few exceptions!) contain minerals that are crystal - line solids composed of the chemical elements. In your chapter on minerals you learned that the most common minerals belong to a group known as the silicate minerals, so it makes sense that magmas form from the melting of rocks that most likely contain abundant silicate minerals. However all minerals (not just the sil - icates) have a certain set of conditions, such as temperature, at which they can melt. Since rocks contain a mixture of minerals it is easy to see how only some of the minerals in a rock may melt, and why others stay as a solid. Furthermore, the temperature conditions are important, as only minerals that can melt at “lower” temperatures (such as 600 ° C) may experience melting, whereas the temperature would have to increase (For example, to 1200 ° C) in order for other minerals to also melt (remember the lower temperature minerals are still melting) and thus add their chemical components to the magma that is being generated. This brings up an important point: even if the same types of rocks are melting, we can generate di±erent magma compositions purely by melting at di±erent temperatures! Once magma is generated, it will eventually start to rise upward through the Earth’s lithosphere, as magma is more buoyant than the source rock that generat- Aphanitic Extrusive • ²elsic (Silicic) Ferromagnesian Glassy Intermediate Intrusive • Mafc Phaneritic Phenocryst Plutonic Porphyritic • Ultramafc Vesicular Volcanic
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