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The Process of

Weathering Rocks

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Weathering

The breaking down of rock into smaller

pieces that remain next to each other. Weathering forms sediments.

There are two types of weathering.

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Two Types of Weathering

1. Mechanical weathering is the breaking down

of rock without any change in the chemical

composition of the rock.

Sometimes called physical weathering

Rock is torn apart by physical force, rather than by

chemical breakdown Smaller pieces do not move to a new location, but

remain next to one another until erosion carries

them away.

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Mechanical Weathering:

Ice Wedging

Water enters the cracks in rocks and then freezes

and expands about 10%.

The ice puts stress on the walls around the cracksand causes cracks to get deeper and wider.

Water can flow deeper into the rock as the ice

thaws. As the cycle repeats, rocks grow weakerover time.

Along the cracks, the rock will break into angular

pieces over time.

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Ice Wedging

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Rocks formed deep in the Earth are under a huge amount

of pressure.

When the overlying rock is removed by erosion, pressureis released and the once buried rocks are exposed.

Expansion occurs along the outer parts of rocks. This

stress will cause fractures to form parallel to the rock

surface.

Along the exposed rock fractures, sheets of rock will break

away.

Mechanical Weathering :

Exfoliation

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Exfoliation

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Thermal weathering is a result of extreme

changes in temperature that causes the expansion

or contraction of rock.

Common in deserts, where it is hot in the day and

cold at night; different minerals expand andcontract at different rates causing the rock to split

The outer layers peel off into thin sheets.

MechanicalWeathering:

Thermal

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Thermal

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Biotic weathering is caused by living

organisms. Most often, plant roots are the cause.

They act as a wedge, widening andextending the cracks.

Digging animals can also cause weathering.

Mechanical Weathering :

Biotic (Force)

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Biotic(Physical Force)

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Two Types of Weathering

2. Chemical weathering is the breaking down of

rock into smaller pieces because of chemical

changes within the rock.

Rock reacts with water, gases, and solutions (may

be acidic); these reactions will add or remove

elements from minerals.

Rocks fall apart because the bonds holding them

together are broken down by chemical reactions.

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Chemical Weathering:

Oxidation When oxygen combines with other elements

in rocks, a chemical reaction known as

oxidation occurs and new types of rock are

formed.

The new types of rock are easier to break apart

because they are much softer than the original

substances.

A reddish-brown coloration on the surface is

called rusting.

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Oxidation

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Chemical Weathering:

Hydrolysis

Hydrolysis is the weathering reaction that

occurs when water and chemical

compounds in a rock meet.

This results in the decomposition of the rock

surface by forming new compounds. The most common reaction is the hydrolysis

of feldspars producing clay (kaolinite).

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Hydrolysis

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Chemical Weathering:

Carbonation

When carbon dioxide reacts with water or rain,

carbonation occurs and a weak carbonic acid is

formed. This is the same acid found in soda.

The acid is too weak to harm plants and

animals, but slowly causes feldspars andlimestone to decompose.

New types of softer substances are formed within

the rocks.

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Carbonation

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Chemical Weathering:

Biotic (Acid)

Lichens and similar plants live on the

surfaces of rocks. Plants lower the local pH to make it more acidic.

Their roots give off a chemical that dissolves

rocks and minerals.

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Chemical Weathering:

Biotic (Acid)