Chapter 6Weathering and Soil

Types of Weathering

• Mechanical Weathering – Breaks apart rock without changing its chemical composition

• Examples– Plant roots grow into the cracks of rocks– Animals burrowing– Ice Wedging

Types of Weathering

• Chemical Weathering – Breaks up rocks due to a change in their chemical composition. (Water and Oxygen are factors)

• Examples– Acid Rain– Rust

Mechanical vs. Chemical

Mechanical

Chemical

Ice Wedging

• Type of mechanical weathering

• This happens when water gets inside rocks, freezes, and then expands causing the rock to break.

Carbonic Acid

• When water mixes with carbon dioxide in the air, carbonic acid forms.

• Type of chemical weathering

• Comes in contact with different rocks, and reacts with the minerals in them.

Oxidation• Oxidation occurs when a material is exposed

to water and the oxygen in the air.

• This causes “rust”

• Type of chemical weathering.

Climate and Weathering

• Cold climates – mechanical weathering breaks down rocks rapidly through ice wedging

• Chemical weathering is more rapid in warm, wet climates. (tropical areas)

• Lack of moisture and cold climates keeps chemical weathering at a minimum.

What is Soil?

• Soil – A mixture of weathered rocks, decaying organic matter (plants & animals), mineral fragments, water, and air.

• Sediments + Organic Matter= Soil

What is Humus?

• Humus – Dark colored organic matter found in soil; made of decayed plants and animals.

• Soil Profile – Vertical section of the different soil layers.

• Horizon – Each layer in the soil profile. (Three Horizons: A, B, and C)

Horizons

• Horizon A– Most evolved– Known as topsoil– More humus and smaller rock and mineral

particles than other layers.• Horizon B– Less evolved– Lighter in color– Some plant roots reach here

Horizons Continued

• Horizon C– Bottom layer of soil profile– Contains partly weathered rock– Solid rock

Layers of Soil

Process of

Soil Formation

Leaching

• Removal of soil materials dissolved in water.

• Occurs throughout all three Horizons.

When does Desertification occur?

• When soil is dry due to lack of rain combined with poor farming practices and over – grazing.