chapter 12 section 3 erosion and deposition by...

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Interactive Textbook 221 Agents of Erosion and Deposition

SECTION

3 Erosion and Deposition by IceAgents of Erosion and Deposition

Name Class Date

CHAPTER 12

After you read this section, you should be able to answer these questions:

• What are glaciers?

• How do glaciers affect the landscape?

What Are Glaciers?A glacier is a huge piece of moving ice. The ice in gla-

ciers contains most of the fresh water on Earth. Glaciers are found on every continent except Australia.

There are two kinds of glaciers: continental and alpine. Continental glaciers are ice sheets that can spread across entire continents. Alpine glaciers are found on the tops of mountains. Both continental and alpine glaciers can greatly affect the landscape.

Glaciers form in areas that are so cold that snow stays on the ground all year round. For example, glaciers are common in polar areas and on top of high mountains. In these areas, layers of snow build up year after year. Over time, the weight of the top layers pushes down on the lower layers. The lower layers change from snow to ice.

HOW GLACIERS MOVEGlaciers can move in two ways: by sliding and by

flowing. As more ice builds up on a slope, the glacier becomes heavier. The glacier can start to slide downhill, the way a skier slides downhill. Glaciers can also move by flowing. The solid ice in glaciers can move slowly, like soft putty or chewing gum.

Thick glaciers move faster than thin glaciers. Glaciers on steep slopes move faster than those on gentler slopes.

This is McBride Glacier in Alaska.

BEFORE YOU READ National Science Education StandardsES 1c, 2a

STUDY TIPCompare As you read, make a table comparing the landforms that glaciers can produce.

READING CHECK

1. Identify What are the two kinds of glaciers?

TAKE A LOOK 2. Defi ne What is a glacier?



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Erosion and Deposition by Ice continued

How Do Glaciers Affect the Landscape?Glaciers can produce many different features as they

move over Earth’s surface. As a glacier moves, it can pick up and carry the rocks in its path. Glaciers can carry rocks of many different sizes, from dust all the way up to boulders. These rocks can scrape grooves into the land below the glacier as the glacier moves.

Continental glaciers tend to flatten the land that they pass over. However, alpine glaciers can produce sharp, rugged landscapes. The figure below shows some of the features that alpine glaciers can form.

Horns are sharp, pyramid-shaped peaks that form when three or more cirque glaciers erode a mountain.

Cirques are bowl-shaped depressions where glacial ice cuts back into the mountain walls.

Hanging valleys are smaller glacial valleys that join the deeper main valley. Many hanging valleys form waterfalls after the ice is gone.

Arêtes are jagged ridges that form between two or more cirques that cut into the same mountain.

U-shaped valleys form when a glacier erodes a river valley. The valley changes from its original V shape to a U shape.

GLACIAL DEPOSITSAs a glacier melts, it drops all of the material that it is

carrying. The material that is carried and deposited by glaciers is called glacial drift. There are two kinds of glacial drift: till and stratified drift.

Critical Thinking3. Identify Relationships How is erosion by glaciers an example of water shaping the landscape?

TAKE A LOOK 4. Explain How are horns, cirques, and arêtes related?



SECTION 3

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Erosion and Deposition by Ice continued

TILL DEPOSITSTill is unsorted rock material that is deposited by

melting glacial ice. It is called “unsorted” because the rocks are of all different sizes. Till contains fine sediment as well as large boulders. When the ice melts, it deposits this material onto the ground.

The most common till deposits are moraines. Moraines form ridges along the edges of glaciers. There are many types of moraines. They are shown in the figure below.

Lateral moraines form along each side of a glacier.

Ground moraines form from unsorted materials left beneath a glacier.

Medial moraines form when valley glaciers that have lateral moraines meet.

Terminal morainesform when sediment is dropped at the front of the glacier.

STRATIFIED DRIFTWhen a glacier melts, the water forms streams that

carry rock material away from the glacier. The streams deposit the rocks in different places depending on their size. Larger rocks are deposited closer to the glacier. The rocks form a sorted deposit called stratified drift. The large area where the stratified drift is deposited is called an outwash plain.

In some cases, a block of ice is left in the outwash plain as the glacier melts. As the ice melts, sediment builds up around it. The sediment forms a bowl-shaped feature called a kettle. Kettles can fill with water and become ponds or lakes.

READING CHECK

5. Explain Why is till considered unsorted?

Say ItLearn New Words Look up the words lateral, medial,and terminal in a dictionary. In a group, talk about why these words are used to describe different kinds of moraines.

READING CHECK

6. Defi ne Write your own defi nition for stratifi ed drift.



Name Class Date

Section 3 Review NSES ES 1c, 2a

SECTION VOCABULARYglacial drift the rock material carried and

deposited by glaciersglacier a large mass of moving ice

stratifi ed drift a glacial deposit that has been sorted and layered by the action of streams or meltwater

till unsorted rock material that is deposited directly by a melting glacier

1. List Give two kinds of glacial drift.

2. Identify What are four kinds of moraines?

3. Compare How are continental glaciers different from alpine glaciers?

4. Explain How do glaciers form?

5. Describe How does a kettle form?

6. Infer How can a glacier deposit both unsorted and sorted material?



SECTION

4 The Effect of Gravity on Erosion and DepositionAgents of Erosion and Deposition

Name Class Date

CHAPTER 12

After you read this section, you should be able to answer these questions:

• What is mass movement?

