Photo/Illustration Credits: Cover: PHOTO 24/Getty Images; 2 Arnold Fisher/ Photo Researchers, Inc.; 3 Julie Woodhouse/Alamy; 4 (gypsum) Visuals Unlimited/Getty Images, (calcite) Julie Woodhouse/Alamy, (fluorite) Gary Retherford/Photo Researchers, Inc., (apatite) Mark Al Schneider/Photo Researchers, Inc., (talc) Ben Johnson/Photo Researchers, Inc.; 5 (feldspar) Visuals Unlimited/Corbis, (corundum) M. Claye/Jacana/ Photo Researchers, Inc., (quartz) Siede Preis/Photodisc/Getty Images, (gold) Charles D. Winters/Photo Researchers, Inc., (topaz) Astrid & Hanns Frieder-Michler/Photo Researchers, Inc., (diamond) Arnold Fisher/Photo Researchers, Inc.; 6 (granite) Joyce Photographics/Photo Researchers, Inc., (pumice) Alamy; 7 (feldspar) Visuals Unlimited/ Corbis, (basalt) Mark A. Schneider/Photo Researchers, Inc.; 8 Biophoto Associates/ Photo Researchers, Inc.; 9 George Bernard/Photo Researchers, Inc.; 10 Dirk Wiersma/ Photo Researchers, Inc.; 11 Siede Preis/Photodisc/Getty Images; 12 Corbis; 13 Corbis; 14 John Wang/Photodisc/Getty Images. If you have received these materials as examination copies free of charge, Harcourt School Publishers retains title to the materials and they may not be resold. Resale of examination copies is strictly prohibited and is illegal. Possession of this publication in print format does not entitle users to convert this publication, or any portion of it, into electronic format.
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Rocks are everywhere. Whether you walk up over a mountain or down into a valley, you are walking over rocks. Rocks are always under your feet, but sometimes they are above your head, too! People use rocks to build many of the bridges, buildings, statues, roads, and walls that you see every day.
You probably know that not all rocks are alike. Rocks are found in different sizes, shapes, and colors. But all rocks have one thing in common they are natural substances made up of one or more minerals. What is a mineral? A mineral is a naturally occurring, nonliving solid that has a specific chemical makeup and a repeating structure. Scientists call this repeating structure crystalline. You may already be familiar with several kinds of minerals. For example, you may know that salt is a mineral found in certain foods. You may also know that the mineral quartz is often used in watches and clocks, and that copper, a metal from a mineral ore, is used for electric wiring and pipes. Some minerals, such as diamonds and rubies, are gems, and are considered to be rare and valuable! These pictures illustrate just a few of the more than 2,000 different minerals identified on Earth.
Minerals are the building blocks of rocks. In fact, you can think of rocks as mineral mixtures. For this reason, the properties of a particular rock depend on the properties of the minerals that make up the rock. One property of a mineral is streak. Streak is the color of the powder left behind when you rub a mineral against a rough white tile, or streak plate. Usually the streak is the same color as the mineral. However, sometimes it is not. Streak is a useful property for mineral identification, because streak does not vary. For example, the mineral quartz is found in a variety of colors, but each color makes the same streak.
Luster describes the way a mineral s surface reflects light. Some minerals, such as pyrite and galena, have a metallic luster. They are shiny. Other minerals, such as talc and gypsum, have a nonmetallic luster. They appear dull or dark. Hardness is a mineral s ability to resist being scratched. Hardness is measured on the Mohs scale. The scale was developed by a German scientist named Frederick Mohs. In this scale, talc, the softest mineral, is classified as a 1. Diamond, the hardest mineral, is classified as a 10. Any mineral with a higher number on the scale can scratch any mineral with a lower number.
Scientists classify rocks based on how they form. There are three basic types. Rocks that form when melted rock cools and hardens are called igneous rocks. There are many different kinds of igneous rocks, containing different amounts and different kinds of minerals. For example, granite is an igneous rock made up of the minerals feldspar, quartz, and mica. Basalt is another igneous rock. It is made up mostly of the minerals feldspar and pyroxene. Different amounts and kinds of minerals are not the only differences in igneous rocks. Some igneous rocks are formed when melted rock cools and hardens below Earth s surface. Other igneous rocks are formed when melted rock flows out onto Earth s surface, where it cools and hardens very quickly.
You ve learned that minerals are substances with unique crystalline structures. You ve also learned that minerals make up rocks. When melted rock cools slowly, the crystals that make up the rock have time to grow. For this reason, igneous rocks that form underground have large crystals. An example of this type of rock is granite. When melted rock cools quickly, it hardens before mineral crystals can grow very large. As a result, igneous rocks that form above ground have small crystals. An example of an igneous rock that forms above ground, and therefore has small crystals, is basalt.
Some rocks form when little bits of rock in the bottoms of rivers, lakes, and oceans pile up over time. These bits of rock, called sediment, are pieces of larger rocks that have worn away over time. Sediment is carried away by wind and water and is set down, or deposited, in another place. The process by which sediment settles at the bottom of water or is dropped by wind is called deposition. Over a long time, layers of sediment can pile up, one layer over another. As the layers build up, the sediment at the bottom of the pile begins to change. Water dissolves some of the minerals in the sediment pile, forming a kind of cement. Cemented sediments form a type of rock called sedimentary rock.
