The Process of

Weathering Rocks

Weathering

• The breaking down of rock into smaller pieces that remain next to each other.

• Weathering forms sediments.

• There are two types of weathering.

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.

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 cracks and causes cracks to get deeper and wider.

• Water can flow deeper into the rock as the ice thaws. As the cycle repeats, rocks grow weaker over time.

• Along the cracks, the rock will break into angular pieces over time.

Ice Wedging

• Rocks formed deep in the Earth are under a huge amount of pressure.

• When the overlying rock is removed by erosion, pressure is 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

Exfoliation

• 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 and contract at different rates causing the rock to split

• The outer layers peel off into thin sheets.

Mechanical Weathering: Thermal

Thermal

• Biotic weathering is caused by living organisms.

• Most often, plant roots are the cause.

• They act as a wedge, widening and extending the cracks.

• Digging animals can also cause weathering.

Mechanical Weathering : Biotic (Force)

Biotic (Physical Force)

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.

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.

Oxidation

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).

Hydrolysis

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 and limestone to decompose.

• New types of softer substances are formed within the rocks.

Carbonation

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.

Chemical Weathering: Biotic (Acid)