Physical (Mechanical) Weathering

We are looking for descriptions and examples of destructive processes:•Ice wedging•Root pry•Exfoliation•Abrasion

Ice Wedging• Water seeps into cracks in rocks• Temperature decreases and the water in the rocks

freezes• As water freezes, it expands, forcing the rock apart

– making the crack larger (deeper & wider)• When the ice melts, the water travels deeper into

the cracks of the rock and eventually the rock breaks apart

• Wedge: a simple machine that forces things apart

Root Pry

• Roots of trees and other plants enter cracks in rocks

• As the root grows, they force the cracks deeper and further apart.

• Eventually, the rock cracks and thus is a form of physical weathering

Exfoliation

• Rocks weather by peeling off in sheets• Erosion moves overlying heavy rock material

and releases pressure• The rocks can then expand more and crack

parallel to the surface – eventually these parallel cracks fall off in “sheets”

Abrasion

• The grinding away of rock by rock particles carried by water, ice, wind, or gravity.

• Example: wind driven sand

Chemical Weathering

(Chemical reactions that break down bonds that hold rocks together)

We are looking for descriptions and examples of destructive processes:•Oxidation•Carbonation•Hydrolysis•Acids•Lichens (biological)

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.

Oxidation makes rocks softer. It is similar to an iron bar rusting. Since there is a lot of iron in many rocks, oxidation often happens.

Carbonation

• Takes place when carbon dioxide reacts with certain types of rocks forming a solution that can easily be carried away by water.

• Key element: carbon dioxide• Carbonation occurs on rocks which contain

calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide to form a weak acid which reacts with calcium carbonate

Hydrolysis

• Occurs when water combines with the substances in rocks to form new types of substances, which are softer than the original rock types. This allows other forces, such as mechanical weathering, to more easily break them apart.

• Hydrolysis usually causes rocks to expand and then mechanical weathering can begin

Acids

• Rain water contains acids• Dissolved carbon dioxide mixes with water

which produces weak carbonic acid which breaks down rocks

Lichens• (Biological weathering) is the actual molecular

breakdown of minerals. There are things called lichens (combinations of fungi and algae) which live on rocks. Lichens slowly eat away at the surface of rocks.

• You might find more activities like lichens near oceans where the air is humid and cooler.

Rate of Weathering• Depends on two factors:1. Type of rock2. Climate

- Chemical and mechanical weathering occur faster in ____________ climates. Why?

- Chemical weathering occurs faster in __________ temperature. Why?

The fastest rate of weathering is in what type of climate? (temperature and humidity)