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LECTURE 12. WEATHERING & MASS WASTING
Introduction
Rock exposed at the Earth's surface is under almost constant
attack by processes which break it down into fragments and
transport it away; 3 stages are involved: Weathering - the
initial breakdown of solid rock; mass wasting - downslope
movement of the fragments; erosion - long distance
transportation of the fragments. The combined effect is the
overall lowering of the earth's surface, termed
DENUDATION.
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IN SITU
DOWN SLOPE
LONG DISTANCE
TRANSPORT
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WEATHERING: includes both MECHANICAL processes,
which exert force and physically break a rock apart, and
CHEMICAL processes, which decompose a rock by altering
its chemical composition.
Mechanical weathering:
1. Frost wedging:
9% expansion
Bare cliff faces are vulnerable to frost wedging.
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Resulting fragments accumulate as a
TALUS SLOPE. Frost wedging is
most effective where temperature
oscillates above and below freezing.
2&3. SALT WEDGING &
HYDRATION: the first is similar to
frost wedging, except it involves the
growth of salt crystals in cracks
from salts dissolved in water; most
effective in semi-arid and coastal
areas. The second refers to the
absorption of water by some
minerals, making them swell and
exert force, breaking the rock. Frost
and salt wedging are aided by
natural joints (cracks) in rock. These
result from contraction of cooling
magma or tectonic stresses.
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4. EXFOLIATION. This applies to large bodies of igneous rocks,
particularly granite batholiths, that are exposed at the surface of the
Earth when the overlying rock is eroded away. The reduction of
overburden pressure allows the igneous rock to expand upwards,
which causes it to crack, usually parallel to the surface, producing an
EXFOLIATION DOME. The sheets of rock produced are often
peeled off in onion-like layers by other weathering processes, such as
frost wedging.
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5. THERMAL
EXPANSION: Thermal
expansion weathering is
caused by large daily
temperature changes,
which are especially
pronounced in hot dry
areas such as deserts.
Different minerals expand
and contract at different
rates; this sets up
DIFFERENTIAL
STRESSES within a rock,
which eventually can cause
the rock to shatter,
producing angular
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fragments.
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6. ORGANIC ACTIVITY:
The growth of plant roots has
a similar effect to the growth
of ice - the growing root
wedges a rock apart, breaking
it up into fragments. The
effect of tree roots on city
sidewalks clearly
demonstrates the effectiveness
of this process.
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CHEMICAL WEATHERING: Refers to the decomposition of
rock by various chemical reactions, which involve either
adding or removing elements from minerals in the rock.
Water is nearly always required for chemical weathering and
so this form of weathering is not very effective in dry areas,
such as deserts. Most
chemical reactions
are more effective at
higher temperatures
and so chemical
weathering is most
pronounced in hot,
wet areas, such as the
Tropics. Chemical
weathering “rounds
off” angular edges
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of boulders.
1. SOLUTION: certain minerals (but not many) simply
dissolve in water. Atoms of minerals forming the rock are
simply pulled out and dispersed into the water surrounding the
mineral; if the water then flows away, the atoms are carried
away with it. A good example is HALITE (rock salt).
2. OXIDATION: oxygen
from the atmosphere
combines with certain
minerals - especially ironbearing ones - to form
new softer minerals. This
is essentially the same as
rust and produces colorful
“desert varnish” on rocks.
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3. HYDROLYSIS: This is the commonest form of chemical
weathering and is particularly effective against feldspar
minerals - the commonest mineral group. Hydrolysis refers
to the chemical combination of minerals with ACIDIC
WATER, resulting in the formation of new minerals. Most
natural SOIL WATER is slightly acidic - the acid comes
from 2 main sources:
1. water dissolves a certain amount of carbon dioxide as it
falls through the atmosphere, forming weak carbonic acid.
2. decaying organic matter at the ground surface releases
weak organic acids.
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E.g. Granite: feldspars -> clays; quartz particles drop out
-> sand.
This chemically weathered
granite boulder has been
“rounded off” (spheroidal
weathering). The feldspars
have been chemically altered
to clay minerals; the quartz
is resistant to chemical
weathering - quartz particles
simply drop out and become
sand grains. The clays also
become detached - many end
up as a component of soil.
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4. Carbonation: involves
carbon dioxide from the air
dissolved in rain water to
form weak carbonic acid; this
dissolves carbonate rocks,
especially limestones, often
leaving no residue (this is why
caves are common in
limestone areas). Chemical
weathering depends on the
presence of water and most
reactions are more intense
under higher temperatures;
for these reasons, chemical
weathering is most
pronounced in the hot wet
Tropics; mechanical
weathering is most
pronounced in cold wet
climates, or at high altitudes.
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MASS WASTING
(DOWNSLOPE movement
of sediment by gravity):
Controls On Mass
Wasting:
Mass wasting occurs on
slopes (even very gentle
slopes) because weathering
attacks the surface rock
allowing the resulting
weak soil and regolith to
be pulled downslope by
gravity. Since weathering
is a continuous process,
Rapid mass wasting events can be triggered
many slopes become
weaker and weaker until by earthquakes, heavy rain or snow, erosion,
eventually they can no
mining, construction, etc. These movements
longer withstand the pull
are considered “natural hazards”.
of gravity and move
downslope.
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Types of Mass Wasting:
Based on:
1. MATERIAL TYPE: Rock,
Soil (debris=coarse sediment;
earth & mud =fine sediment).
Note: the deposits of mass
wasting are collectively known
as COLLUVIUM.
2. TYPE OF MOTION:
A. Falls: Free-fall of material
under gravity - forms cliffs and
talus slopes.
Rockfall from a steep cliff;
talus slope at base of cliff.
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Slides: movement of slope material over a well-defined planar
surface. These commonly occur where there are joints,
bedding planes or fractures parallel to the surface of a slope.
1925 Gros Ventre rockslide, Wyoming. Sandstone slid over a saturated clay layer.
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Sometimes the slide is rotational - producing a slump.
scarp
Note “stepped”
appearance of
slope
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Flows: High fluid content; behaves like a viscous liquid flow.
Coarse material mixed with mud = DEBRIS FLOW. Fine
material alone referred to as an EARTHFLOW
(unconfined) or MUDFLOW if confined to a channel. When
mudflows emerge from channels they build ALLUVIAL
FANS.
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Earthflow (saturated soil, unconfined to channel).
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Mudflow: stream choked
with mud/clay. Common
in flash floods in arid
regions. Where
mudflows emerge from
channels onto flat valley
floors, alluvial fans are
built.
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Alluvial fans on Mars.
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Creep: Slow movement of nearsurface material (e.g. cms/year).
Common on most slopes above
a few degrees. Caused by
gravity, freeze/thaw,
wetting/drying, rainsplash,
burrowing. Shows up as curved
tree trunks, bent walls,
"wrinkled" slope surface
(terracettes).
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