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
Frost action or frost wedging -
Water freezes in joints or small fractures. Its expansion forces the crack open further.
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.
Chemical Weathering Processes
• Hydrolysis – combination with water
• Oxidation – combination with oxygen
• Carbonation – acid rainwater attacks minerals
• Organic acids – from decaying plant matter attack minerals
Animals Contribute to Soil Formation
The above solution of limestone is caused by repeated urination by territorial animals.
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