Weathering and soil
Chemical weathering
• Primarily done by water that is acidic.
• Rain picks up material from the atmosphere
to become acidic.
– Carbon dioxide Æ Carbonic acid
– Sulfur oxides Æ Sulfuric acid
– Nitrogen oxides Æ Nitric acid
• Once rain enters the ground it usually picks
up more carbon from decaying organic
material and becomes more acidic.
1
2
Chemical Weathering of Feldspar
Potassium feldspar hydrogen ion
4KAlSi3O8
+
H+
+
water
2H2O
Potassium ion kaolinite
+ Al4Si4O10(OH)8
4K+
silica
+ 8SiO2
Kaolinite
Chemical Weathering of Mafic Minerals
• The chemical weathering of mafic (dark
colored) minerals like pyroxenes,
amphiboles, biotite, etc. (all contain Fe
+/- Mg) releases iron which can then
form iron oxide (Fe2+O) and goethite
(Fe3+)(OH).
• These minerals give the weathered rock
a red (“rusty”) color.
Kspar
The reaction of carbonic acid with groundwater and feldspar is a
common weathering reaction in most temperate climates (as in
Colorado).
Typical “Fresh”
Igneous Rock
50%
quartz
33%
Kfeldspar
17% Fe
Silicates
Products of
Weathering
pore space
or loss to
solution
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
–Impact
75%
quartz
17% clay
8% Rust
3
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
– Impact
–Abrasion
Balanced rock from sand abrasion in the desert
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
– Impact
– Abrasion
–Frost Wedging
4
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
– Impact
– Abrasion
– Frost Wedging
–Root Wedging
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
– Impact
– Abrasion
– Frost Wedging
– Root Wedging
–Salt Wedging
Mechanical Weathering
• Forces act to break rock apart without
chemically altering it.
– Impact
– Abrasion
– Frost Wedging
– Root Wedging
– Salt Wedging
–Unloading
5
Soil vs. Regolith
• Regolith = the layer of rock and
mineral fragments at the Earth’s
surface.
• Soil = a combination of mineral
and organic matter, water, and
air.
–The portion of regolith that can
support rooted plant growth.
Soil Horizons
• The most important characteristics
used to define soil horizons are:
1. Color
2. Texture
3. Structure
4. Organic Matter Content
5. Moisture Content
6
Soil Horizons - Color
• Organics turn soil black to dark
brown
• Ferric Iron turns the soil yellowbrown to red
• Ferrous Iron turns soil dark greygreen
• SiO2 or CaCO3 turns the soil light
gray to white
Carbon rich
Reduced (ferrous
– Fe+2) iron rich
CaCO3 rich
7
Soil Horizons - Texture
Soil horizons - structure
• Shape of aggregates
of soil particles called
peds
• In clay rich material
the spaces between
peds may be the only
way water can travel
down the profile
Soil Horizons – Moisture
Content
Soil Horizons – Organic Matter
Content
• Litter (leaves and branches)
• Humus (decomposed litter)
• Both tend to be produced more at higher
temperatures, with an optimum of about
25° C
• Provides chelates, increase water
absorption ability, and makes CO2 to
create carbonic acid
SOIL PROFILES
Five Basic Horizons:
O Horizon = Loose and Partially
Decayed Organic Matter
A Horizon = Loose Rock and Mineral Matter
• Total quantity of water that can
be held in a soil is the available
water capacity (AWC)
• Field capacity
• Permanent wilting point
mixed with Partially Decomposed
Organic Matter (Humus)
E Horizon = FineFine-grained Mineral Particles
that have been carried down by water
(Eluviation & Leaching)
B Horizon = Accumulation of clay
transported from above
C Horizon = Loose and Partially
Altered “Parent”
Parent” Rock Material
Parent Rock
8
O horizon
• Mostly undecomposed or partially
decomposed organic material
• Mineral content minor (<50%)
A horizon
• Mineral horizon with decomposed organic
material where clay and mobile
components are leached downward
• Black, nutrient-rich topsoil
• Zone of eluviation
E horizon
• Intense leaching that removes ferric iron or
organic coatings from mineral grains
leaving the layer bleached grey or tan
Clay tends to get leached from the A and E horizons
and carried down to the B horizon due to the electric
property of water molecules interacting with the
negatively charged surfaces of clay particles.
B horizon
• Transitional between E and C where
material brought down from above
accumulates
• Zone of illuviation
• Commonly has reddish hues, iron and
aluminum concentrations, stable primary
minerals, and a high clay content
9
C horizon
• Underlying, unconsolidated parent
materials that have been
unmodified/slightly modified, by soil
forming processes
R horizon
Solid bedrock
K horizon
• A calcite rich layer generally found in arid
region
• Caliche
Soil Forming Factors
•
•
•
•
•
Climate
Topography (slope)
Parental material
Organics
Time
10
11
12
13
Soil erosion (of bauxite soils) in Madagascar: clearly unsustainable.
14