Chapter: Erosional Forces Table of Contents Section 3: WindWind Section 1: Erosion by Gravity Section 2: GlaciersGlaciers
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. Erosion and Deposition— What wears away sediments? Erosion by Gravity 1 1 Other causes of erosion, also called agents of erosion, are water, wind, and glaciers.
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. Erosion and Deposition— What wears away sediments? Erosion by Gravity 1 1 As ice melts, sediment is deposited, or dropped.
Agents 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. Dropping Sediments Erosion by Gravity 1 1
Rocks 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. Mass Movement Erosion by Gravity 1 1
Common types of mass movement include slump, creep, rockfalls, rock slides, and mudflows. Mass Movement Erosion by Gravity 1 1 Landslides are mass movements that can be one of these types or a combination of these types of mass movement.
When a mass of material slips down along a curved surface, the mass movement is called slump. Slump Erosion by Gravity 1 1
Sometimes a slump happens when water moves to the base of a slipping mass of sediment. Slump Erosion by Gravity 1 1 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.
Leaning trees and human-built structures show another mass movement called creep. Creep Erosion by Gravity 1 1 Creep occurs when sediments slowly shift their positions downhill. Creep is common in areas of frequent freezing and thawing.
Rockfalls happen when blocks of rock break loose from a steep slope and tumble through the air. Rockfalls and Rock Slides Erosion by Gravity 1 1 As they fall, these rocks crash into other rocks and knock them loose. More and more rocks break loose and tumble to the bottom.
During the winter, when ice freezes in the cracks of rocks, the cracks expand and extend. Rockfalls and Rock Slides Erosion by Gravity 1 1 In the spring, the pieces of rock break loose and fall down the mountainside.
Rock slides occur when layers of rock— usually steep layers—slip downslope suddenly. Rockfalls and Rock Slides Erosion by Gravity 1 1 They commonly occur on mountainous areas or in areas with steep cliffs.
A mudflow is a thick mixture of sediments and water flowing down a slope. Mudflows Erosion by Gravity 1 1
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. Mudflows usually occur in areas that have thick layers of loose sediments. Mudflows Erosion by Gravity 1 1 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.
The water adds mass and creates fluid pressure between grains and layers of sediment. Mudflows, rock slides, rockfalls, creep, and slump are most likely to occur on steep slopes, and they all depend on gravity to make them happen. Mudflows Erosion by Gravity 1 1 Also, all types of mass movement occur more often after a heavy rain.
When people build homes on steep slopes, they constantly must battle naturally occurring erosion. Consequences of Erosion — Building on Steep Slopes Erosion by Gravity 1 1 Some steep slopes are prone to slumps because of weak sediment layers underneath.
One of the best ways to reduce erosion is to plant vegetation. Making Steep Slopes Safe Erosion by Gravity 1 1 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.
Section Check 1 1 Question 1 What is erosion? Answer Erosion is a process that wears away surface materials and moves them from one place to another.
Section Check 1 1 Question 2 What type of mass movement is shown in this illustration? A. creep B. deposition C. mudflow D. slump
1 1 Section Check Answer The 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.
Section Check 1 1 Question 3 How does a rock slide differ from creep?
Section Check 1 1 Answer Rock slides occur suddenly, are fast and move layers of rock down steep slopes. Creep is the slow shifting of position of sediments on a slope.
How Glaciers Form and Move Glaciers 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. Glaciers 2 2
How Glaciers Form and Move The mass slowly begins to flow in a thick, plasticlike lower layer, and ice slowly moves away from its source. Glaciers 2 2 A large mass of ice and snow moving on land under its own weight is a glacier.
Ice Eroding Rock Glaciers are agents of erosion. Glaciers 2 2 Glaciers then carry eroded material along and deposit it somewhere else. Click image to view movie. As glaciers pass over land, they erode it, changing features on the surface.
Plucking When glacial ice melts, water flows into cracks in rocks. Glaciers 2 2 Later, the water refreezes in the cracks, expands, and fractures the rock. Pieces of rock then are lifted out by the ice.
Plucking Glaciers 2 2 This process, called plucking, results in boulders, gravel, and sand being added to the bottom and sides of a glacier.
Transporting and Scouring A glacier can transport huge volumes of sediment and rock. Glaciers 2 2 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.
Transporting and Scouring These marks, called grooves, are deep, long, parallel scars on rocks. Glaciers 2 2 Shallower marks are called striations.
This mixture of different-sized sediments is called till. As it retreats, a jumble of boulders, sand, clay, and silt is left behind. Ice Depositing Sediment When glaciers begin to melt, sediment drops or is deposited, on the land. Glaciers 2 2 When a glacier melts and begins to shrink back, it is said to retreat.
Such a ridge is called a moraine. Because of this, a big ridge of material piles up. Moraine Deposits Till is also deposited at the end of glacier when it is not moving forward. Glaciers 2 2 Rocks and soil are moved to the end of the glacier, much like items on a grocery store conveyor belt.
Outwash Deposits Material deposited by the meltwater from a glacier, most often beyond the end of the glacier, is called outwash. Glaciers 2 2 Meltwater carries sediments and deposits them in layers.
