weathering, erosion & soil
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Weathering, erosion &
Dr. Aman Zalama
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Alteration of Minerals and Rocks
Thermal Expansion and Contraction
Growth of Salt Crystals
Organisms and Weathering
Chemical Weathering Decomposition of Earth Materials
Expansive Soils and Soil Degradation
Weathering and Natural Resources
Soil and Its Origin
The Composition of Soil and Soil Horizons
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Exogenetic processes deals with
weathering, erosion, and soil.
Weathering is a pervasive phenomenon
that alters rocks and minerals so that
they are more nearly in equilibrium
with a new set of environmental
Erosion removes weathered materials
which are transported elsewhere and
deposited as sediment that may be
converted to sedimentary rock.
Earth materials are not structurally
and compositionally homogeneous, so
they are attacked by weathering
processes at different rates, even in
the same area.
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Differential Weathering In this example
of differential weathering, the rock
layers have been tilted so that the
planes separating adjacent layers are
vertical. Weathering and erosion along
these surfaces have yielded the
pillars, spires, and isolated knobs at
this rock exposure in Montana.
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Mechanical weathering processes
break parent material into
smaller pieces but do not change
Frost action, pressure release,
thermal expansion and
contraction, salt crystal
growth, and the activities of
organisms account for mechanical
is a form of mechanical weathering
(that is, weathering that involves
physical, rather than chemical change).
Frost wedging is caused by the repeated
thaw cycle of water in extreme -freeze
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ll cracks climates. Most rocks have sma
in them, called joints (or, tectonic
joints). When it rains, rainwater seeps
into these joints. As the day cools and
temperatures at night drop below
freezing, the water inside the joints
freezes. As water freezes into ice, it
ing ice places expands. The expand
pressure on the joints in the rock.
Finally, when the pressure is too much,
the joint expands. In some cases, the
rock will split, though this usually
happens after repeated freeze and
thaws. As new water is added during the
e ice is created at warmer days, mor
night, wedging the joints apart
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Some rocks form at depth and are
stable under tremendous pressure.
Granite, for instance, crystallizes
far below the surface, so when it
is uplifted and the overlying
material is eroded, its contained
energy is released by outward
expansion, a phenomenon called
Outward expansion results in the
origin of fractures called sheet
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joints that more or less parallel
the exposed rock surface.
Sheet-jointbounded slabs of rock
slip or slide off the parent rocka
process known as exfoliation
leaving large, rounded masses of
rock called exfoliation domes.
During thermal expansion and
contraction, the volume of rocks
changes in response to heating
In a desert, where the
temperature may vary as much as
30C in one day, rocks expand
when heated and contract as they
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Rock is a poor conductor of
heat, so its outside heats up
more than its inside; the
surface expands more than the
interior, producing stresses
that may cause fracturing.
Under some circumstances, salt crystals
that form from solution cause
disaggregation of rocks.
Growing crystals exert enough force to
widen cracks and crevices or dislodge
particles in porous, granular rocks
such as sandstone.
Even in crystalline rocks such as
granite, salt crystal growth may pry
loose individual minerals.
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Animals, plants, and bacteria
all participate in the
mechanical and chemical
alteration of rocks.
Burrowing animals, such as
worms, termites, reptiles,
rodents, and many others,
constantly mix soil and sediment
particles and bring material
from depth to the surface where
further weathering occurs.
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Chemical weathering processes include
solution, oxidation, hydrolysis, and
the activities of organisms. They
result in a change in the structure,
composition, or both of parent
Particle size, climate, and type of
parent material determine how quickly
chemical weathering takes place.
To a chemist, oxidation is any
chemical reaction in which a compound
or ion loses electrons, whether oxygen
is present or not.
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But in chemical weathering it refers to
reactions with oxygen to form oxides
(one or more metallic elements combined
with oxygen) or, if water is present,
hydroxides (a metallic element or
radical combined with the hydroxyl, OH
For example, iron rusts when it
combines with oxygen and forms the iron
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When hydrolysis takes place, hydrogen
ions (H) or the hydrogen in (OH) of
water reacts with and replaces positive
ions in minerals, thereby changing
their composition and liberating
soluble compounds and iron that may
then be oxidized.
Potassium feldspars such as orthoclase
(KAlSi3O8) and plagioclase feldspars
(which vary from CaAl2Si2O8 to
NaAlSi3O8) are framework silicates, but
when altered by hydrolysis they yield
materials in solution and clay
minerals, which are sheet silicates.
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Erosion is the removal of weathered
materials from their source area.
Erosion is the removal of weathered
rocks and minerals by moving water,
wind, ice, and gravity.
Deflation is erosion by wind. Silt and
sand are removed selectively, leaving
larger stones on the surface and
creating desert pavement.
The Western Desert of Egypt is the land
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Sand grains are carried short distances
and a meter or less above the ground by
saltation, but silt can be transported
great distances at higher elevations.
A dune is a mound or ridge of wind-
deposited sand. Most dunes are
asymmetrical, with gently sloping
windward sides and steeper slip faces
on the lee sides.
Types of dunes include barchan dunes,
transverse dunes, longitudinal dunes,
and parabolic dunes.
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Expansive soils contain clay minerals
that expand when wet and shrink when
Soil degradation is any loss of soil or
decrease in soil fertility resulting
from erosion, compaction, and
contamination by pollutants.
Weathering is responsible for the origin of
some natural resources and for the
concentration of others, such as bauxite
and other residual concentrations. And of
course soils are one product of weathering.
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Soil is made up of weathered materials,
air, water, and organic matter and can
Soil profiles for pedalfer, pedocal,
and laterite show horizons designated
O, A, B, and C that differ in
structure, composition, and color.
Climate, parent material, organisms,
angle of slope, the direction a slope
faces, and time are the factors that
control the rate of soil formation.
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