Chapter: Weathering and Erosion Table of Contents Section 1: Weathering and Soil Section 2: Erosion...

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Chapter: Weathering and Erosion

Table of ContentsTable of Contents

Section 1: Weathering and Soil

Section 2: Erosion of Earth’s Surface

Formation

• Weathering is a mechanical or chemical surface process that breaks rocks into smaller pieces.

• Freezing and thawing, oxygen in the air, and even plants and animals can affect the stability of rock.

Weathering

Weathering and Soil FormationWeathering and Soil Formation

• Mechanical weathering breaks rocks into smaller pieces without changing them chemically.

• The small pieces are identical in composition to the original rock.

• Two of the many causes of mechanical weathering are ice wedging and living organisms.

Mechanical Weathering

• Water seeps into cracks.• The deeper the cracks are, the deeper water

can seep in.

Ice Wedging

• The water freezes and expands, forcing the cracks to open further.

Ice Wedging

• The ice melts.• If the temperature falls below freezing again,

the process will repeat itself.

Ice Wedging

• Plant roots grow deep into cracks in rock where water collects.

• As they grow, roots become thicker and longer, slowly exerting pressure and wedging rock apart.

Plants and Animals

• Gophers and prairie dogs also weather rock—as do other animals that burrow in the ground.

• As they burrow through sediment or soft sedimentary rock, animals break rock apart.

Plants and Animals

• Chemical weathering occurs when the chemical composition of rock changes.

• This kind of weathering is rapid in tropical regions where it's moist and warm most of the time.

Chemical Weathering

• The table summarizes the rates of chemical weathering for different climates.

Chemical Weathering

• Some rocks react chemically with natural acids in the environment.

• When water mixes with carbon dioxide in air or soil, for example, carbonic acids forms.

Natural Acids

• Although carbonic acid is weak, it reacts chemically with many rocks.

• When carbonic acid comes in contact with rocks like limestone, dolomite, and marble, they dissolve.

Natural Acids

• Many plants produce a substance called tannin.

• In solution, tannin forms tannic acid.

• This acid dissolves some minerals in rocks.

Plant Acids

• Moss growing on rocks can cause chemical weathering.

• Oxidation is the effect of chemical changes caused by oxygen.

• When iron-containing materials such as steel are oxidized, a chemical reaction causes the material to rust.

Effect of Oxygen

• When some iron-containing minerals are exposed to oxygen, they can weather to minerals that are like rust.

• This leaves the rock weakened, and it can break apart.

Effect of Oxygen

• Soil is a mixture of weathered rock, organic matter, water, and air that supports the growth plant life.

• Organic matter includes decomposed leaves, twigs, roots, and other material.

• One factor affecting soil formation is the kind of parent rock that is being weather.

• For example, in areas where sandstone is weathered, sandy soil forms.

Parent Rock

Click image to view movie.

• The topography, or surface features, of an area also influence the types of soil that develop.

• On steep hillsides, soil has little chance of developing.

• This is because rock fragments move downhill constantly.

The Slope of the Land

• Climate affects the amount of organic material in soil.

• Soils in desert climates contain little organic material.

• However, in warm, humid climates, vegetation is lush and much organic material is present.

Climate

• The result is the formation of a dark-colored material called humus.

• Most of the organic matter in soil is humus.

• Humus helps soil hold water and provides nutrients that plants need to grow.

Climate

• It can take thousands of years for some soil to form.

• As soils develop, they become less like the rock from which they formed.

• Thicker, well-developed soils often are found in areas where weathering has gone on undisturbed for a long period of time.

• Organisms influence soil development.• Lichens can grow directly on rock.• As they grow,

they take nutrients from the rock that they are starting to break down, forming a thin soil.

Organisms

• After a soil has formed, many types of plants such as grasses and trees can grow.

• The roots of these plants further break down the parent rock.

Organisms

• Dead plant material such as leaves accumulates and adds organic matter to the soil.

Organisms

Section CheckSection Check

11Question 1

Explain how a tree can break apart rock.

As the tree grows, its roots become thicker and longer. The roots exert pressure on the rocks eventually breaking them apart.

Answer

IN: 6.3.15

11Section CheckSection Check

Question 2

How is a rusty nail an example of chemical weathering?

AnswerWhen iron-containing materials, such as a nail, are exposed to oxygen, a chemical reaction occurs and rust is produced. The rust has a different chemical composition than the nail.

IN: 6.3.18

Question 3

Which of the following does NOT affect soil formation?

A. climateB. carbonic acidC. organismsD. topography

IN: 6.3.15

The answer is B. Many factors affect soilformation. Carbonic acid is responsible forchanging the chemical composition of mineralsin rock.

Answer

IN: 6.3.15

Agents of Erosion

• Erosion is the wearing away and removal of rock or sediment.

• Erosion occurs because gravity, ice, wind, and water sculpt Earth's surface.

Erosion of Earth’s SurfaceErosion of Earth’s Surface

Gravity

• Gravity pulls everything on Earth toward its center.

• When gravity alone causes rock or sediment to move down a slope, the erosion is called mass movement.

• Mass movements can occur anywhere there are hills or mountains.

• The process in which sediments move slowly downhill, is called creep.

• Creep is common where freezing and thawing occur.

Creep• When soil freezes, particles are lifted.

• When it thaws, the particles are pulled downhill by gravity.

• Eventually, large amounts of sediment are moved by this process.

