2Chapter: Erosional Forces Table of ContentsChapter: Erosional ForcesSection 1: Erosion by GravitySection 2: GlaciersSection 3: Wind
3Erosion and Deposition— What wears away sediments? Erosion by Gravity1Erosion and Deposition— What wears away sediments?Erosion is a process that wears away surface materials and moves them from one place to another.An important erosional force is gravity.Gravity is the force of attraction that pulls all objects toward Earth’s center.Other causes of erosion, also called agents of erosion, are water, wind, and glaciers.
4Erosion and Deposition— What wears away sediments? Erosion by Gravity1Erosion and Deposition— What wears away sediments?Water and wind erode materials only when they have enough energy of motion to do work. A strong wind can move dust and even larger particles.Glacial erosion works by slowly moving sediment that is trapped in solid ice.As ice melts, sediment is deposited, or dropped.
5Erosion by Gravity1Dropping SedimentsAgents of erosion drop the sediments they are carrying as they lose energy. This is called deposition.When sediments are eroded, they are not lost from Earth—they are just relocated.
6Erosion by Gravity1Mass MovementRocks and other materials, especially on steep slopes, are pulled toward the center of Earth by gravity.A mass movement is any type of erosion that happens as gravity moves materials downslope.
7Erosion by Gravity1Mass MovementCommon types of mass movement include slump, creep, rockfalls, rock slides, and mudflows.Landslides aremass movements that can be one of these types or a combination of these types of mass movement.
8Erosion by Gravity1SlumpWhen a mass of material slips down along a curved surface, the mass movement is called slump.
9Erosion by Gravity1SlumpSometimes a slump happens when water moves to the base of a slipping mass of sediment.Or, if a strong rock layer lies on top of a weaker layer—commonly clay—the clay can weaken further under the weight of the rock.A curved scar is left where the slumped materials originally rested.
10Erosion by Gravity1CreepLeaning trees and human-built structures show another mass movement called creep.Creep occurs when sediments slowly shift their positions downhill.Creep is common in areas of frequent freezing and thawing.
11Rockfalls and Rock Slides Erosion by Gravity1Rockfalls and Rock SlidesRockfalls happen when blocks of rock break loose from a steep slope and tumble through the air.As they fall, these rocks crash into other rocks and knock them loose.More and more rocks break loose and tumble to the bottom.
12Rockfalls and Rock Slides Erosion by Gravity1Rockfalls and Rock SlidesDuring the winter, when ice freezes in the cracks of rocks, the cracks expand and extend.In the spring, the pieces of rock break loose and fall down the mountainside.
13Rockfalls and Rock Slides Erosion by Gravity1Rockfalls and Rock SlidesRock slides occur when layers of rock—usually steep layers—slip downslope suddenly.They commonly occur on mountainous areas or in areas with steep cliffs.
14Erosion by Gravity1MudflowsA mudflow is a thick mixture of sediments and water flowing down a slope.
15Erosion by Gravity1MudflowsMudflows usually occur in areas that have thick layers of loose sediments.When heavy rains fall on these areas, water mixes with sediment, causing it to become thick and pasty.Gravity causes this mass to flow downhill.When a mudflow finally reaches the bottom of a slope, it loses its energy of motion and deposits all the sediment and everything else it has been carrying.
16Erosion by Gravity1MudflowsMudflows, rock slides, rockfalls, creep, and slump are most likely to occur on steep slopes, and they all depend on gravity to make them happen.Also, all types of mass movement occur more often after a heavy rain.The water adds mass and creates fluid pressure between grains and layers of sediment.
17Consequences of Erosion— Building on Steep Slopes Erosion by Gravity1Consequences of Erosion— Building on Steep SlopesWhen people build homes on steep slopes, they constantly must battle naturally occurring erosion.Some steep slopes are prone to slumps because of weak sediment layers underneath.
18Making Steep Slopes Safe Erosion by Gravity1Making Steep Slopes SafeOne of the best ways to reduce erosion is to plant vegetation.Deep trees roots and fibrous grass roots bind soil together, reducing the risk of mass movement.Plants also absorb large amounts of water.Drainage pipes or tiles inserted into slopes can prevent water from building up, too.Walls made of concrete or boulders also can reduce erosion by holding soil in place.
19Question 1 Answer 1 What is erosion? Section Check1Question 1What is erosion?AnswerErosion is a process that wears away surface materials and moves them from one place to another.
20Question 2 1 What type of mass movement is shown in this illustration? Section Check1Question 2What type of mass movement is shown in this illustration?A. creepB. depositionC. mudflowD. slump
21Section Check1AnswerThe answer is D. If a slope becomes too steep, the base material may not be able to support the rock and soil above it. Slump occurs when material slips down slope as one large mass.
22Section Check1Question 3How does a rock slide differ from creep?
