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

• 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.

FREEZE-THAW

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 areproducts of weathering

Clay minerals are phyllosilicates …

• They have sheet structures composed of silica tetrahedra and aluminum octahedra

Aluminumoctahedron

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 surface becomes rich in organic plant debris, known as the HUMUS LAYER

Soil Horizons Animation

Seasonal freezing allows vegetation debris to accumulate in the soil.

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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