Eskers Another type of outwash deposit looks like a long, winding ridge. Glaciers 2 2 This deposit forms in a melting glacier when meltwater forms a river within ice.
Eskers This river carries sand and gravel and deposits them within its channel. Glaciers 2 2 When the glacier melts, a winding ridge of sand and gravel, called an esker, is left behind.
Continental Glaciers Today, continental glaciers cover ten percent of Earth, mostly near the poles in Antarctica and Greenland. Glaciers 2 2 Continental glaciers are huge masses of ice and snow. Continental glaciers are thicker than some mountain ranges.
Climatic Changes In the past, continental glaciers covered as much as 28 percent of Earth. Glaciers 2 2
Climatic Changes These periods of widespread glaciation are known as ice ages. Glaciers 2 2 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.
Valley Glaciers—Evidence of Valley Glaciers Valley glaciers erode bowl-shaped basins, called cirques, into the sides of mountains. Glaciers 2 2 If two valley glaciers side by side erode a mountain, a long ridge called an arête forms between them.
Valley Glaciers — Evidence of Valley Glaciers Glaciers 2 2 If valley glaciers erode a mountain from several directions, a sharpened peak called a horn might form.
Valley Glaciers — Evidence of Valley Glaciers Glaciers 2 2 Valley glaciers flow down mountain slopes and along valleys, eroding as they go.
Valley Glaciers — Evidence of Valley Glaciers Glaciers 2 2 Glacially eroded valleys are U-shaped because a glacier plucks and scrapes soil and rock from the sides as well as from the bottom.
Importance of Glaciers Today, glaciers in polar regions and in mountains, continue to change the surface features of Earth. Glaciers 2 2 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.
Section Check 2 2 Question 1 The mixture of different-sized sediments left by a retreating glacier is called a __________. A. moraine B. outwash C. pluck D. till
Section Check 2 2 Answer The answer is D. The sediments in outwash are deposited in layers, with the heaviest sediments dropping first.
Section Check 2 2 Question 2 __________ are winding ridges of sand and gravel deposited by glacial meltwater streams. A. aretes B. cirques C. eskers D. moraines
Section Check 2 2 Answer The answer is C. Eskers are glacial deposits formed by meltwater rivers and streams.
Section Check 2 2 Question 3 In which country would you expect to find continental glaciers? A. Australia B. Denmark C. Greenland D. South Africa
Section Check 2 2 Answer The answer is C. Continental glaciers are found mostly near the poles in Antarctica and Greenland.
Wind Erosion Air differs from other erosional forces because it usually cannot pick up heavy sediments. Wind carries and deposits sediments over large areas. Wind 3 3
Deflation 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. Wind 3 3
Abrasion When 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. Wind 3 3 The rocks becomes pitted and are worn down gradually.
Abrasion These sand grains strike against rock and break off small fragments. The rocks becomes pitted and are worn down gradually. Wind 3 3
Abrasion Deflation 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. Wind 3 3 When winds blow over them, they can be eroded rapidly.
Sandstorms When 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. Wind 3 3
Dust Storms Silt 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. Wind 3 3 However, after they are airborne, the wind can carry them long distances. Where the land is dry, dust storms can cover hundreds of kilometers.
Dust Storms These storms blow topsoil from open fields, overgrazed areas, and places where vegetation has disappeared. Wind 3 3
Reducing Wind Erosion — Windbreaks One of the best ways to slow or stop wind erosion is to plant vegetation. Wind 3 3 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.
Roots Plants with fibrous systems, such as grasses, work best at stopping wind erosion. Wind 3 3 Grass roots are shallow and slender with many fibers. They twist and turn between particles in the soil and hold it in place.
Deposition by Wind — Loess Wind deposits of fine-grained sediments are known as loess. Wind 3 3 Strong winds that blew across glacial outwash areas carried the sediments and deposited them.
Deposition by Wind — Loess Wind 3 3 The 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.
Dunes Wind 3 3 A dune is a mound of sediments drifted by the wind. Dunes are common in desert regions.
Moving Dunes Wind 3 3 A 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.
Dune Shape Wind 3 3 The 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.
Dune Shape Wind 3 3 Another common type of dune, called a transverse dune, forms where sand is abundant. In regions where the wind changes, star dunes, form pointed structures.
Shifting Sediments Wind 3 3 When 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.
Section Check 3 3 Question 1 __________ is erosion by wind in which small particles are removed, leaving heavier material behind. A. abrasion B. deflation C. glaciation D. loess
Section Check 3 3 Answer The answer is B. When wind erodes by deflation, it blows across loose sediment and removes small particles.
Section Check 3 3 Question 2 Mounds of sediment drifted by the wind are called __________. A. dunes B. loess C. shifters D. windbreaks
Section Check 3 3 Answer The answer is A. Dunes form when wind sweeps around an obstacle and drops sediment behind it.
Section Check 3 3 Question 3 Large wind deposits of fine grained sediments are known as __________. A. dunes B. loess C. outwash D. till
Section Check 3 3 Answer The answer is B. Loess deposits are made up of tightly-packed, fine-grained sediments.
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