23Answer 1 Rock slides occur suddenly, are fast and move layers of rock Section Check1AnswerRock slides occur suddenly, are fast and move layers of rockdown steep slopes. Creep is the slow shifting of position of sediments on a slope.
24How Glaciers Form and Move 2How Glaciers Form and MoveGlaciers form in regions where snow accumulates.When snow doesn’t melt, it piles up.As it accumulates slowly, the increasing weight of the snow becomes great enough to compress the lower layers into ice.
25How Glaciers Form and Move 2How Glaciers Form and MoveThe mass slowly begins to flow in a thick, plasticlike lower layer, and ice slowly moves away from its source.A large mass of ice and snow moving on land under its own weight is a glacier.
26Ice Eroding Rock 2 Glaciers are agents of erosion. As glaciers pass over land, they erode it, changing featureson the surface.Glaciers then carry eroded material along and deposit it somewhere else.Click image to view movie.
27Plucking 2 When glacial ice melts, water flows into cracks in rocks. Glaciers2PluckingWhen glacial ice melts, water flows into cracks in rocks.Later, the water refreezes in the cracks, expands, and fractures the rock.Pieces of rock then are lifted out by the ice.
28Glaciers2PluckingThis process, called plucking, results in boulders, gravel, and sand being added tothe bottom and sides of a glacier.
29Transporting and Scouring Glaciers2Transporting and ScouringA glacier can transport huge volumes of sediment and rock.Plucked rock fragments and sand at its base scour and scrape the soil and bedrock, eroding the ground below even more.When bedrock is gouged deeply by rock fragments being dragged along, marks are left behind.
30Transporting and Scouring Glaciers2Transporting and ScouringThese marks, called grooves, are deep, long, parallel scars on rocks.Shallower marks are called striations.
31Ice Depositing Sediment Glaciers2Ice Depositing SedimentWhen glaciers begin to melt, sediment drops or is deposited, on the land.When a glacier melts and begins to shrink back, it is said to retreat.As it retreats, a jumble of boulders, sand, clay, and silt is left behind.This mixture of different-sized sediments is called till.
32Glaciers2Moraine DepositsTill is also deposited at the end of glacier when it is not moving forward.Rocks and soil are moved to the end of the glacier, much like items on a grocery store conveyor belt.Because of this, a big ridge of material piles up.Such a ridge is called a moraine.
33Glaciers2Outwash DepositsMaterial deposited by the meltwater from a glacier, most often beyond the end of the glacier, is called outwash.Meltwater carries sediments and deposits them in layers.
34Glaciers2EskersAnother type of outwash deposit looks like a long, winding ridge.This deposit forms in a melting glacier when meltwater forms a river within ice.
35Glaciers2EskersThis river carries sand and gravel and deposits them within its channel.When the glacier melts, a winding ridge of sand and gravel, called an esker, is left behind.
36Glaciers2Continental GlaciersToday, continental glaciers cover ten percent of Earth, mostly near the poles in Antarctica and Greenland.Continental glaciers are huge masses of ice and snow.Continental glaciers are thicker than some mountain ranges.
37Glaciers2Climatic ChangesIn the past, continental glaciers covered as much as 28 percent of Earth.
38Glaciers2Climatic ChangesThese periods of widespread glaciation are known as ice ages.During this time, glaciers advanced and retreated many times over much of North America.The average air temperature on Earth was about 5°C lower during these ice ages than it is today.
39Valley Glaciers—Evidence of Valley Glaciers 2Valley Glaciers—Evidence of Valley GlaciersValley glaciers erode bowl-shaped basins, called cirques, into the sides of mountains.If two valley glaciers side by side erode a mountain, a long ridge called an arête forms between them.
40Valley Glaciers—Evidence of Valley Glaciers 2Valley Glaciers—Evidence of Valley GlaciersIf valley glaciers erode a mountain fromseveral directions, a sharpened peak called a horn might form.
41Valley Glaciers—Evidence of Valley Glaciers 2Valley Glaciers—Evidence of Valley GlaciersValley glaciers flow down mountain slopesand along valleys, eroding as they go.
42Valley Glaciers—Evidence of Valley Glaciers 2Valley Glaciers—Evidence of Valley GlaciersGlacially eroded valleys are U-shaped because a glacier plucks and scrapes soil and rock fromthe sides as well as from the bottom.
43Importance of Glaciers 2Importance of GlaciersToday, glaciers in polar regions and in mountains, continue to change the surface features of Earth.In addition to changing the appearance of Earth’s surface, glaciers leave behind sediments that are economically important.The sand and gravel deposits from glacial outwash and eskers are important resources.
44Section Check2Question 1The mixture of different-sized sediments left by a retreating glacier is called a __________.A. moraineB. outwashC. pluckD. till
45Section Check2AnswerThe answer is D. The sediments in outwash are deposited in layers, with the heaviest sediments dropping first.
