Weathering and Erosion
I
• Weathering alters rocks at the earth's surface
and breaks them down over time into finegrained particles of sediment and soil.
• Erosion is the movement of the particles by ice,
wind, or water.
• The particles are then transported by that
agent until they are deposited to form
sedimentary deposits, which can be later eroded
again or transformed into sedimentary rocks.
Types of Weathering
• Mechanical weathering is the process
by which rocks are broken down into
smaller pieces by external conditions, such
as the freezing of water in cracks in the
rock.
• Chemical weathering when it reacts
with rain, water, and the atmosphere to
destroy chemical and mineralogical bonds
and form new minerals.
Mechanical Weathering: no change in
chemical composition--just
disintegration into smaller pieces
Chemical Weathering: breakdown as a
result of chemical reactions
2+
3---> Ca2+
++
2HCO3CaCOCaCO3+CO2+H2O
+CO
+H
O
--->
Ca
2HCO
3
2
2
Mechanical weathering
Physical breakup
• pressure release
• water: freeze - thaw cycles
• crystallization of salt in cracks
• thermal expansion and contraction
All this increases the total surface area
exposed to weathering processes.
• spherically weathered boulder forms when
the corners of an angular rock are broken down more
quickly than the flat surfaces , forming rounded shapes.
Differential weathering results when some rocks
resist weathering more than other rocks, creating
uneven rates
Processes of Mechanical
Weathering
• Ice. The formation of ice in the myriad of tiny cracks
and joints in a rock's surface slowly pries it apart over
thousands of years .
• Frost wedging results when the formation of ice widens
and deepens the cracks, breaking off pieces and slabs .
Frost wedging is most effective in those climates that
have many cycles of freezing and thawing.
• Frost heaving is the process by which rocks are lifted
vertically from soil by the formation of ice . Water freezes
first under rock fragments and boulders in the soil; the
repeated freezing and thawing of ice gradually pushes
the rocks to the surface .
Frost Wedging: rock breakdown caused by
expansion of ice in cracks and joints
Frost Heaving
Frost heaving is the
process by which rocks
are lifted vertically from
soil by the formation of
ice
. Water freezes first
under rock fragments
and boulders in the soil;
the repeated freezing
and thawing of ice
gradually pushes the
rocks to the surface .
Shattered rocks are common
in cold environments where
repeated freeze-thaw cycles
gradually pry rocks apart.
• unloading. If a large intrusion is brought to the surface
through tectonic uplift and the erosion of overlying
rocks, the confining pressure above the intrusion has
been released, but the pressure underneath is still being
exerted, forcing the rock to expand.
• , or
• Sheet joints, develop that parallel the curved outer
surface of the rock because the outer layers expand the
most, cracks
• Sheet joints become surfaces along which curved pieces
of rock break loose, exposing a new surface . This
process is called exfoliation ; large rounded landforms
(usually intrusive rocks) that result from this process are
called exfoliation domes.
Mechanical Weathering
Exfoliation:
Rock breaks apart in layers that are
parallel to the earth's surface; as
rock is uncovered, it expands (due to the
lower confining pressure) resulting in
exfoliation.
Weathering
Mechanical Weathering
Sheet Joints
(Exfoliation)
Sheet joints, develop
that parallel the curved
outer surface of the rock
because the outer layers
expand the most, cracks
Stone Mountain, GA
Half Dome,
Yosemite, CA
Exfoliated Domes, Yosemite
• Other processes. include the
burrowing of animals, plant roots that
grow in surface cracks, and the digestion of
certain minerals, such as metal sulfides, by
bacteria .
• Daily temperature changes, especially in
those regions where temperatures can vary by
30 degrees centigrade, result in the expansion
and contraction of minerals, which weaken rocks
. Extreme
• temperature changes, such as those
produced by forest fires, can force rocks to
shatter
Thermal expansion due to the extreme range of temperatures can
shatter rocks in desert environments.
Repeated swelling and shrinking of minerals with different expansion
rates will also shatter rocks.
Biological Weathering
Can be both chemical and
mechanical in nature.
• roots split rocks apart
• roots produce acids
that dissolve rocks.
• tree throw
• burrowing animals
Burrowing of Animals
Living Organisms
• Lichens that grow on rocks produce weak
acids that chemically weather rock
Role of Physical Weathering
1) Reduces rock material to smaller
fragments that are easier to
transport
2)
Increases the exposed surface area
of rock, making it more vulnerable to
further physical and chemical
weathering
Surface Area and Weathering
Rates of weathering
Joints in a rock are a
pathway for water – they
can enhance mechanical
weathering