• A slump occurs when a mass of rock or sediment moves downhill, leaving a curved scar.

• Slumps frequently occur on slopes that have been undercut by erosion, such as those above the bases of cliffs that have been eroded by waves.

Rock Slides

• During a rock slide layers of rock break loose from slopes and slide to the bottom.

• The rock layers often bounce and break apart during movement.

• This produces a huge, jumbled pile of rocks at the bottom of the slope.

Mudflows

• A mudflow is a mass of wet sediment that flows downhill over the ground surface.

• Some mudflows can be thick and flow slowly downhill at rates of a few meters per day.

Mudflows• Other mudflows can be much more fluid and

move down slope at speeds approaching 160km/h.

Ice• When the ice in a

glacier becomes thick enough, its own weight causes it to flow downhill under the influence of gravity.

• A glaciers move over Earth's surface, they erode materials from some areas and deposit sediment in other areas.

Ice• Continental glaciers in polar regions cover

about ten percent of Earth.• These glaciers are so large and thick that they

can bury mountain ranges.

Ice• Valley glaciers are much smaller and are

located in high mountains where the average temperature isn't warm enough to melt the ice sheets.

Glacial Erosion

• Glaciers can erode rock in two different ways.

• Ice can pull out pieces of rock.

• This causes the rock to erode slowly. Loose pieces of rock freeze into the bottom of the glacier and are dragged along as the glacier moves.

Glacial Erosion• This scratching is the second way that

glaciers can erode rock.• Scratching

produces large grooves or smaller striations in the rock underneath.

Effects of Glacial Erosion

• In mountains, valley glaciers can remove rock from the mountaintops to form large bowls, called cirques (SURKS), and steep peaks.

• When a glacier moves into a stream valley, it erodes rock along the valley sides, producing a wider, U-shaped valley.

Glacial Deposition

• When stagnant glacier ice melts or when ice melts at the bottom of a flowing glacier or along its edges, the sediment the ice was carrying gets left behind on Earth's surface.

• This sediment, deposited directly from glacier ice, is called till.

Glacial Deposition

• A lot of melting occurs around glaciers.

• So much water can be produced that streams often flow away from the glacier.

• These steams carry and deposit sediment.

• Sand and gravel deposits laid down by these streams are called outwash.

• When wind blows across loose sediments like silt and sand, it lifts and carries it.

• Wind often leaves behind particles too heavy to move.

• This erosion of the land by wind is called deflation.

• Abrasion is a form of erosion that can make pits in rocks and produce smooth, polished surfaces.

• Abrasion is common in some deserts and in some cold regions with strong winds.

• If sand deposit continues to grow, a sand dune might form.

• Sand dunes move when wind carries sand up one side of the dune and it avalanches down the other.

• Sometimes, wind carries only fine sediment called silt.

• When this sediment is deposited, an accumulation of silt called loess (LOOS) can blanket Earth's surface.

• Loess often is deposited downwind of large deserts and deflated glacial outwash deposits.

• Water that flows over Earth's surface is called runoff.

• Runoff is an important agent of erosion.

• The more speed water has, the more material it can carry with it.

Sheet Flow

• When water flows downhill as a thin sheet, it is called sheet flow.

• This thin sheet of water can carry loose sediment grains with it, causing erosion of the land.

• This erosion is called sheet erosion.

Rills and Gullies

• Where a sheet of water flows around obstacles and becomes deeper, rills can form.

• Rills are small channels cut into the sediment at Earth's surface.

• These channels carry more sediment than can be moved by sheet flow.

Rills and Gullies

• As runoff continues to flow through the rills, more sediment erodes and the channel widens and deepens.

• When the channels get to be about 0.5m across, they are called gullies.

Streams

• Gullies often connect to stream channels.

• Most streams have water flowing through them continually, but some have water only during part of the year.

Streams

• In mountainous and hilly regions, streams flow down steep slopes.

• This type of stream typically has white-water rapids and may have water falls.

Streams• As streams move out of the mountains and

onto flatter land, they begin to flow more smoothly.

• The streams might snake back and forth across their valley, eroding and depositing sediments along their sides.

Shaping Earth’s Surface

• Over long periods of time, water moving in a stream can have enough power to cut large canyons into solid rock.

• Many streams together can sculpt the land over a wide region, forming valleys and leaving some rock as hills.

Shaping Earth’s Surface

• When rivers enter oceans or lakes, the water slows and sediment is deposited.

• This can form large accumulations of sediment called deltas.

22Question 1

Compare and contrast continental glaciers and valley glaciers.

22Answer

Continental glaciers are located in Polar Regions and are very large and thick. Valley glaciers are much smaller and are found high up in the mountains where the average temperature is not warm enough to melt the ice. Glaciers can erode rock by pulling out pieces of rock underneath them and dragging them along the surface as the glacier moves.

22Question 2

Which is NOT caused by water erosion?

A. abrasionB. canyonsC. gulliesD. sheet erosion

22Answer

The correct answer is A. Abrasion results when wind carrying sediment wears down other rocks.

22Question 3

Identify and describe the process being illustrated by this diagram?

IN: 6.7.2

22Answer

The process shown is called creep. Creep occurs when sediments slowly move downhill. It is common in areas where freezing and thawing occur. As the ice expands in the soil, sediments move up. When the soil thaws, the sediments move further downslope by gravity.

IN: 6.7.2

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Chapter: Weathering and Erosion Table of Contents Section 1: Weathering and Soil Section 2: Erosion...

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