12 s060902 e_the process of weathering rocks

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 strains the walls around the cracks and causes cracks to deepen and widen.

• When the ice thaws, water can flow further into the rock. Repeated freeze-thaw cycles weaken the rocks.

• Over time, the rock breaks up along the cracks into angular pieces.

Ice Wedging

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

• When erosion removes the overlying rock, the once buried rocks are exposed and the pressure on them is released.

• The outer rock parts tend to expand. The expansion sets up stresses which cause fractures to form parallel to the rock surface.

• Over time, sheets of rock break away from the exposed rocks along the fractures.

Mechanical Weathering :Exfoliation

Exfoliation

• Thermal weathering results from expansion or contraction of rock, caused by extreme changes in temperature.

• 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. – Chemical reactions break down the bonds

holding the rocks together, causing them to fall apart.

Chemical Weathering: Oxidation

• Oxidation takes place when oxygen combines with other elements in rocks to form new types of rock. • These new substances are usually much

softer and thus, easier for other forces to break apart. • 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.–Most common reaction is the hydrolysis of

feldspars producing clay (kaolinite)

Hydrolysis

Chemical Weathering: Carbonation

• Carbonation takes place when carbon dioxide reacts with water or rain, forming a weak carbonic acid.– 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.

• The substances in rocks form new types of softer substances.

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)