Download Rocks and minerals are disintegrated and decomposed by the

Chapter 6
Rocks and minerals are disintegrated and decomposed
by the processes of mechanical and chemical
weathering. This breakdown occurs because the parent
material reacts with its new physical and chemical
environment
Fig. 6-CO, pp. 168-169
Introduction
• How does weathering differ from erosion?
• Weathering is the physical disintegration and
chemical decomposition of rocks at or near
Earth's surface
– Erosion involves the removal and
transportation of weathered materials from
their place of origin-by running water or
wind, for example.
Fig. 6-3a, p. 172
Mechanical Weathering –
Breaking Down of Earth Materials
•
Mechanical weathering includes the processes of
– Frost action
– Pressure release
– Thermal expansion and contraction
– Crystal growth (salts)
– Activities of organisms.
• The products of mechanical weathering are
chemically the same as their parent materials.
Fig. 6-3a, p. 172
Mechanical Weathering –
• How do freezing and thawing contribute to weathering?
– When water freezes in
cracks in rocks it expands
and then it contracts when
it thaws, thus exerting
pressure and opening the
cracks wider.
– Repeated freezing and
thawing breaks rocks into
angular pieces that may
tumble downslope and
accumulate as talus.
Fig. 6-3a, p. 172
Talus
Frost
Wedging
As water freezes
(changes from a liquid
to a solid state). The
expanding volume
creates pressures
which may break rocks
and minerals into
smaller pieces.
Mechanical Weathering –
• What are sheet joints and how do they form?
– Sheet joints are fractures that more or less parallel exposed
rock surfaces,
• especially rocks now at the surface that formed under great pressure
at depth.
– These joints form in response to pressure release;
•
when the rocks formed deep underground, they
contained energy that is released by outward
expansion when exposed at the surface.
Fig. 6-4a, p. 173
Mechanical Weathering –
• How do organisms contribute to mechanical and chemical
weathering?
– Any organic activity
such as
burrowing/particle
mixing/root wedging/
reduction of particle
size contribute to
mechanical weathering
– Organic acids aid in the
chemical alteration of
parent material.
Fig. 6-6b, p. 174
Organisms and Mechanical
Weathering
Thermal Expansion and
Contraction
CRYSTAL
GROWTH
The expansion of
water upon freezing,
salts dissolved in
water, its support of
organic growth all
facilitate the
mechanical
weathering
processes.
• precipitates that form as water in cracks evaporates. Such
compounds are called SALTS. Ice crystals and other minerals
that form in near-surface environments can also do the work.
Chemical Weathering –
Decomposition of Earth Materials
•
•
•
Chemical weathering processes include
– Solution
– Oxidation
– Hydrolysis
Hot and wet environments
accelerate chemical weathering.
Chemical weathering occurs in all environments,
except, possibly, permanently frozen polar regions.
Fig. 6-10, p. 180
Chemical Weathering –
•
These processes cause a change in the chemical composition
• Water is the dominant agent because it initiates
chemical weathering.
Rainfall (H20) falls through our atmosphere that
contains carbon dioxide (CO2). The reaction of water
and carbon dioxide results in an acid called carbonic
acid (H2CO3)
Fig. 6-7, p. 175
Chemical Weathering –
Decomposition of Earth Materials
• Solution
– – rocks dissolve
• Carbonate Rocks
– Rocks such as limestone
(CaCO³) are nearly
insoluble in neutral or
alkaline solutions, but
they rapidly dissolve in
acidic solutions
p. 194
Chemical Weathering –
Decomposition of Earth Materials
• Oxidation
– – rocks rust
– Rocks such as sandstone
may contain iron minerals
that will breakdown when
exposed to the atmosphere
– The atoms making up the
minerals dissociate, that is,
they separate as the rock
rusts away.
Concept Art, p. 177
Chemical Weathering –
Decomposition of Earth Materials
• Hydrolysis
– – breakdown to clays
• Potassium feldspar
– During hydrolysis hydrogen ions react with and
replace positive ions in potassium feldspar
– The result is clay minerals and substances in
solution such as potassium and silica.
water is necessary for hydrolysis and the
formation of carbonic acid. Water serves to
hold and transport the dissolved products.
How does mechanical weathering
contribute to the rate of chemical
weathering?
– Mechanical weathering enhances
chemical weathering by
breaking material into
smaller pieces, thereby
increasing the surface area
for chemical reactions.
– Because chemical weathering is a
surface process, the
more
surface exposed, the faster
the weathering.
Fig. 6-12, p. 181
Mechanical and
Chemical
Weathering
For both chemical and
mechanical weathering,
water also serves to
transport/erode the
weathering product.
Concept Art, p. 176