CHAPTER 5 - WEATHERING AND SOILS
WEATHERING - the disintegration and decompositionof rock at or
near surface.
MASS WASTING - the transfer of rock material downslope via
influence of gravity.
EROSION - the incorporation and transportation of material by a
mobile agent:
WATER, WIND, or ICE.
Weathering falls into two catagories: MECHANICAL WEATHERING
and
CHEMICAL WEATHERING.
MECHANICAL WEATHERING - "making big pieces into smaller
pieces"
Types of Mechanical Weathering:
- FROST WEDGING - ice forms in cracks, splits rocks.
- UNLOADING - removal of rock overburden causes rocks that wer
under pressure to
expand: this process of expansion by "sheeting" forms exfoliation
domes (such as in
Yosemite National Park, CA).
- THERMAL EXPANSION - daily heatingcycle causes 30 C variation;
this causes
expansion pressure on the surface of rocks which creates
fractures.
- ORGANIC ACTIVITY - plants (roots), burrowing animals, humans.
Roots expand
after a rain storm.
Mechanical weathering acts to increase the surface area available
to chemical attach. For
example, a fresh block of granite with a volume of 1 cubic meter
has six sides with a total of
6 square meters of surface area. The surface area within a cubic
meter of sand approaches a
square kilometer! For each cubic meter of sand on the beach
about 10 cubic meters of clay
are deposited offshore. Clay particles are "flat"- similar to
chips of mica, and have up to 100
times the surface area as an equal volume of sand. The greater
the
CHEMICAL WEATHERING - involves chemical transformation into 1
or more new
materials.
- H2O + CO2 = H2CO3 (carbonic acid) from atmosphere and plant
acids dissolve most
rocks through time.
- H2CO3 + CaCO3 (calcite) = Ca+2 + -1HCO3 is a reversible
process (land and oceans).
Calcite is the primary mineral in Limestone. Calcite also acts a
a cement in sediments to
form sedimentary rocks.
- Quartz (SiO2 ) dissolves and reprecipitates as silica, which
acts as a cement in sediments
to form sedimentary rocks.
- Feldspar weathers to become clay (reversible under
metamorphic conditions)
- Mafic Minerals become clay, silica, and iron oxide minerals
(limonite/hematite).
- Cations (such as +K, +Na, ++Ca) and anions (such as -Cl,
-HSO4 ,-HCO3) stay in
solution and add to the saltiness of the ocean, precipitate as
salts, or are incorporated into
clays in sediments.
WEATHERING FEATURES
Rock exposures, such as on cliffs or barren hilltops, have
characteristic appearances due to
SPHEROIDAL WEATHERING (the "rounding of rough edges") and
DIFFERENTIAL
WEATHERING (some rocks and rock layers resist erosion better than
others; creates a rough
weathered surface).
As both chemical and mechanical weathering proceeds rock material
accumulates as a
REGOLITH - layer of rock and mineral fragments produced by
weathering. If water is
available, plant (and animal) activity affect the material and
dead organic matter accumulates.
This results in the FORMATION OF SOIL.
SOIL - mineral matter (~45%), HUMUS - organic matter (~5%), air &
water (~50%).
CONTROLS OF SOIL FORMATION
- PARENT MATERIAL - the original mineral matter can be either:
RESIDUAL
(regolith that accumulates "in place") or TRANSPORTED mineral
material by water, wind,
or ice.
- TIME - important to all geologic processes; longer
exposure=more weathering.
- CLIMATE - water and temperature affect weathering processes.
Hot & humid climates
produce deep weathered soils lacking essential nutrients. Cold &
dry climates results in very
slow weathering which increases chances of wind erosion before
soil can form.
- PLANTS & ANIMALS: microscopic organisms do most of the work;
the acculation of
organic mater forms HUMUS; roots, worms, rodents, and insects
rework the regolith and
humus to form soil.
- SLOPE - soils tend to be thickest on flat surfaces (due to
mass wasting & erosion.
Sunny slopes tend to be warmer and dry faster.
SOIL PROFILE
- O horizon - "fresh" organic matter that accumulates on
the ground surface.
- A horizon - consists of mixed organic and rock
fragments: water washes
out fines (by a process called ELUVIATION); mineral nutrients
dissolved away by process
called LEACHING.
- B horizon - zone of "accumulation" of fines; forms a
"hardpan"
- C horizon - (below "solum" or true soil) is a zone of
weathering bedrock
above the actual unweathered bedrock - still considered a
regolith
THREE GENERAL KINDS OF SOILS
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