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Transcript
Earth Sciences Standards Preview 5.b. Students know natural processes, including freezing and thawing and the growth of roots, cause rocks to break down into smaller pieces. Standard Set 5. Earth Sciences 5. Waves, wind, water, and ice shape and reshape Earth’s land surface. As a basis for understanding this concept: 5.c. Students know moving water erodes landforms, reshaping the land by taking it away from some places and depositing it as pebbles, sand, silt, and mud in other places (weather, transport, and deposition). 5.a. Students know some changes in the earth are due to slow processes, such as erosion, and some are due to rapid processes, such as landslides, volcanic eruptions, and earthquakes. by Lillian Duggan Genre Nonfiction Comprehension Skill Sequence Text Features • • • • Captions Labels Maps Glossary Science Content Earth’s Surface Scott Foresman Science 4.7 ISBN 0-328-23559-8 ì<(sk$m)=cdf j < +^-Ä-U-Ä-U Vocabulary deposition earthquake erosion fault landform landslide soil transport volcano weathering Changing Earth by Lillian Duggan Picture Credits Every effort has been made to secure permission and provide appropriate credit for photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its attention in subsequent editions. Unless otherwise acknowledged, all photographs are the copyright of Dorling Kindersley, a division of Pearson. ISBN: 0-328-23559-8 Copyright © Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to Permissions Department, Scott Foresman, 1900 East Lake Avenue, Glenview, Illinois 60025. 1 2 3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 How does Earth’s surface slowly change? Weathering Earth’s Crust Chemical Weathering Earth’s outer surface is called the crust. The crust is a layer of rock that covers all of Earth. In some places the crust is beneath the ocean. Earth’s surface has many natural features, or landforms. Mountains, flat plains, and peninsulas are landforms. Peninsulas stick out into the water from coasts. Landforms have different shapes and sizes. It can take millions of years for a landform such as a mountain to form. But what if many rocks roll down its side? That can change the mountain quickly. Chemicals cause rocks to change into different materials and break down. This is chemical weathering. Acid in rain can cause chemical weathering. When the acid touches rock, it combines with the rock. This forms a new substance. It causes the rock to break down. Landforms can change only when their rocks break apart. Weathering is the process that breaks rocks in Earth’s crust into smaller pieces. Weathering is caused by water, ice, temperature changes, chemicals, and living things. It can change the land quickly or slowly. Acid rain has worn away the stone from Cleopatra’s Needle. This mountain is a landform. How do you think it has changed over time? 2 3 What causes physical weathering? Physical Weathering Physical weathering causes rock to break down into smaller pieces. During chemical weathering rock changes into another material. This does not happen during physical weathering. Physical weathering is caused by plants, ice, and water. Plants cause physical weathering by growing in cracks in rocks. Their growing roots break up the rocks. Glaciers are huge sheets of ice. They move slowly over land. As glaciers move, they cause physical weathering. They drag rocks along the ground. The rocks scrape the ground. The scraping forms valleys and ridges. Glaciers also grind against rocks. They break the rocks into smaller pieces. Freezing and Thawing Ice can also break rocks. Water from rain or melted snow gets into cracks in rocks. The water expands when it freezes into ice. This makes the ice push against the sides of the crack. The crack becomes bigger. It keeps growing bigger as more ice forms. Eventually the rock splits. Plant roots caused cracks to form in this rock. This rock split apart because of water freezing and thawing. 4 5 Water and Weathering Soil The landforms along coasts are being reshaped all the time. Waves and flowing water cause these changes. Crashing waves contain pieces of sand and gravel that wear away rocks. Beach sand also gets carried away by crashing waves. Water shapes landforms in other ways. Rivers carry pebbles, sand, mud, and other materials. The materials rub against rocks on the bottom of the river. As they rub, they slowly wear away the rocks. This causes valleys and canyons to form. Weathering breaks down rocks into tiny pieces. These tiny pieces are part of soil. Soil is the thin layer of loose, weathered material that covers most of the land surface of Earth. Scientists have named the rocks in soil based on their sizes. Sand, silt, and clay make up most soil. Soil also contains humus. Humus is decaying plant and animal material. Different soils have different kinds of rocks and humus. The spaces between the bits of sand, silt, clay, and humus fill with water and air. Different plants do best in different kinds of soil. Waves carrying sand and pebbles formed this cut in the rocks. Sizes of Rocks Cobble 100 mm Pebble 30 mm Clay You cannot see clay particles with your eyes. 6 Sand 1 mm Silt Silt looks like tiny specks. 7 More Weathering Weathering breaks some rocks into tiny pieces. Larger rocks are affected by weathering too. The weathering of large rocks can form huge arches, towers, sand dunes, and other shapes. Some small pieces of weathered rock are moved by wind or other forces. Blowing winds can pick up tiny pieces of sand, soil, or dust. When the pieces blow against a rock, they scrape it. Tiny bits of the rock can break off and then blow away. These weathered rocks are in California’s Death Valley. 8 Pieces of rock and soil in open areas are constantly weathering. The top layer of soil is called topsoil. Crops grow best in it. Topsoil gets weathered in farm fields. Waves Moving water and waves can break down large rocks. Pieces of rock, soil, and sand can be carried away by water and wind. They gradually become smaller and smaller. Later these pieces can end up in other places. The water or wind may bring them there. Weathering shaped these rocks in California’s Alabama Hills. 