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INSTRUCT Forming and Transporting Rock Particles A sandy ocean beach, a gravel bar in a river, and a muddy lake bottom all consist mainly of rock particles. These particles were broken away from rocks by the action of water or wind or a combination of both. Such particles may vary in size from boulders to sand to tiny bits of clay. Teach from Visuals To help students interpret the visual showing sediment particles, ask: • As water currents slow down, what size of particles settles first? the ? A largest, gravel • What size of particles settles next? the next smallest, sand, and then silt and clay RESOURCE CENTER CLASSZONE.COM • What does the type of sedimentary rock that forms depend on? the size Find information on sedimentary rocks. Just as water washes mud off your hands as it runs over them, rainwater washes away rock particles as it flows downhill. The water carries these rock particles to streams and rivers, which eventually empty into lakes or oceans. Strong winds also pick up sand and rock dust and carry them to distant places. As winds or water currents slow down, rock particles settle on the land or at the bottom of rivers, lakes, and oceans. The sediments form layers as larger particles settle first, followed by smaller ones. of rock particles from which it forms Forming Loose Sediments into Rocks If you have ever watched workers building a road, you know that they first put down layers of gravel and other materials. Then they press the layers together, using a huge roller. In a similar way, layers of sediments Integrate the Sciences The faster a current flows, the larger the particles it can carry. Gravity works against the ability of water to carry particles. A basic law of physics states that all particles, no matter what their size, fall to Earth at the same speed in a vacuum. But in a fluid such as water, larger particles settle faster than smaller ones. Density is also a factor; however, the most common minerals in sediments have similar densities. Sorting Sediments by Size Fast-moving water can move large particles of sediment. As the water slows, the sediment particles settle from it by size. 1 Water in a lake usually moves fastest near the shore or where a river enters. In deeper areas, water moves slower. gravel sand silt and clay Ongoing Assessment Identify the types of materials that make up sedimentary rocks. 2 Ask: How does most sedimentary rock develop? Sedimentary rock develops as Gravel settles near the shore. Rock containing large sediment particles, such as gravel, is known as conglomerate. layers of sediments (rock particles, minerals, and plant and animal remains) build up on land or underwater. 3 4 Sand is carried farther from shore. Rock that forms from sand-sized particles is known as sandstone. Is shale more likely to form near the shore or near the middle of a big lake or ocean? Answer: in the middle, because silt and clay particles can settle only from very slow-moving water A 90 Unit: Earth’s Surface DIFFERENTIATE INSTRUCTION ? More Reading Support A How do rock particles get into streams and rivers? Rainwater washes away rock particles as it flows downhill, and it carries these particles into streams and rivers. A 90 Unit: Earth’s Surface Silt and clay are carried into deep water. Rock that forms from silt- and clay-sized particles is known as shale. English Learners Point out the introductory clauses beginning with If on pp. 90–91 and p. 93. Tell English learners that these clauses require them to recall something or imagine a hypothetical situation. Point out that while the clauses beginning with If are fragments, the main clauses that follow contain the subject and are complete sentences. English learners might also lack prior knowledge about the white cliffs of Dover, mentioned on p. 92. Integrate the Sciences Both geologists and biologists have used fossils to learn about Earth’s history. The British naturalist Charles Darwin based his theory of evolution partly on his study of fossil organisms and his understanding of Earth’s geologic history. He also observed animals and plants in their native environments. composed of rock particles may get pressed together to form rock. One layer gets buried by another, and then another. The overlying layers apply pressure to, or press down on, the sediments underneath. Small particles of sediment, such as silt and clay, may be formed into rock by pressure alone. In other sedimentary rocks the particles are held together by minerals that have crystallized between them, acting as cement. Over a long time, these processes transform loose sediments into sedimentary rocks. Check Your Reading What are two processes that can change sediments into rocks? Teach from Visuals To help students interpret the photographs of coal: Some rocks form from plants or shells. Processes similar to the ones that produce sedimentary rocks from rock particles also produce rocks from shells or plant remains. These remains are fossils. A fossil is the remains or trace of an organism from long ago. MAIN IDEA WEB • Ask: What kind of plant appears to be preserved in the sample of coal? fern Add a web to your notebook for each main idea. • Tell students this coal formed in an ancient swamp. Ask: Does the land in the photograph appear to be a swamp? no Coal If you look