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Download File - 6th Grade Science with Mrs. Harlow
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What is a Rock? Scientists define rock as a naturally occurring solid mixture of one or more minerals and organic matter. It may be hard to believe, but rocks are always changing. The continual process by which new rock forms from old rock material is called the rock cycle. The Value of Rock Rock has been an important natural resource as long as humans have existed. Early humans used rocks as hammers to make other tools. They discovered that they could make arrowheads, spear points, knives, and scrapers. Value of Rocks Rock has also been used for centuries to make buildings, monuments, and roads. Figure 1 shows how rock has been used as a construction material by both ancient and modern civilizations. Buildings have been made out of granite, limestone, marble, sandstone, slate, and other rocks. Question Which rock would be the weakest building material? A. B. C. D. Granite Sandstone Slate Marble Why? Processes That Shape the Earth Certain geological processes make and destroy rock. These processes shape the features of our planet. These processes also influence the type of rock that is found in a certain area of Earth’s surface. Weathering, Erosion, and Deposition The process in which water, wind, ice, and heat break down rock is called weathering. Weathering is important because it breaks down rock into fragments. These rock and mineral fragments are the sediment of which much sedimentary rock is made. Process of Weathering The process by which sediment is removed from its source is called erosion. Water, wind, ice, and gravity can erode and move sediments and cause them to collect. Figure 2 shows an example of the way land looks after weathering and erosion. Bryce Canyon in Utah Can you think of other landmarks that were shaped by weathering & erosion? More Weathering The process in which sediment moved by erosion is dropped and comes to rest is called deposition. Sediment is deposited in bodies of water and other low-lying areas. In those places, sediment may be pressed and cemented together by minerals dissolved in water to form sedimentary rock. W. E. D. Heat and Pressure Sedimentary rock made of sediment can also form when buried sediment is squeezed by the weight of overlying layers of sediment. If the temperature and pressure are high enough at the bottom of the sediment, the rock can change into metamorphic rock. In some cases, the rock gets hot enough to melt. This melting creates the magma that eventually cools to form igneous rock. How the Cycle Continues Buried rock is exposed at the Earth’s surface by a combination of uplift and erosion. Uplift is movement within the Earth that causes rocks inside the Earth to be moved to the Earth’s surface. When uplifted rock reaches the Earth’s surface, weathering, erosion, and deposition begin. Question A rock at the Earth’s surface is primarily affected by forces of A. B. C. D. Heat and pressure Pressure only Weathering and Erosion Cooling Question A rock deep underground is primarily affected by forces of A. B. C. D. Extreme heat and pressure Cooling Weathering and Erosion Heat only Illustrating the Rock Cycle You have learned about various geological processes, such as weathering, erosion, heat, and pressure, that create and destroy rock. The diagram on the next slide illustrates one way that sand grains can change as different geological processes act on them. In the following steps, you will see how these processes change the original sand grains into sedimentary rock, metamorphic rock, and igneous rock. Round and Round It Goes You have seen how different geological processes can change rock. Each rock type can change into one of the three types of rock. For example, igneous rock can change into sedimentary rock, metamorphic rock, or even back into igneous rock. This cycle, in which rock is changed by geological processes into different types of rock, is known as the rock cycle. Hmm… What would form? Sediment + Compaction & Cementation =________ Metamorphic Rock + heat=_______________ Sedimentary Rock + heat & pressure =_________ Igneous Rock + heat=__________________ Rock Classification You have already learned that scientists divide all rock into three main classes based on how the rock formed: igneous, sedimentary, and metamorphic. But did you know that each class of rock can be divided further? Rock Classification These divisions are also based on differences in the way rocks form. For example, all igneous rock forms when magma cools and solidifies. But some igneous rocks form when magma cools on the Earth’s surface, and others form when magma cools deep beneath the surface. Composition The minerals a rock contains determine the composition of that rock, as shown in Figure 4. For example, a rock made of mostly the mineral quartz will have a composition very similar to that of quartz. A rock made of 50% quartz and 50% feldspar will have a very different composition than quartz does. Texture The size, shape, and positions of the grains that make up a rock determine a rock’s texture. Sedimentary rock can have a fine-grained, mediumgrained, or coarse-grained texture, depending on the size of the grains that make up the rock. The texture of igneous rock can be fine-grained or coarsegrained, depending on how much time magma has to cool. Based on the degree of temperature and pressure a rock is exposed to, metamorphic rock can also have a finegrained or coarse-grained texture. Sedimentary Rock Texture Think about it! Scientists classify rocks using what important criteria? A. B. C. D. Composition & texture The depth at which they form Elevation The pressure under which they formed Clues on how the Rock Formed Think about it! What factors can affect the texture of an igneous rock? A. The length of time the magma had to cool B. The size of the rock C. The minerals that cement the rock together D. The pressure and temperature the rock was exposed to