There are different kinds of sedimentary rock. Some kinds form from large pieces of sediment, such as pebbles. Others contain small pieces of sediment, such as grains of sand. Some sedimentary rock forms mostly from one particular mineral or in one particular location. Sedimentary rock, like all rock, can be worn away by wind and water. Sediment from a sedimentary rock may be deposited in a different location or in a different way, forming a completely different sedimentary rock!
High heat and great pressure can change the texture of rock. A rock s texture is the way it looks and feels. When rock is changed in this way, the new rock is called metamorphic rock. Metamorphic rock can form from any other kind of rock igneous, sedimentary, or even other metamorphic rock. Some metamorphic rock is formed when mountains are built up on Earth. Suppose there is some granite somewhere under a huge mountain. The weight of the mountain squashes the rock. The pressure of the mountain also makes the rock very hot, since heat increases with pressure. The heat and pressure change the granite into a new kind of rock gneiss.
Metamorphic rock also forms on the floor of the ocean. Here, layers of sediment are piled up on top of one another to form sedimentary rock. In time, there will be enough pressure on the bottom layers of sedimentary rock to change them into metamorphic rock. Metamorphic rock also forms near some volcanoes. At volcanoes, melted rock rises above Earth s surface. You ve already learned how igneous rock forms from melted rock. Melted rock rising to Earth s surface sometimes melts rocks that have already formed, changing them into metamorphic rock.
Did you know that rocks are always changing? You might think that rocks never change. After all, a rock that you saw a year ago probably looks the same today. Rocks do change, but the changes usually happen so slowly that you don t notice them. Rocks at Earth s surface are exposed to wind, rain, heat, snow, and ice. These forces wear away rocks. They may cause rocks to crumble, crack, and break. The process of wearing away rocks by these natural processes is called weathering. You may have seen the effects of weathering on old tombstones or statues. Although all rocks on Earth s surface are weathered, not all rocks weather at the same rate. You ve learned that minerals have different degrees of hardness. Some minerals are soft and easily scratched, while others are hard and not easily scratched. Rocks that contain mostly hard minerals weather much more slowly than rocks that contain mostly soft minerals.
Weathered material or sediment, does not stay in one place. It is moved by erosion. Erosion is the process of moving sediment by wind, water, or ice. Erosion happens all around you. Rivers and streams carry sediment over great distances. Winds pick up and move great amounts of soil and sand. Large masses of ice, called glaciers, pick up sediment as they move over land.
You probably know of many cycles in nature. For example, there is the cycle of day and night, the cycle of the four seasons, and the cycle of the phases of the moon. Rocks also have a cycle of change and renewal. The rock cycle is the continuous process in which one type of rock changes into another type. The effects of weathering, erosion, deposition, heat, and pressure are all part of the rock cycle. As rocks move through the rock cycle, the materials that make them up are used over and over again. The steps of the rock cycle can take millions of years. As with all cycles, the rock cycle has no beginning or end. The diagram on the following page shows how the rock cycle works.
Melting Erosion and deposition Igneous rock Temperature & pressure Melting Sedimentary rock Metamorphic rock Erosion and deposition Temperature and pressure Rocks are made up of one or more minerals. Minerals are naturally occurring substances with a crystalline structure and unique properties of streak, luster, and hardness. Three types of rock are igneous, sedimentary, and metamorphic. Rocks change from one kind to another in the rock cycle. The processes that make up the rock cycle are weathering, erosion, deposition, heat, and pressure.
deposition (depuhzishshuhn) The process in which sediment settles at the bottom of water or is dropped by wind (8, 14, 15) erosion (eerohzhuhn) The process of moving sediment by wind, water, or ice (13, 14, 15) hardness (HARDnis) A mineral s ability to resist being scratched (5, 12, 15) igneous rock (IGneeuhs RAHKZ) Rocks that form when melted rock cools and hardens (6, 7, 10, 11, 15) luster (LUSter) The way a mineral s surface reflects light (5, 15) metamorphic rock (metuhmawrfik RAHKZ) Rock formed when high heat and great pressure change existing rock into a new form (10, 11, 15) mineral (MINeruhl) A naturally occurring, nonliving solid that has a specific chemical makeup and a repeating structure (2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15) rock cycle (RAHK CYkuhl) The continuous process in which one type of rock changes into another type (14, 15) sedimentary rock (seduhmentuhree RAHKZ) Rock formed when sediments are cemented together (8, 9, 11, 14, 15) streak (STREEK) The color of the powder left behind when you rub a mineral against a rough white tile, or streak plate (4, 15) weathering (WETHering) The process of wearing away rocks by natural processes (12, 13, 14, 15)
What is the rock cycle? How are igneous rocks formed? Describe the steps by which sedimentary rock may become igneous rock. Suppose you are given an unidentified mineral sample. What three properties could you evaluate to help you identify the mineral? Work with a small group of classmates to design another way to communicate the steps of the rock cycle. You might make a chart, write a list, create a song, or draw another diagram. Share your work with the class. Share with a family member what you have learned about different types of rocks. Discuss ways you use rocks at school, work, and home. Together, write a paragraph that describes the ways you use rocks each day.
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