Weathering Processes
Formation of Soils
By
Diana L. Duckworth
Rustburg High School
Campbell 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 Profile
Topsoil
A-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.
Rock + Air +
Water
heat
Rock fragments + clay +
rust + dissolved salt + quartz
• 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
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