at a piece of coal through a magnifying glass, you may be able to make out the shapes of bits of wood or leaves. That is because coal is made up of remains of plants—dead wood, bark, leaves, stems, and roots. Coal is an unusual sedimentary rock because it forms from plants instead of earlier rocks. ? B • Ask: What does this tell you about the area? The environment has changed over time. The coal people use today started forming millions of years ago in swamps. As plants died, their remains fell upon the remains of earlier plants. Then layers of other sediments buried the layers of plant remains. The weight of the sediments above pressed the plant material into coal. The dark layer in these rocks is coal. Ongoing Assessment Answer: Sediments can be put under pressure or cemented together by new minerals to form rock. Here, you can see fossils of ancient plants preserved in coal. Chapter 3: Rocks 91 A DIFFERENTIATE INSTRUCTION ? More Reading Support B How is a rock like coal formed? Plant remains are buried under layers of sediment. The weight of the sediments above presses the plant material into coal. Below Level Have students draw a diagram of the process by which coal forms. The diagram should show layers of plant remains, with the layers becoming more and more compacted as the depth increases. Chapter 3 91 A Teach from Visuals To help students interpret the photographs showing limestone formation, ask: Limestone Limestone is made up of carbonate minerals, such as calcite. The shells and skeletons of ocean organisms are formed of these minerals. When the organisms die, the shells and skeletons settle on the ocean floor as layers of sediment. Over time, the layers become buried, pressed together, and cemented to form limestone. The photographs below show how loose shells can become limestone. • In the three photographs, what overall change do you see in the rock? The shells become harder to see. • What caused this change? The minerals in the shells dissolved and re-formed to produce limestone. • How does the limestone in the coral reef pictured at the bottom of the page differ from the limestone in the top photographs? The limestone in the coral reefs is added directly to the reef. It is not loose sediment first. These shells were made by ocean organisms. Integrate the Sciences Limestone is a biochemical sedimentary rock. Many marine organisms take in dissolved calcium carbonate from seawater and then secrete it to form their shells. Coral organisms, called polyps, do the same thing. Most coral polyps live in colonies, and the ones that form coral reefs attach themselves to one another. Coral reefs contain the skeletons of millions of coral polyps. reading tip Notice that limestone made up of cemented shells and the limestone in coral reefs were both formed by ocean organisms. ? C 1 2 3 The shells get cemented together into limestone as some of their minerals dissolve and re-form. Individual shells become harder to see as minerals in the limestone continue to dissolve and re-form. Over time, what was once loose sediment becomes limestone with no recognizable shells. The famous white cliffs of Dover, England, consist of a type of limestone called chalk. The limestone began to form millions of years ago, when the land was under the ocean. The rock developed from shells of tiny organisms that float in the ocean. Most limestone comes from shells and skeletons of ocean organisms. The materials the organisms use to build their shells and skeletons are present in ocean water because they were dissolved from earlier rocks. Like almost all sedimentary rock, limestone forms from material that came from older rocks. Coral reefs also consist of limestone that comes from organisms. However, in the case of reefs, the limestone is produced directly as coral organisms build their skeletons one on top of another. In the formation of coral, the rock does not go through a loose-sediment stage. EXPLORE Revisit “How Can Rocks Disappear?” on p. 73. Have students relate their results to the formation of limestone. The skeletons of these tiny coral organisms eventually make huge coral reefs. A 92 Unit: Earth’s Surface DIFFERENTIATE INSTRUCTION ? More Reading Support C Where does most limestone come from? the shells and skeletons of ocean organisms Advanced Have students explain how the formation of limestone demonstrates the rock cycle. Minerals from existing rocks dissolve in rainwater and are carried by rivers to the oceans. Ocean organisms use dissolved carbonate minerals to form their shells. When the organisms die, their shells build up on the ocean floor and form layers of sediment. The layers get buried and then pressed and cemented together to form limestone. R A 92 Unit: Earth’s Surface Challenge and Extension, p. 175 Teach from Visuals To help students interpret the photographs of limestone formations, ask: Some rocks form when dissolved minerals re-form from water. • How do the stalactites pictured in the top right photograph form? Water that If you have grown crystals in a container, you know that some substances can dissolve in water and then re-form as the water evaporates. The same process happens in nature. Some sedimentary rocks are made up of minerals that crystallized as water dried up. ? D drips from the roof of a cave leaves behind minerals to form stalactites. • What do