Chapter 4

Weathering and Soil Formation

•

Weathering breaks down rocksinto smaller pieces

1. Mechanical weathering

2. Chemical weathering

Mechanical weathering produces physical changes in rocks:

• Ice wedging• Pressure release-causes rock to expand,

cracking and leading to exfoliation

Exfoliation-process in which layers of exfoliation

rock gradually break off.• Plant root growth• Abrasion-wearing down by friction, the rubbing

of one object or surface against another

Chemical weathering

• Breakdown of rocks by chemical reactions that change the rocks composition (what it is made of)

• Water (dissolving)

• Rusting

Weathering occurs at different rates

• Surface area-the more exposed area, the faster the break down of rock (chemical)

• Rock composition ex: Granite compared to limestone

• Climate:• chemical weathering occurs in wet, hot

areas faster than cold, dry regions• Mechanical weathering caused by freezing

and thawing

Weathering and organic processes form soil

• Soil- a mixture of weathered rock particles and other materials

• Humus-decayed organic (living things) in the soil

Soil formation is affected by:

• Rock in an area

• Climate: tropical, desert, temperate, artic

• Landforms: mountains and valleys

• Plant cover: provide organic matter

• Living organisms: microorganisms and animals

• Time

Soil composition

• Soil horizon-a layer of soil with properties that are different than the layers above and below it

• Soil profile- soil horizons in a specific area

Soil Properties

• Texture-determined by size of weathered rock particles it contains

• Color-most comes from iron and humus

• Pore space-the spaces between the particles

• Chemistry-minerals and organic nutrients

Human activities

• Soil is a valuable resource for humans

1. Farming-soil loss and overgrazing results in desertification- expansion of desert in areas where natural plant cover has been destroyed

2. Construction and development

3. Mining• Strip mining• Open pit

Soil conservation

• Crop rotation

• Conservation tillage

• Terraces

• Contour plowing

• Windbreaks

Info for Soil lab

• Climate in Missouri- Temperate soils form in region with moderate rainfall and temperatures. Some temperate soils are dark-colored, rich in organic matter and minerals, and good for growing crops.

Soil Horizons

• The A horizon- is the upper layer of soil and is commonly called topsoil.

• It contains the most organic matter out of all the horizons due to the humus that it contains gives it the dark color.

The B Horizon

• This layer lies just below the A horizon. It has a little organic matter and is usually brownish or reddish color and contains clay and minerals.

The C horizon

• This is the deepest layer of soil. This layer contains the largest and least-weathered rock particles. It is usually light yellowish brown.