• How does mass movement shape Earth’s surface?

• How can mass movement affect living things?

What Is Mass Movement?Gravity can cause erosion and deposition. Gravity

makes water and ice move. It also causes rock, soil, snow, or other material to move downhill in a process called mass movement.

ANGLE OF REPOSEParticles in a steep sand pile move downhill. They stop

when the slope of the pile becomes stable. The angle of repose is the steepest angle, or slope, at which the loose material no longer moves downhill. If the slope of a pile of material is larger than the angle of repose, mass move-ment happens.

The slope of this pile of sand is equal to the sand’s angle of repose. The sand pile is stable. The sand particles are not moving.

The slope of this pile of sand is larger than the angle of repose. Therefore, particles of sand move down the slope of the pile.

60°

35°

The angle of repose can be different in different situations. The composition, size, weight, and shape of the particles in a material affect its angle of repose. The amount of water in a material can also change the material’s angle of repose.

BEFORE YOU READ National Science Education StandardsES 1c, 2a

STUDY TIPAsk Questions As you read this section, write down any questions you have. Talk about your questions in a small group.

READING CHECK

1. Defi ne What is the angle of repose?

TAKE A LOOK 2. Explain Why are sand particles moving downhill in the bottom picture?



SECTION 4

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The Effect of Gravity on Erosion and Deposition continued

What Are the Kinds of Mass Movement?Mass movement can happen suddenly and quickly.

Rapid mass movement can be very dangerous. It can destroy or bury everything in its path.

LANDSLIDESA landslide happens when a large amount of rock and

soil moves suddenly and rapidly downhill. Landslides can carry away or bury plants and animals and destroy their habitats. Several factors can make landslides more likely.

• Heavy rains can make soil wet and heavy, which makes the soil more likely to move downhill.

• Tree roots help to keep land from moving. Therefore, deforestation, or cutting down trees, can make land-slides more likely.

• Earthquakes can cause rock and soil to start moving.• People may build houses and other buildings on unsta-

ble hillsides. The extra weight of the buildings can cause a landslide.

The most common kind of landslide is a slump.Slumps happen when a block of material moves downhill along a curved surface.

ROCK FALLSA rock fall happens when loose rocks fall down a

steep slope. Many such slopes are found on the sides of roads that run through mountains. Gravity can cause the loose and broken rocks above the road to fall. The rocks in a rock fall may be many different sizes.

MUDFLOWSA mudflow is a rapid movement of a large amount of

mud. Mudflows can happen when a lot of water mixes with soil and rock. The water makes the slippery mud flow downhill very quickly. A mudflow can carry away cars, trees, houses, and other objects that are in its path.

Mudflows are common in mountain regions when a long dry season is followed by heavy rain. Mudflows may also happen when trees and other plants are cut down. Without plant roots to hold soil in place and help water drain away, large amounts of mud can quickly form.

READING CHECK

3. Describe How can landslides affect wildlife habitats?

READING CHECK

4. Identify Give three factors that can make landslides more likely.

Critical Thinking5. Infer Does water probably increase or de-crease the angle of repose of soil? Explain your answer.



SECTION 4

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The Effect of Gravity on Erosion and Deposition continued

LAHARSVolcanic eruptions can produce dangerous mudflows

called lahars. A volcanic eruption on a snowy peak can suddenly melt a great amount of snow and ice. The water mixes with soil and ash to produce a hot flow that rushes downhill. Lahars can travel faster than 80 km/h.

CREEPNot all mass movement is fast. In fact, very slow mass

movement is happening on almost all slopes. Creep is the name given to this very slow movement of material down-hill. Even though creep happens very slowly, it can move large amounts of material over a long period of time.

Many factors can affect creep. Water can loosen soil and rock so that they move more easily. Plant roots can cause rocks to crack and can push soil particles apart. Burrowing animals, such as moles and gophers, can loosen rock and soil particles. All of these factors may make creep more likely.

Type of Mass Movement Description

Landslide Material moves suddenly and rapidly down a slope.

Rock fall

Mudfl ow

Water mixes with volcanic ash to produce a fast-moving, dangerous mudfl ow.

Material moves downhill very slowly.

READING CHECK

6. Compare How is creep different from the other kinds of mass movement that are discussed in this section?

TAKE A LOOK 7. Describe Fill in the blank spaces in the table.



Name Class Date

Section 4 Review NSES ES 1c, 2a

SECTION VOCABULARYcreep the slow downhill movement of

weathered rock materiallandslide the sudden movement of rock and

soil down a slope mass movement the movement of a large mass

of sediment or a section of land down a slope

mudfl ow the fl ow of a mass of mud or rock and soil mixed with a large amount of water

rock fall the rapid mass movement of rock down a steep slope or cliff

1. List What are four kinds of mass movement?

2. Infer Why is it important for people to think about mass movement when they decide how to use land?

3. Identify Relationships How is mass movement related to the angle of repose?

4. Identify What force causes mass movements?

5. Compare How are landslides different from mudflows?

6. List Give four things that can affect a material’s angle of repose.

chapter 12 section 3 erosion and deposition by...

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