WEATHERING
AND SOILS
The photo shows
Bryce Canyon, Utah which formed from weathering and erosion--weathering is an
important process in the formation of soils
I. Nature of
weathering and erosion
A.
Weathering
- weathering is simply the chemical and/or physical
breakdown of a rock material--weathering involves specific processes
acting on rock materials at or near the surface of the Earth
B.
Erosion
- is the removal or
transportation of material by agents as running water, ice, wind, etc.
- different degrees of weathering in rocks and erosion
work together to form some interesting landforms such as Bryce Canyon,
National Park, Utah
- click here to see Bryce Canyon or
see page 130 in text
II. Kinds of weathering
A.
Physical or mechanical
- is the breakdown of a substance by disintegration---the
material broken down is the same material and has not been chemically
altered
·
is
the alternating freezing and thawing of moisture in the openings or cracks of
rocks resulting in a breakdown of the rock--this is the single most abundant
form of physical weathering
·
a
slope of gravel material called talus can form at the base of a cliff or
mountain--this accumulation of material is an example of mass wasting--frost
wedging loosens pieces of bedrock which then fall and accumulate
·
click here to see a talus slope
·
in
humid northern latitudes during the late Autumn or early Spring frost wedging
causes "potholes" in streets and highways
·
click here to see frost wedging
·
erosion
of upper rock portions causes remaining underlying rocks to expand (like an inflated
balloon after pressure on it is released) resulting in cracking and peeling off
in slabs similar to onion skin layers
·
click here to see exfoliation
·
activities
of plants and burrowing animals can cause a rock material to disintegrate
·
click here to see tree roots causing rocks to disintegrate
B.
Chemical weathering
- is the breakdown of a substance by decomposition
resulting in the formation of new mineral matter--water and acids
especially carbonic acid (H2CO3) are important agents in this type of
weathering
- Kinds of chemical weathering
- hydrolysis--is the
chemical reaction of water with the substance in which the (OH) in water
(HOH) becomes part of the new formed substance-- often silicate minerals
will weather this way to form clays
- solution (leaching)--is
the dissolving of substances by acid in waters usually leaving residual
non-solubilized materials--carbonate minerals are especially affected by
leaching
- oxidation--is the
reaction of a substance with oxygen causing a rust to form--manganese (Mn) and especially iron (Fe) minerals are affected
- although there are individual chemical and physical
weathering types often combinations of these can affect the total weathering
of a substance
III. Factors which affect the rate and/or extent of weathering
A.
Total surface area of mineral or rock
- chemical and physical weathering increases
proportionately with the amount of open space (cracks, holes, etc.) at the
surface of the rock and extending through the rock
B.
Climate
- temperature and the amount of moisture can influence
the kind and magnitude of weathering--a humid hot climate should
experience alot of hydrolysis, leaching,
oxidation and physical weathering from biological activity but there would
be very little or no frost wedging
- a good example of climate affecting weathering is a
granite obelisk called Cleopatra's Needle--for almost 3500 years it was in
Egypt with barely a weathered trace but present in New York City for only
about 80 years it has been weathered (pitted) alot
- click here to see Cleopatra's Needle
in Egypt
- click here to see Cleopatra's Needle
in New York
C.
Composition of rock or mineral substance
1. Rate and extent of chemical weathering
·
as
mentioned earlier silicate composition substances weather extensively by
hydrolysis while carbonate substances do so primarily by leaching-solution and
iron and manganese minerals weather primarily by oxidation
2. Silicate mineral
weathering series
·
silicate
minerals higher in Bowen's mineral series will decompose at a greater rate and
extent than those lower in the series based on composition alone--a rock
containing olivine, pyroxene and plagioclase as does gabbro should weather at a
greater rate and more extensively than granite which contains quartz,
orthoclase and hornblende, all other weathering factors being the same
·
which should weather more based on composition alone, a rhyolite
or a basalt?
·
click here to see the silicate weathering series
D.
Time
- obviously time is a factor in all geologic processes
including weathering
IV. Soil
A.
Definition and basic nature of soil
- is an unconsolidated material formed from the chemical
and/or physical weathering of substances as bedrock or outcrops and
varying degrees of organic material present--soil can be found in situ (in
place) or transported
- obviously soil is an important product of weathering
and based on the factors of weathering there is a classification of soil
types (pedocals and pedalfers)
- click
here to see definition of soil types
B. Soil profile and soil horizons
- a typical soil profile includes 3 horizons--the 2 upper
horizons A and B are known as the solum--the A
horizon is concentrated in organic matter and acids and represents an area
of high leaching or solution effects--the B horizon is concentrated in
clay size materials and rich in iron (Fe)--the C or bottom horizon is
comprised primarily of fragmented bedrock
- eluviation is the removal of clay and soluble materials
from the A horizon and illuviation is the precipitation of eluviated materials from the A horizon in the B
horizon
- click here to see the soil profile
or page 136 in text
- is a form of mass
wasting of
earth materials--creep is often initiated if the ground becomes saturated
with water allowing gravity to pull the water logged soil
downslope--freezing and thawing of soil can also cause heaving thus
loosening the soil—includes debris slide, rock and debris flow and
large landslides
- click here to see soil creep
D.
Solifluction
- is a form of mass wasting and soil creep that occurs in
regions underlain by permafrost in arctic climates--this form of creep
occurs in the layer called the active layer above the permafrost
zone---the active layer becomes saturated with water during the summer
months since the underlying permafrost layer is impermeable--this results
in a creep down-slope of the active layer