Chapter: Weathering and Erosion

Section 1: Weathering and Soil

Formation

Minerals form rocks

Rocks form soil

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

Weathering

Weathering and Soil Formation

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

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

Mechanical Weathering

Weathering and Soil Formation

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

Chemical Weathering

Weathering and Soil Formation

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

• Organic matter includes decomposed leaves, twigs, roots, and other materials such as decaying animals.

Soil

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Causes of Mechanical

Weathering

1.Ice Wedging – read page 317 – figure 2

1

Freezing and thawing break up rocks

Ice crystals occupy more space than water

Exerting pressure on the rock .

• Plant roots grow deep into cracks in rock

• Roots become thicker and longer, exerting pressure and breaking the rock apart.

2. Plants

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• Animals can break rocks apart when digging burrows

3. Animals

Weathering and Soil Formation

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Causes of Chemical

Weathering

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

• EX: carbonic acid

• Carbonic acid is the result of the reaction between water and carbon dioxide

• Carbonic acid can dissolve rocks

1. Natural Acids

Weathering and Soil Formation

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• Many plants produce a substance called tannin.

• In solution, tannin forms tannic acid.

• This acid dissolves some minerals in rocks and the

rock can break

2. Plant Acids

Weathering and Soil Formation

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• Oxidation is the effect of chemical changes caused by oxygen, when in contact with iron

• Some minerals contain IRON, that can react with the oxygen in the air

• This weakens the rock and it can break

3. Effect of Oxygen

Weathering and Soil Formation

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

Soil

Weathering and Soil Formation

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Soil Formation1

• The type of soil will depend on the type of rock .

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

• Limestone will form a soil rich in clay

Parent Rock – is the rock that will be weathered, forming the soil

Weathering and Soil Formation

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

• This is because rock fragments move downhill constantly.

The Slope of the Land

Weathering and Soil Formation

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• In areas where the land is flat, water and wind will help deposit sediments that will form rocks and soil

The Slope of the Land

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1) Affects the chemical weathering of Rocks –important for soil formation

Chemical weathering happens faster when the weather is wet and hot

Climate affects soil formation for two reasons

Weathering and Soil Formation

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2) Climate affects the amount of organic material in soil.

• In warm, humid climates, vegetation is lush

• There is a lot of decomposition of the dead plants and animals resulting in a lot of organic matter in the soil

• This organic matter is called Hummus

Climate

Weathering and Soil Formation

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

• Hummus makes the soil rich.

Climate

Weathering and Soil Formation

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

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

Time

Weathering and Soil Formation

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• Organisms influence soil development.

Ex: Lichens

Organisms

Weathering and Soil Formation

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• Lichens can remove nutrients from rocks that are breaking down

• This action of the lichens will start forming a thin soil , allowing other plants to grow.

• The roots of these plants further break down the parent rock, creating more soil

Organisms

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• Important aspects of soil

formation and maintenance:

Minerals are considered non-

renewable resources.

It takes millions of years to form

minerals

It is important to recycle all

materials obtained from

minerals…..

Minerals form rocks and

Rocks form soil

• Unplanned use of soil, mining and

deforestation can increase erosion.

the consequences are:

1) loss of land that is good for agriculture

2) polluted streams and other waterways

3) increased flooding

Importance of vegetation for soils

• Plant roots hold the soil in position

and prevent it from being washed away

by erosion agents such as wind and

water.

Importance of vegetation for soils

• The presence of vegetation increases

the quality of the soil ( decomposition

of dead plants will contribute to the

formation of hummus, that fertilizes the

soil)