Weathering and Erosion

Definition: weathering

Weathering is the physical or chemical alteration of rocks near or at the Earth’s surface

As a rule: ROCKS are NOT HAPPY BEING EXPOSED TO THE AIR!

Types of weathering:

Physical weathering: when a rock is broken down into smaller parts by being acted on physically.

There are several types of physical weathering:

1. Frost wedging (aka frost action)

2. Root wedging

3. Abrasion (aka sandblasting)

Frost Wedging

Frost wedging occurs as a result of a special property of water. Nearly all matter contracts or shrinks when it is cooled – water, however EXPANDS!

• Definition: Frost wedging is physical weathering where rock is split apart by the expansion of water freezing in small cracks.

Some examples of frost wedging:

Root Wedging

Plants and trees send out roots to seek water and nutrients from the ground. These roots can move into small spaces and cracks in their search.

When roots end up in cracks in rocks, they eventually grow larger and can split the rock apart.

Definition: root wedging is physical weathering caused by plant and tree roots splitting rock apart.

Abrasion (including sand-blasting)

Wind and water can carry tiny particles of debris – rock and dust.

When these particles are washed against or blown past rock, it can wear the rock down like sanding wood.

• Definition: abrasion is physical weathering where small particles moving in wind or water abrade rock, smoothing it out.

Some rocks are abraded by wind-blown sand in arid regions.

Abrasion also happens at beaches and in rivers and streams – here the particles doing the weathering are carried by the water.

Types of weathering:

Chemical weathering: when a rock is broken down into smaller parts by having its chemical structure changed.

There are several types of chemical weathering. We will be most concerned wtih:

1. Oxidation

2. Dissolution

Oxidation:

When oxygen is added to the chemical structure of a mineral or rock, it changes the physical structure of the mineral or rock. This is best seen in rust.

Definition: oxidation is chemical weathering where oxygen is added to other elements.

A chemical formula change might look like this:

Iron + oxygen = iron oxide or…. Fe + O -> Fe2O3

Iron oxide is rust. Notice that while iron is hard and strong, rust is not – it easily breaks apart… (Chemical weathering)

Dissolution

Many materials are dissolved by water. From salt and sugar to carbon dioxide and other gases, water dissolves many things.

Acids can also dissolve materials.

Definition: dissolution is chemical weathering where rock is dissolved by water or acid.

Factors that increase weathering:

• Exposure• Particle size• Mineral composition• ClimateWhy????? Think about it!

Meet with a partner, and talk about why each of these things would increase weathering. Then find another two people and share your ideas. We’ll talk about them!