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Characteristics of Igneous Rocks Earth is made of thousands of types of rocks, which have been made by a number of different of processes and are used for countless purposes. The world of rocks is comprised of an amazing variety of textures, colors, and patterns. Some rocks are smooth and have a uniform color, while others are rough, speckled, and multi-colored. All rock is made of different combinations of minerals, inorganic materials found in nature. The mineral composition of a rock determines many of its properties. The process of rock formation also influences the rock’s physical properties. During the rock cycle, different conditions determine how the rock forms. Sedimentary rock is formed when Rocks come in an amazing sediment is layered and pressed together over variety of textures and colors. time. Metamorphic rock is formed when rock is subjected to intense heat and pressure under Earth’s surface. Igneous rock is formed when molten rock, magma, or lava cools and hardens. Intrusive and Extrusive Igneous Rock There are two main types of igneous rock. Intrusive igneous rock, also known as plutonic igneous rock, forms beneath Earth’s surface. As magma deep inside Earth moves closer to the surface, it cools. The cooling process occurs very slowly because the temperatures under Earth’s surface are still very high. The magma eventually solidifies. As this happens, the minerals that make up the magma crystallize. Since under Granite’s texture is coarse because its crystals most conditions magma takes a long developed slowly, over a long period of time. time to solidify, the crystals have a long time to grow. This process gives intrusive igneous rock a coarse texture with large, visible crystals. Granite, with its speckled appearance, is a common example of intrusive igneous rock. Discovery Education Science © Discovery Communications, LLC Characteristics of Igneous Rocks Extrusive igneous rock, also known as volcanic rock, forms when magma rises slowly or erupts through Earth’s crust. Once above Earth’s surface, the molten rock is called lava. Since temperatures are much lower on Earth’s surface than deep beneath it, lava cools and hardens very quickly. There is not much time for the minerals in the lava to form large crystals. As a result, the texture of extrusive igneous rock has a very fine grain. Extrusive igneous rock may be rough like pumice or smooth and shiny like obsidian. Obsidian owes its smooth texture to the short time in which it cooled. Igneous Rock Shapes Earth’s Surface Extrusive igneous rock plays a large part in shaping and changing Earth’s surface. For example, a large percentage of the land making up the Hawaiian Islands is volcanic rock. Repeated volcanic eruptions have caused ocean-floor mountains to rise above the surface of the ocean, forming the chain of islands. Evidence of Hawaii’s volcanic beginnings can be observed at its many beaches. Eroded volcanic rock gives Hawaii’s beaches various colors. Some beaches are covered by black or green sand made from weathered and eroded volcanic rock. Other dramatic landforms are made of intrusive igneous rock. Devil’s Tower National Monument in Wyoming is made of a type of igneous rock called phonolite porphyry. The term "porphyry" describes the texture of the rock in Devil’s Tower. Igneous rocks with a porphyritic texture have both larger and smaller crystals. Geologists believe that this variation has occurred because the magma that solidified into Devil’s Tower began to cool and Devil’s Tower National Monument in crystallize deep under Earth’s surface. The Wyoming is made of igneous rock. larger crystals formed during this stage. As the partially solidified magma moved closer to Earth’s surface, it cooled quickly. This quick cooling caused the smaller crystals to form. Discovery Education Science © Discovery Communications, LLC