46Section Check2Question 2__________ are winding ridges of sand and gravel deposited by glacial meltwater streams.A. aretesB. cirquesC. eskersD. moraines
47Section Check2AnswerThe answer is C. Eskers are glacial deposits formed by meltwater rivers and streams.
48Section Check2Question 3In which country would you expect to find continental glaciers?A. AustraliaB. DenmarkC. GreenlandD. South Africa
49Section Check2AnswerThe answer is C. Continental glaciers are found mostly near the poles in Antarctica and Greenland.
50Wind3Wind ErosionAir differs from other erosional forces because it usually cannot pick up heavy sediments.Wind carries and deposits sediments over large areas.
51Deflation 3 Wind erodes Earth’s surface by deflation and abrasion. When wind erodes by deflation, it blows across loose sediment, removing small particles such as silt and sand.
52Wind3AbrasionWhen windblown sediment strikes rock, the surface of the rock gets scraped and worn away by a process called abrasion.Wind acts like a sandblasting machine, bouncing and blowing sand grains along.The rocks becomes pitted and are worn down gradually.
53Wind3AbrasionThese sand grains strike against rock and break off small fragments.The rocks becomes pitted and are worn down gradually.
54Wind3AbrasionDeflation and abrasion happen to all land surfaces but occur mostly in deserts, beaches, and plowed fields.These areas have fewer plants to hold the sediments in place.When winds blow over them, they can be eroded rapidly.
55Wind3SandstormsWhen the wind blows forcefully in the sandy parts of deserts, sand grains bounce along and hit other sand grains, causing more and more grains to rise into the air.These windblown sand grains form a low cloud just above the ground.
56Dust Storms 3 Silt and clay particles are small and stick together. Wind3Dust StormsSilt and clay particles are small and stick together.A faster wind is needed to lift these fine particles of soil than is needed to lift grains of sand.However, after they are airborne, the wind can carry them long distances.Where the land is dry, dust storms can cover hundreds of kilometers.
57Wind3Dust StormsThese storms blow topsoil from open fields, overgrazed areas, and places where vegetation has disappeared.
58Reducing Wind Erosion— Windbreaks 3Reducing Wind Erosion— WindbreaksOne of the best ways to slow or stop wind erosion is to plant vegetation.For centuries, farmers have planted trees along their fields to act as windbreaks that prevent soil erosion.As the wind hits the trees, its energy of motion is reduced.It no longer is able to lift particles.
59Wind3RootsPlants with fibrous systems, such as grasses, work best at stopping wind erosion.Grass roots are shallow and slender with many fibers.They twist and turn between particles in the soil and hold it in place.
60Deposition by Wind—Loess 3Deposition by Wind—LoessWind deposits of fine-grained sediments are known as loess.Strong winds that blew across glacial outwash areas carried the sediments and deposited them.
61Deposition by Wind—Loess 3Deposition by Wind—LoessThe sediments settled on hilltops and in valleys.Once there, the particles packed together, creating a thick, unlayered, yellowish-brown-colored deposit.Loess is as fine as talcum powder.
62Dunes 3 A dune is a mound of sediments drifted by the wind. Dunes are common in desert regions.
63Moving Dunes 3 A sand dune has two sides. Wind3Moving DunesA sand dune has two sides.The side facing the wind has a gentler slope.The side away from the wind is steeper.Most dunes move, or migrate away from the direction of the wind.As they lose sand on one side, they build it up on the other.
64Wind3Dune ShapeThe shape of a dune depends on the amount of sand or other sediment available, the wind speed and direction, and the amount of vegetation present.One common dune shape is a crescent-shaped dune known as a barchan dune.This type of dune forms on hard surfaces where the sand supply is limited.
65Wind3Dune ShapeAnother common type of dune, called a transverse dune, forms where sand is abundant.In regions where the wind changes, star dunes, form pointed structures.
66Shifting Sediments 3 When dunes and loess form, the landscape changes. Wind3Shifting SedimentsWhen dunes and loess form, the landscape changes.Wind, like gravity, running water, and glaciers, shapes the land.New landforms created by these agents of erosion are themselves being eroded.Erosion and deposition are part of a cycle of change that constantly shapes and reshapes the land.
67Section Check3Question 1__________ is erosion by wind in which small particles are removed, leaving heavier material behind.A. abrasionB. deflationC. glaciationD. loess
68Section Check3AnswerThe answer is B. When wind erodes by deflation, it blows across loose sediment and removes small particles.
69Section Check3Question 2Mounds of sediment drifted by the wind are called __________.A. dunesB. loessC. shiftersD. windbreaks
70Section Check3AnswerThe answer is A. Dunes form when wind sweeps around an obstacle and drops sediment behind it.
71Section Check3Question 3Large wind deposits of fine grained sediments are known as __________.A. dunesB. loessC. outwashD. till
72Section Check3AnswerThe answer is B. Loess deposits are made up of tightly-packed, fine-grained sediments.
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