Sedimentary Rock. LOOKING CLOSER 2. Compare How is compaction different from cementation?

Transcription

1 CHAPTER 6 SECTION 3 Rocks Sedimentary Rock KEY IDEAS As you read this section, keep these questions in mind: How do compaction and cementation happen? How do chemical and organic sedimentary rocks form? How does clastic sedimentary rock form? What are seven features that sedimentary rock can have? What Are Compaction and Cementation? Recall that sedimentary rock is made up of fragments of other rocks, minerals, and organic matter. These fragments are called sediment. Most sedimentary rock forms when sediment is laid down, or deposited, by water, wind, or ice. Over time, two processes can change the sediment into sedimentary rock: compaction and cementation. After a layer of sediment is laid down, more sediment may bury it. The weight of the top layers of sediment presses down on the bottom layers. The pressure squeezes water and air out of the spaces between the sediment particles. The particles get closer together. The sediment becomes smaller, or more compact. Therefore, this process is called compaction. Sometimes, water moves through sediment. The water may have minerals, such as calcite and quartz, dissolved in it. As the water moves through the sediment, those minerals may crystallize between the sediment particles. The minerals glue, or cement, the sediment particles together. Therefore, this process is called cementation. Compaction Pressure squeezes water out of the sediment % water READING TOOLBOX Summarize Cause and Effect As you read this section, mark examples of cause and effect or a chain of events. Then, make chain-of-events charts to summarize each example. 1. Describe What happens during compaction? LOOKING CLOSER 2. Compare How is compaction different from cementation? 50-60% water Cementation There are empty spaces between sediment particles. Water moves through the empty spaces. Minerals crystallize between the particles and glue them together. Holt McDougal Earth Science 79 Rocks

2 What Are the Different Kinds of Sedimentary Rock? Scientists put sedimentary rocks into three different groups: chemical sedimentary rocks, organic sedimentary rocks, and clastic sedimentary rocks. The rocks in the different groups form in different ways. 3. Explain What causes chemical sedimentary rocks to form? Critical Thinking 4. Apply Concepts Some kinds of limestone form when calcium carbonate crystallizes from shallow ocean water. Are these limestones chemical sedimentary rocks or organic sedimentary rocks? CHEMICAL SEDIMENTARY ROCK The water in natural water bodies, such as lakes and oceans, contains many dissolved minerals. As the water evaporates, the minerals are left behind. As the minerals crystallize, they form a type of sedimentary rock called chemical sedimentary rock. ORGANIC SEDIMENTARY ROCK Remember that organic matter is matter that comes from living things. Some kinds of sedimentary rock are made from organic matter. These sedimentary rocks are called organic sedimentary rocks. Coal is one example of an organic sedimentary rock. Coal forms from the remains of plants that live in swamps. When the plants die, their remains fall to the bottom of the swamp. Over time, they are buried. Pressure changes the plant remains into coal, which is made up mostly of the element carbon. Some kinds of limestone are also organic sedimentary rocks. These kinds of limestone form from the shells of animals that live in the oceans. The animals use the compound calcium carbonate to make their shells. When the animals die, their shells sink to the bottom of the ocean. Over time, the shells are pressed together. They form the rock limestone. Formation of Organic Sedimentary Rock Talk About It Describe Discuss this figure with a partner. Take turns describing to each other what is happening in each part of the figure. Holt McDougal Earth Science 80 Rocks

3 CLASTIC SEDIMENTARY ROCK The most common kind of sedimentary rock is clastic sedimentary rock. Clastic sedimentary rock is made up of pieces of rocks that have been carried from their source and deposited. The rock pieces in a clastic sedimentary rock may be pieces of igneous, metamorphic, or other sedimentary rocks. Scientists classify clastic sedimentary rock based on the sizes of the rock pieces in the rock. If most of the rock pieces are more than 2 mm in diameter, the rock is a conglomerate. If the rock pieces have sharp angles, the rock is a breccia. If most of the rock pieces are the size of sand grains, the rock is a sandstone. If most of the rock pieces are very small, the rock is a shale. No matter what size the particles in the rock are, all clastic sedimentary rocks form the same way. First, water, wind, or ice deposit the sediment in layers. As the layers build up, the sediment at the bottom is compacted. Over time, the sediment may be cemented together to form clastic sedimentary rock. What Are Some Characteristics of Sediment? The sediment in clastic sedimentary rocks can have different characteristics. The characteristics of the sediment can tell scientists about how the sediment formed and how it was laid down. Two important characteristics of sediment are sorting and angularity. 5. Identify Where do the sediments that form clastic sedimentary rock come from? Math Skills 6. Calculate The rate at which sediment collects, or accumulates, is called the sedimentation rate. Suppose the sedimentation rate for an area is 1.5 mm per year. How many years will it take for 10 cm of sediment to accumulate? SORTING Some sediment is made up of particles that are all about the same size. Other sediment contains particles of many different sizes. The range of particles sizes in a sediment determines how well the sediment is. LOOKING CLOSER 7. Compare What is the main difference between well- sediment and poorly sediment? Very well Well Moderately Poorly Very poorly Holt McDougal Earth Science 81 Rocks