the towers in the photograph at the bottom of the page have in common with these stalactites? They The water in oceans, lakes, rivers, and streams contains minerals that came from rocks. Some of these minerals are in solid form. As rainwater washes over rocks, it picks up pieces of minerals and rock particles and carries them into streams and rivers, where many of them settle to the bottom. However, some of the minerals dissolve in the water and are carried along with it. both consist of limestone that had been dissolved in water. Teach Difficult Concepts Water is shaping this limestone cavern. Water dissolves and transports minerals, then leaves the minerals behind as it evaporates. Students may not understand how the limestone towers in Mono Lake formed. Streams from the Eastern Sierras flow into Mono Lake, carrying salts and dissolved minerals. As a result, the lake water contains large amounts of carbonates. The lake is also fed by underground springs that carry calcium. In the lake, the calcium combines with carbonate, and the compound precipitates out as limestone. Over a long period of time, the limestone builds up into underwater towers around the mouths of the springs. Water often flows through cracks in rock that is near Earth’s surface. As water moves through limestone, some of the rock dissolves. A large open space, or cave, can be left in the rock. As the water flows and drips through the cave, some of it evaporates. The new limestone that forms can take many odd and beautiful shapes. Sometimes minerals crystallize along the edges of lakes and oceans where the climate is dry and a lot of water evaporates quickly. Over time, the minerals build up and form layers of sedimentary rock. Rock salt and gypsum form in this way. Under the city of Detroit, for example, is a large bed of rock salt that developed when part of an ancient ocean dried up. check your reading How are the origins of rock salt and some limestone similar? These limestone towers in Mono Lake, California, formed underwater. They are now above the surface because the lake level has dropped. Real World Example The deposits that form on a water faucet are an everyday example of minerals precipitating out of water. Ongoing Assessment Describe the three groups of sedimentary rocks. Ask: How do the three groups of sedimentary rocks form? from rock particles Chapter 3: Rocks 93 A DIFFERENTIATE INSTRUCTION ? More Reading Support D Where do the minerals dissolved in water come from? from rainwater washing over rocks Alternative Assessment Have students draw and label diagrams showing the three ways sedimentary rock is formed: rock particles that build up and get pressed together, shells or plant remains that build up and get pressed together, dissolved minerals that re-form from water. that build up in layers and get pressed together; from shells or plant remains that build up in layers and get pressed together; from dissolved minerals that re-form from water Answer: Both can dissolve in water, be carried to a new location, and re-form as the water evaporates. Chapter 3 93 A INVESTIGATE Rock Layers PURPOSE Make a model to show how sedimentary rocks form in layers Rock Layers TIP 20 min. Divide the class into small groups for this activity. How do sedimentary rocks form in layers? SKILL FOCUS Modeling PROCEDURE WHAT DO YOU THINK? Layers of sediment are deposited at different times and contain sediments of different sizes. Layers of sediment made up mainly of pebbles or gravel would be deposited when water is moving very quickly, such as during a flood or where a mountain river spreads out and water slows down. The middle layer of sand-size sediment would be deposited when water has slowed down but is still moving fairly quickly. The plaster of Paris is similar to minerals that grow between the particles of sediment and cement them together. 1 Prepare the plaster of Paris by mixing it with the water. 4 Mix the rest of the gravel with the rest of the plaster of Paris. Add the mixture to the paper cup, on top of the sand mixture. 2 Mix 2 tablespoons of the gravel with 2 tablespoons of the plaster of Paris and pour the mixture into the paper cup. 5 After the mixtures harden for 3 Mix the sand with 2 tablespoons about 5 minutes, tear apart the paper cup and observe the layers. of the plaster of Paris and the food coloring. Add the mixture to the paper cup, on top of the gravel mixture. MATERIALS • 1 paper cup • 3 mixing cups • 6 tbs plaster of Paris • 3 tbs water • 4 tbs gravel • 2 tbs sand • 3 drops food coloring TIME 20 minutes WHAT DO YOU THINK? • How is the procedure you used to make your model similar to the way sedimentary rock forms? • Describe how similar layers of real rock could form. CHALLENGE A model of how limeCHALLENGE How would you create stone forms could be made in a similar way, using shells in place of sand or pebbles. R a model to show the formation of fossil-rich limestone? Datasheet, Rock Layers, p. 176 Sedimentary rocks show the action of wind and water. Technology Resources Customize this student lab as needed or look for an alternative. Print rubrics to assess student lab reports. reading tip Notice that sedimentary rocks are laid down in layers. As conditions in an area change, so do the characteristics of the layers. Lab Generator CD-ROM ? E Sedimentary rocks are laid down in layers, with