CHAPTER 7:
WEATHERING
The Hoodoos
• near to Drumheller, Alberta
What weathering processes likely contributed to the development
of these remarkable rock formations?
Weathering
• Weathering is the result of a
series of physical, biological, and
chemical processes.
• Modifies rocks and the minerals
within them
• Produces sediment which is
subsequently removed by
erosion and transported
elsewhere
• Enables growth of new minerals
through crystallization
• Contributes to the production of
soil
Weathering Mechanisms
• Physical and
biological
weathering
causes rock to
fracture and
fragment
• Chemical
weathering
attacks exposed
surfaces and
weakens the rock
Explain how physical (a.k.a. mechanical) and chemical weathering
complement each other in breaking rock into smaller fragments.
Physical Weathering
• Physical Weathering causes Fragmentation of Rock
• Pressure release, abrasion, freeze-thaw, hydraulic action, growth of salt
crystals, and other physical means
• Physical weathering is aided by bedding, rock joints and other types of
fractures
Pressure release jointing - Exfoliation
Exfoliation contributed to jointing of granitic rock
adjacent to the Coquihalla Highway, north of Hope, BC
What other mechanical weathering processes may
also be important in this area?
Wind abrasion forming Ventifacts
The photo to the left was
taken on Earth. Which other
planet has similar
weathering features? Can
you explain the connection
with the area shown here?
Growth of Salt Crystals
•
Saline water seeps into permeable rock and then evaporates, causing
interstitial growth of salt crystals, which pry the rock grains or crystals apart.
Honeycomb weathering of sandstone on a rocky ocean shore, Gabriola Island, BC
FREEZE-THAW
FREEZE-THAW
• Hydraulic Action
• On rocky shorelines, the powerful
force of breaking waves forces water
into cracks and fractures in the rock
• Wind Erosion
• Growth of Salt Crystals
• Other physical means
– Insolation – daily temperature
changes cause expansion and
contraction of rocks.
– Slaking – interstitial water (due to
alternate wetting and drying of
rocks) creates tension.
Talus slope created by ice-wedging
Hydrolysis, Oxidation and Dissolution are
chemical weathering processes
• Water molecule is polarized (H is
positive, O is negative)
• The negative O ions attract cations
• Water is known as the “Universal
Solvent” because so many
substances are water soluble.
Hydrolysis
• Hydrolysis is the chemical reaction (cation exchange) of a compound with
acidic water.
• Example: Feldspar (either orthoclase or plagioclase) is converted to the clay
mineral kaolinite
Carbonic
acid
Oxidation
• Oxidation is the loss of electrons by cations and their use by free oxygen.
The red coloration here is the result of oxidation. Which element,
that turns red in its oxidized state, is responsible for this colour?
Dissolution
• Dissolution occurs when carbonic acid
(produced when atmospheric CO2 dissolves
in water) dissolves the mineral calcite,
which is the main constituent of limestone.
• Widespread dissolution of carbonate rock
leads to the development of:
Karst Topography
Biological Weathering
• Biological Weathering involves both chemical and physical processes
Moving, mixing, decomposition and CO2
production (resulting in carbonic acid)
Growing plants exert pressure
on the rock
Sedimentary products result from all three
types of weathering
Rocks and minerals can be ranked by their
vulnerability to weathering
Weathered granite outcrop
Which mineral in granite is most susceptible to chemical weathering?
Natural bridges are
products of weathering
Clay minerals are phyllosilicates …
• They have sheet structures composed of silica tetrahedra and aluminum
octahedra
Aluminum
octahedron
Kaolinite under a scanning electron microscope
Weathering can contribute to climate change
• When the feldspar in a rock like this
gneiss is weathered to clay minerals,
carbon dioxide from the atmosphere
is consumed and ends up as
dissolved bicarbonate which is
transported to and stored in the
oceans.
What type of sedimentary
rock forms from the
dissolved bicarbonate?
The Uplift - Weathering hypothesis
• The global rate of chemical
weathering is dependent on
availability of fresh rock
• Mountain chains (formed during
orogenesis resulting from
continental collisions) lead to
significantly enhanced weathering
• As new silicate-rich crust is
exposed to weathering,
atmospheric carbon dioxide is
consumed and the climate cools.
The formation of the Himalayan
Mountains and the Tibetan Plateau
took place over the past 40 million
years, and contributed to the cooling
that resulted in the Pleistocene
glaciations.
Weathering produces soil
SOIL FORMATION
• influenced by:
•
biological processes
•
nature of parent rock
•
climate
•
topography
•
time
Climate is critical in soil formation
• Physical Weathering
dominates in regions of low
temperature and low rainfall.
• Chemical Weathering
dominates in regions of high
temperature and high rainfall.
Why are cold humid
climates not common?
Environments
HOT, ARID ENVIRONMENTS
• Salts accumulate at the surface due to evaporation.
• Frost, abrasion, and slaking break the rocky surface into sand or gravel.
• Wind blows smaller particles away – creating a desert pavement of cobbles
and boulders.
• COLD ENVIRONMENTS
• Mechanical breakdown (by ice wedging)
is the major weathering process.
Hot and humid environments
• Hot and humid environments lead to the formation of insoluble residues with
crusts of iron oxides (laterite) and aluminum oxides (bauxite)
Soil Horizons
Animation
Seasonal freezing allows vegetation
debris to accumulate in the soil.
Soil surface becomes rich in
organic plant debris, known
as the HUMUS LAYER
Soil erosion is a significant problem
• Fluvial Erosion – particles of sediment
removed by flowing water in a channel:
– Suspension
– Bed Load
– Saltation
• Mass Wasting – sediment moved by
gravity
– Debris flows
– Mudflows
• Eolian Erosion – sediment moved by
wind
Fluvial erosion involves several different
processes
The power of wind – eolian erosion!
Which parts of Canada are susceptible to wind erosion?
There are 10 orders in
the Canadian soil classification system
What is the main soil type in
your region of Canada?
Which climate, weathering processes and soil profiles
are likely in each of these five settings?
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