9 How does weathered material move? Erosion and Transport Small pieces of rock sometimes are picked up and carried to other places. This movement of weathered materials is called erosion. Erosion is caused by wind, water, glaciers, living things, and gravity. Water carries, or transports, weathered materials. First, rain picks up small pieces of rock. It carries them into rivers and streams. The rivers move the pieces to other places. The pieces might settle on the rivers’ bottoms. Or they might be carried to the ocean. There they might make a new landform. Weathered materials are also transported by wind and gravity. Wind blows dry sand and soil to new places. Rocks and soil get pulled downhill by gravity. Soil is moved by living things as well. Worms carry soil as they move underground. They mix and move it. As tree roots grow down, they push soil away. Rivers carry pieces of rocks to new places. These valleys were made by erosion. Photo A. was taken one minute before Photo B. Waves caused the rock to collapse. Ocean waves may transport the pieces of broken-up rock from place to place. 10 11 Transport and Deposition Pieces of eroded rock and soil are transported and left in new places. The laying down of pieces of rock and soil is called deposition. Fast-moving water can transport particles a long distance. When the water slows down, the particles begin to settle out. Larger pieces of rocks are deposited first. Then sand particles will sink. The clay and silt are last to be deposited. This water is transporting silt. The silt makes it muddy. 12 Deltas can form where rivers deposit sediment. Ocean waves carry around sediment. They create sandy islands near coasts when they deposit the sediment. Erosion moves these islands around too! Wind deposits a layer of silt and clay on top of soil. Sand dunes are formed by wind depositing grains of sand in deserts and other places. Blowing winds constantly change the shape of sand dunes. Glaciers deposit rocks and soil when they melt. Ridges of broken rocks and soil are evidence of glaciers. Rivers have transported and deposited silt here. They have created a new landform. 13 What causes rapid changes to landforms? Rapid Changes The side of a hill slides down a cliff wall. Suddenly the landforms have changed. Changes to Earth’s landforms can happen in an instant. Landslides, earthquakes, and volcanoes cause rapid changes. A landslide changed the shape of this coastline. Gravity and Landslides Objects are pulled from higher places to lower places by gravity. Gravity can cause loose rocks and dirt to roll slowly or quickly downhill. Heavy rain can loosen a steep hill’s materials. Gravity pulls down the materials. They land in piles at the bottom. The rapid downhill movement of a large amount of rock and soil is a landslide. Freezing and thawing can loosen rock. Sometimes rock gets loosened during winter. It may slide downhill in spring. Landslides can cause a lot of damage. As they slide downhill, they can carry very heavy objects. Sliding mud can bury anything in the landslide’s path. Homes, roads, and bridges can be destroyed by landslides. This boulder fell during a landslide. It completely blocked the road. A landslide caused rocks and soil to slide down this hill. 14 15 Volcanoes A volcano is a cone-shaped landform that forms at a weak spot in Earth’s crust where magma reaches the surface. Deep underground, hot molten rock called magma mixes with gases. The gases force it toward the surface. If magma reaches a weak spot in Earth’s crust, a volcano will form. As more magma comes to the surface, the volcano erupts. Lava is magma that has reached the surface. Some eruptions are more violent than others. Sometimes the magma’s gases explode. Hot rocks, gases, and ash burst from the volcano’s openings. Other times, magma oozes out slowly. Mount St. Helens was once almost 3,000 meters tall. 16 When a volcano erupts, lava and ash can spread over a large area. This changes the volcano’s shape. It also changes the surrounding land. Dormant and Active Volcanoes An active volcano is one that erupts often or shows signs that it will soon erupt. Kilauea is a volcano in Hawaii. It has not stopped erupting since 1983. A dormant volcano is one that has not erupted for a long time. Mount Lassen is a dormant volcano. Located in California’s Cascade Mountains, it has not erupted since 1921. Scientists do not know when it will erupt again. A volcano is extinct if scientists believe it can never erupt again. Africa’s Mount Kenya is extinct. The 1980 eruption of Mount St. Helens reduced the height of the mountain by 400 meters. 17 Earthquakes Earth’s upper mantle lies beneath the crust. The crust and upper mantle move around as plates. Plates are very large. They move constantly. Many earthquakes and volcanic eruptions happen where plates meet. A fault is a break or crack in rocks where Earth’s crust can move. Rocks sometimes get stuck in faults. But the plates continue to move. As they move, they push on the rocks. If they push hard enough, the rocks will break. This causes the plates to move suddenly. The sudden movement that causes Earth’s crust to shake is an earthquake. The San Andreas fault (below) runs for hundreds of kilometers through California. 18 Earthquakes can change Earth’s surface quickly. They cause vibrations which move through Earth as waves. The waves can cause cracks in Earth’s surface. Plates Meeting The San Andreas fault is mostly in California. It is where two plates meet. Many towns and cities are built along it. Earthquakes happen frequently near the fault. Some earthquakes are very powerful. In 1906, a strong earthquake broke gas lines in San Francisco. Another strong earthquake did the same thing to San Francisco in 1989. The broken gas lines caused fires each time. Earthquakes can also cause landslides. The San Andreas fault runs through San Francisco. It caused earthquakes there in 1906 and 1989. 19 Glossary What did you learn? 1. What is a peninsula? deposition the laying down of pieces of rock and soil earthquake a sudden movement that causes Earth’s crust to shake erosion the movement of weathered materials fault a break or crack where Earth’s crust can move landform natural feature of Earth landslide the rapid downhill movement of a large amount of rock and soil soil the thin layer of loose, weathered material that covers most of the land surface of Earth transport to carry from one place to another volcano place on Earth’s crust where magma reaches the surface weathering the process that breaks down rocks in Earth’s crust into smaller pieces 20 2. What kinds of materials are deposited from water first? 3. What kind of volcano is Mount Lassen? When did it last erupt? 4. You learned about three things that cause rapid changes to landforms: landslides, earthquakes, and volcanoes. Write about all three in a couple of paragraphs. Include an introductory paragraph, supporting paragraphs, and a concluding paragraph. Use correct indentation. 5. Sequence Describe the sequence of events when water physically weathers rock.