4 8. Identify What type of water can drop poorly sediment? WHAT SORTING INDICATES Water and wind can carry sediment over long distances. However, the speed of the water or wind affects the size of the sediment they can carry. Fast-moving water can carry very large sediment pieces. Slow-moving water can carry only small sediment pieces. When fast-moving water slows suddenly, it may drop a lot of sediment all at once. The sediment pieces it drops will have a range of sizes. In other words, water that slows suddenly can drop poorly sediment. However, if the water slows gradually, it will drop the sediment over a larger distance. In other words, water that slows gradually can drop well- sediment. Therefore, the sorting in a sediment can tell a scientist how the sediment was deposited. 9. Explain What causes a sediment particle to become smoother and rounder over time? ANGULARITY Some sediment pieces are smooth and rounded. Others are rough and have sharp angles. A sediment s angularity describes how smooth or rough the sediment particles are. As wind or water carry sediment over Earth s surface, the sediment particles hit one another. They may also hit other objects, such as larger rocks. When the particles hit other objects, they can break apart. They may also become worn down and smooth. The longer the particle is carried along, the smoother and rounder it becomes. Therefore, the angularity of a sediment can tell a scientist how far the sediment has traveled. What Structures Can Sedimentary Rock Form? The areas where sediment is laid down are called depositional environments. Beaches, lakes, deltas, and oceans are examples of depositional environments. Sediment laid down in different depositional environments will form different features. Those features include stratification cross-beds and graded beds ripple marks mud cracks fossils and concretions Holt McDougal Earth Science 82 Rocks

5 STRATIFICATION Most sediment is laid down in layers. Sedimentary rock that forms from layers of sediment generally also has layers. Layering in sedimentary rock is called stratification. CROSS-BEDS AND GRADED BEDS The layers in a sedimentary rock are called beds. Most beds are parallel to one another. However, in some rocks, the beds form angles with one another. Beds that are at different angles are called cross-beds. This photo shows sedimentary rock with cross-beds. Most beds are made up of sediment particles that are all about the same size. However, some beds contain particles of different sizes. The particles in the bed are from top to bottom. A bed made up of particles of different sizes that are from top to bottom is a graded bed. 10. Describe What is stratification? Talk About It Discuss Use a dictionary to look up the word root strat. With a partner, discuss why this word root is found in the word that describes layering in sedimentary rocks. RIPPLE MARKS AND MUD CRACKS When water flows over sediment, it may move some of the sediment to form ripples. Ripples are wavy lines in sediment. Sedimentary rocks that form from sediment that has ripples may show ripple marks. Sedimentary rocks may also show mud cracks. Mud cracks form when wet mud dries. As mud dries, it shrinks and cracks. The cracks may then fill with sediment and form sedimentary rock. FOSSILS AND CONCRETIONS A fossil is a sign that a living thing once existed. The hard parts of organisms can become fossils. Footprints and other marks can also become fossils. Most fossils are found in sedimentary rock. Some sedimentary rocks contain lumps of rock that are different from the rest of the rock. These lumps are called concretions. They form when minerals crystallize out of water that flows through the rock. 11. Explain What causes mud to crack as it dries? Holt McDougal Earth Science 83 Rocks

6 Section 3 Review SECTION VOCABULARY cementation the process in which minerals precipitate into pore spaces between sediment grains and bind sediments together to form rock chemical sedimentary rock sedimentary rock that forms when minerals precipitate from a solution or settle from a suspension clastic sedimentary rock sedimentary rock that forms when fragments of preexisting rocks are compacted or cemented together compaction the process in which the volume and porosity of a sediment is decreased by the weight of overlying sediments as a result of burial beneath other sediments organic sedimentary rock sedimentary rock that forms from the remains of plants or animals 1. Classify A scientist is studying a sedimentary rock that did not form through compaction and cementation. What kind of sedimentary rock is the scientist studying? Explain your answer. 2. Identify List the seven features that sedimentary rocks can have. 3. Infer A clastic sedimentary rock is made up of smooth, round sediment pieces that are all about the same size. What can you infer about how the sediment that formed the rock was transported and deposited? Explain your answer. 4. Synthesize Concepts Which structure are you more likely to find in an organic sedimentary rock, mud cracks or fossils? Explain your answer. Holt McDougal Earth Science 84 Rocks