the oldest layers on the bottom. A geologist studying layers of sedimentary rocks can tell something about what conditions were like in the past. For instance, fossils of fish or shells in a layer of rock show that the area was covered by a lake or an ocean long ago. Fossils are not the only way to tell something about what past conditions were like. The sediments themselves contain a great deal of information. For example, a layer of sedimentary rock may contain sediment particles of different sizes. The largest particles are at the very bottom of the layer. Particles higher in the layer become increasingly smaller. A layer like this shows that the water carrying the sediment was slowing down. The largest particles dropped out when the water was moving quickly. Then smaller and smaller particles dropped out Metacognitive Strategy Ask students to write a few sentences at the end of their lab reports describing how making this model helped them understand the process of sedimentary rock formation. A 94 Unit: Earth’s Surface Teaching with Technology If you have access to a video camera, you might tape groups of students showing the models they created and explaining the answers to the Investigate questions. You could show the videotape at an open house or play it during the wait time at parent conferences. A 94 Unit: Earth’s Surface DIFFERENTIATE INSTRUCTION ? More Reading Support E What do fossils of fish in a layer of rock indicate? that the area was covered by a lake or ocean long ago Below Level After completing the Investigate in roughly 20 minutes, some students may think that actual sedimentary rock layers are also formed over short time periods. Stress that the layers of rock are formed over thousands to millions of years and continue to change. Ongoing Assessment Crossbeds Ripples Explain how sedimentary rocks record past conditions. Mud Cracks Ask: What can geologists learn about past conditions by examining dunes, bars, and ripples? the direction that wind or water currents were traveling and carrying sediments The tilted layers in these sandstone rocks are called crossbeds. The layers were once moving sand dunes. The surface of this sandstone preserves ancient sand ripples. Answer: Ripples preserved in rocks show that sand was once moving along that surface. Mud cracks show that an area was once wet and then became very dry. As wet silt and clay dry out, cracks develop on the surface of the sediment. as the water slowed. This type of layer is often created by a flood, when a large amount of water is at first moving quickly. Reinforce Sedimentary rocks can give information about the directions in which long-ago wind or water currents were traveling when sediments settled from them. Sand can be laid down in tilted layers on the slopes of sand dunes or sandbars. Sand can also form ripples as water or wind moves over its surface. If the sand has been buried and cemented into sandstone, a geologist can examine it and tell the direction in which the water or wind was moving. Have students relate the section to the Big Idea. R Reinforcing Key Concepts, p. 177 ASSESS & RETEACH Some rocks made of clay or silt have cracks that developed when the mud from which they formed dried out. Mud cracks show that the rocks formed in areas where wet periods were followed by dry periods. check your reading What could a geologist learn by finding rocks that have ripples or mud cracks? Assess A Section 3.3 Quiz, p. 44 Reteach Copy the following information on the board and use it for review. KEY CONCEPTS CRITICAL THINKING 1. What types of material can make up sediments? 4. Infer Why is coal called a fossil fuel? 2. Describe the three processes by which sedimentary rocks form. 5. Analyze How could the speed of flowing water change to lay down alternating layers of sand and mud? 3. Describe how a sedimentary rock can show how fast water was flowing when its sediments were laid down. How sedimentary rock forms CHALLENGE 6. Synthesize How is it possible for a single sedimentary rock to contain rock particles, animal shells, and minerals that crystallized from water? • From layers of rock particles that are pressed and cemented together • From layers of shells or plants that pressed and cemented together • From dissolved minerals that are left behind as water evaporates What sedimentary rocks can tell us about past conditions Chapter 3: Rocks 95 2. pieces of minerals and rocks are cemented together; shells and skeletons of ocean animals are cemented together; dissolved minerals are left behind as water evaporates • Fossils of fish indicate an area was once covered with water. • Layering with the largest particles first might indicate that a flood occurred. ANSWERS 1. pieces of minerals, rocks, plant and animal remains A 3. A rock made of large pieces shows that water was moving quickly. silt and clay that make up mud settle from slowermoving water. 4. It is made of the remains of ancient plants. 6. A rock that forms from sediment on a beach could have pieces of shells and rocky sand cemented together by minerals that crystallized from water. 5. Weather patterns cause flow rates of rivers to vary. Sand settles from faster-moving water; smaller pieces of Technology Resources Have students visit ClassZone.com for reteaching of Key Concepts. CONTENT REVIEW CONTENT REVIEW CD-ROM Chapter 3 95 A