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Weathering, Erosion & Soil
A. Weathering Processes:
- chemical weathering: minerals in a rock
are chemically altered or dissolved.
 produce clays and soils
 change chemical composition
of natural waters
- physical weathering: rock becomes
fragmented by physical processes.
- chemical and physical weathering enhance
each other
chemical weathering weakens the rock for
easier physical disintegration.
physical weathering increases the surface
area for fast chemical reaction.
Figure 5.3 (surface area)
B. Erosion:
- the set of processes that loosen and move
earth material downhill or downwind
- erosion carries away weathered material
and exposes fresh rock surface to
weathering
- weathering enhances erosion by breaking
down rock into smaller and weak
fragments.
C. Physical Weathering
- This process tends to be more pronounced
along zones of weakness: e.g. bedding
plane of sedimentary rocks, fractures
called joints in igneous rocks (they form
when rocks cool and contract).
- Frost wedging is the most efficient
mechanisms for widening cracks and
break the rock: as water freezes and turns
into ice, the volume increases by about
9%.
Figure 5.4 insert
- Alternating heat (expansion) and cold
(contraction) could result in breakage of
rock.
- Exfoliation: a large scale physical
weathering that produces flat or curved
sheets of rock (Fig. 5.5)
Spheroidal weathering: similar to exfoliation,
but at a smaller scale. (some newer books,
including your textbook, define spheriodal
weathering as rounding effect of chemical
weathering.)
- No generally accepted explanation for
either exfoliation or spheroidal
weathering.
One possibility is that they are products of
uneven expansion and contraction due to
alternating cooling and heating, and
perhaps with the removal of overlying
rock that reduces the pressure.
D. Chemical Weathering:
Chemical reactions dissolve mineral and
create new chemical compounds.
e.g. K-feldspar weathered to kaolinite
Feldspar + carbonic acid + water
2KAlSi3O8 2H2CO3
H2O
 kaolinite + dissolved+potassium+ bicarbonate
silica
Al2Si2O5(OH)4 4SiO2(aq)
ion
2K+
2 HCO3-
Where did the carbonic acid come from?
Kaolinite: a common weathering product of
silicate minerals in most environments.
Bauxite, an ore composed of aluminum
hydroxide and a major source of aluminum
Many iron silicates, such as iron pyroxene
(FeSiO3), could be altered to iron mineral,
such as hematite (Fe2O3) by a process called
oxidation.
With a few exceptions (e.g. halite), calcite is
the most easily soluble mineral:
calcite+carbonic acid  ca. ion+bicarbonate
CaCO3 H2CO3
Ca+2
HCO3-
Stability of common minerals: minerals
formed under lower temperature tend to be
more stable than those crystallized at higher
temperature. (Bowen’s reaction series)
E. Factors that affect the extent of
weathering
- parent rock: solubility of minerals in the
rock
Fig. 5.11 granite vs. limestone
- climate: rainfall & temperature: hot &
humid environments enhance weathering
freezing of water may speed up physical
weathering
- soil: medium for life (microbes and
plants); water retainer.
- Residual Soils
- Transported Soils
 acid environment enhances chemical
weathering
- time
Transported soil and residual soil
F. Soil profiles: A product of weathering
A soil profile in South Dakota (Figure 5.17)
Figure 5.18: Idealized soil profile (humid
climate)
G. Weathering makes the raw material of
sediment