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Table of Contents. Chapter: Erosional Forces. Section 1: Erosion by Gravity. Section 2: Glaciers. Section 3: Wind. Erosion by Gravity. 1. Erosion and Deposition— What wears away sediments?.
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Table of Contents Chapter: Erosional Forces Section 1: Erosion by Gravity Section 2: Glaciers Section 3: Wind
Erosion by Gravity 1 Erosion 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.
Erosion by Gravity 1 Erosion 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.
Erosion by Gravity 1 Dropping Sediments • 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.
Erosion by Gravity 1 Mass Movement • 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.
Erosion by Gravity • Landslides are mass movements that can be one of these types or a combination of these types of mass movement. 1 Mass Movement • Common types of mass movement include slump, creep, rockfalls, rock slides, and mudflows.
Erosion by Gravity 1 Slump • When a mass of material slips down along a curved surface, the mass movement is called slump.
Erosion by Gravity 1 Slump • Sometimes 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.
Erosion by Gravity 1 Creep • Leaning 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.
Erosion by Gravity 1 Rockfalls and Rock Slides • Rockfalls 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.
Erosion by Gravity 1 Rockfalls and Rock Slides • During 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.
Erosion by Gravity 1 Rockfalls and Rock Slides • Rock slides occur when layers of rock—usually steep layers—slip downslope suddenly. • They commonly occur on mountainous areas or in areas with steep cliffs.
Erosion by Gravity 1 Mudflows • A mudflow is a thick mixture of sediments and water flowing down a slope.
Erosion by Gravity 1 Mudflows • Mudflows 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.
Erosion by Gravity 1 Mudflows • 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. • 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.
Erosion by Gravity 1 Consequences of Erosion— Building on Steep Slopes • When 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.
Erosion by Gravity 1 Making Steep Slopes Safe • One 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.
Section Check 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 Question 2 What type of mass movement is shown in this illustration? A. creep B. deposition C. mudflow D. slump
Section Check 1 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 Question 3 How does a rock slide differ from creep?
Section Check 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. 1 Answer
Glaciers 2 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 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. • A large mass of ice and snow moving on land under its own weight is aglacier.
Glaciers • As glaciers pass over land, they erode it, changing features on the surface. 2 Ice Eroding Rock • Glaciers are agents of erosion. • Glaciers then carry eroded material along and deposit it somewhere else. Click image to view movie.
Glaciers 2 Plucking • When 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.
Glaciers • This process, called plucking, results in boulders, gravel, and sand being added to the bottom and sides of a glacier. 2 Plucking
Glaciers 2 Transporting and Scouring • A 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.
Glaciers 2 Transporting and Scouring • These marks, called grooves, are deep, long, parallel scars on rocks. • Shallower marks are called striations.
Glaciers 2 Ice Depositing Sediment • When 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.
Glaciers 2 Moraine Deposits • Till 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.
Glaciers 2 Outwash Deposits • Material 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.
Glaciers 2 Eskers • Another type of outwash deposit looks like a long, winding ridge. • This deposit forms in a melting glacier when meltwater forms a river within ice.
Glaciers 2 Eskers • This 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.
Glaciers 2 Continental Glaciers • Today, 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.
Glaciers 2 Climatic Changes • In the past, continental glaciers covered as much as 28 percent of Earth.
Glaciers 2 Climatic Changes • These 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.
Glaciers 2 Valley Glaciers—Evidence of Valley Glaciers • Valley 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.
Glaciers • If valley glaciers erode a mountain from severaldirections, a sharpened peak called a horn might form. 2 Valley Glaciers—Evidence of Valley Glaciers
Glaciers • Valley glaciers flow down mountain slopes and along valleys, eroding as they go. 2 Valley Glaciers—Evidence of Valley Glaciers
Glaciers • Glacially eroded valleys are U-shaped because a glacier plucks and scrapes soil and rock from the sides as well as from the bottom. 2 Valley Glaciers—Evidence of Valley Glaciers
Glaciers 2 Importance of Glaciers • Today, 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.
Section Check 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 Answer The answer is D. The sediments in outwash are deposited in layers, with the heaviest sediments dropping first.
Section Check 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 Answer The answer is C. Eskers are glacial deposits formed by meltwater rivers and streams.
Section Check 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 Answer The answer is C. Continental glaciers are found mostly near the poles in Antarctica and Greenland.
Wind 3 Wind Erosion • Air differs from other erosional forces because it usually cannot pick up heavy sediments. • Wind carries and deposits sediments over large areas.
