Weathering ProcessesFormation of Soils

ByDiana L. Duckworth

Rustburg High SchoolCampbell County, VA

Two Types of Weathering

• Physical or Mechanical Weathering– Breaks rock down into smaller pieces which

accelerate chemical weathering

• Chemical Weathering (decomposition)– Minerals are changed by chemical reactions to

form clay and rust and dissolved salts

Joints (cracks)

As pressure from overlying rocks is released by erosion, cracks form in the rocks.

Frost action or frost wedging -

Water freezes in joints or small fractures. Its expansion forces the crack open further.

Tree roots

Tree roots grow into cracks, forcing the crack to enlarge.

Exfoliation Repeated heating during the day and

cooling at night of the top few mm causes

repeated expansion and contraction.

Pieces break off and the edges of rock become rounded. It occurs only in granite.

Exfoliation boulders, typically in granite.

Exfoliation joints occur only in granite.

Exfoliation domes produced by curved joints in granite as pressure is released.

Chemical Weathering Processes

• Hydrolysis – combination with water

• Oxidation – combination with oxygen

• Carbonation – acid rainwater attacks minerals

• Organic acids – from decaying plant matter attack minerals

Lichens & other plants secrete acids

Animals Contribute to Soil Formation

The above solution of limestone is caused by repeated urination by territorial animals.

Soil Horizons in Soil ProfileTopsoilA-horizon

Subsoil B-horizon

C-horizon

Soils do not form all horizons immediately

• Physical weathering breaks rock down

• Chemical weathering changes composition– Chemical processes alter feldspars & iron

minerals to clay minerals– Tiny clay particles wash downward with rain

into subsoil forming the B-horizon last.

• Organic matter added to surface layers from plants

Different soils can also form in different physical circumstances

• Mature soils – contain all soil horizons which are well developed.

• Immature soils – lack the B-horizon because it forms last.

• Residual soils – soils that form on top of and from their parent rocks.

• Transported soils – topsoil is eroded and deposited elsewhere; floodplain soils.

• Extent of weathering depends on climate– amount of heat & moisture present

• Mineral make up of soil will depend on extent of weathering

• Soil type is largely controlled by climate– tropical climate - quartz & rust & clay– colder drier climates - rock fragments & quartz,

little clay

Rock fragments + clay + rust + dissolved salt + quartz

Rock + Air + Water

heat

What happens to soil products in geologic cycle?

• Dissolved salts are carried by groundwater & river runoff to the ocean where they make the ocean salty

• Rock fragments & quartz become sand grains carried in bed load of streams

• Clay and silt can be eroded by wind or water; carried in suspension

• Different sized particles wind up in different places - sorting