weathering_rates_of_weathering_notes.notebook
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February 06, 2014
Weathering Processes
• Weathering: natural process by which atmospheric and environmental agents, such as wind, rain, and temperature changes, disintegrate and decompose rocks
• There are two main types of weathering
> mechanical (physical) weathering > chemical weathering
Mechanical Weathering
• the process by which rocks break down into smaller pieces by physical means
> does not change the composition of the rock
• common agents of mechanical weathering:
> ice
> plants
> animals
> gravity
> running water
> wind
Physical_Weathering.asf
weathering_rates_of_weathering_notes.notebook
February 06, 2014
• Ice wedging:
> occurs when water seeps into crack in rock and then freezes; volume increases about 10% and creates pressure on the surrounding rock
> the cycle of freezing and thawing creates deeper, wider cracks in the rock, and the rock eventually splits apart
• Exfoliation:
> peeling off of rock layers; caused by the release of pressure on bedrock and the expanding bedrock
• Abrasion:
> the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles
> caused by gravity, running water and wind
> gravity causes loose soil and rocks to move down slopes
> running water can carry particles of sand or rock that scrape against each other and stationary rocks
> wind lifts and carries small particles and cause them to strike against other rock surfaces
• Organic Activity:
> plant roots grow and expand to create pressure that wedges rocks apart
> digging activities of burrowing animals can weather rocks – ground squirrels, prairie dogs, ants, earthworms, coyotes, and rabbits
Chemical Weathering
• the process by which rocks break down as a result of chemical reactions
• chemical reactions commonly occur between rocks, water, carbon dioxide, oxygen and acids
• chemical weathering changes the chemical and physical composition of the rock
Chemical_Weathering.asf
weathering_rates_of_weathering_notes.notebook
February 06, 2014
• Oxidation:
> a reaction that removes one or more electrons from a substance such that the substance's valence or oxidation state increases; in geology, the process by which a metallic element combines with oxygen
> commonly occurs in rock that has iron­bearing minerals (hematite, magnetite); iron combines with oxygen dissolved in water to form rust (iron oxide)
4Fe + 3O2 ­­> 2Fe2O3
• Hydrolysis:
> a chemical reaction between water and another substance to form two or more new substances
> minerals affected by hydrolysis often dissolve in water that carries the dissolved minerals to lower layers of rock (leaching)
• Carbonation:
> water and carbon dioxide react to form carbonic acid
H2O + CO2 ­­> H2CO3
> carbonation is the conversion of a compound into a carbonate; occurs when carbonic acid reacts with calcite, a major component of limestone, and converts the calcite into calcium bicarbonate (dissolves easily in water so limestone eventually weathers away)
• Organic Acids:
> acids produced naturally by some living organisms
– lichens and mosses grow on rocks and produce weak acids that weather the surfaces
– they can seep into the rock and break the rock apart
• Acid Precipitation:
> precipitation such as rain, sleet, or snow, that contains a high concentration of acids, often because of the pollution of the atmosphere
> weathers some rock faster than ordinary precipitation
weathering_rates_of_weathering_notes.notebook
February 06, 2014
Rates of Weathering
• processes of mechanical and chemical weathering work very slowly (carbonation dissolves limestone at an average rate of only about 0.2 cm every 100 years)
• rates of weathering depend on
> rock composition
> climate
> topography
• Differential weathering: the process by which softer, less weather resistant rocks wear away at a faster rate than harder, more weather resistant rocks do
• Rock composition: > limestone and other sedimentary rocks that contain calcite are weathered more rapidly because they undergo more carbonation
> rates at which rocks weather depend mostly on the material that holds the sediment grains together
• Amount of exposure:
> the more exposure to weathering agents the faster the rock will weather
> mechanical weathering breaks up rock into smaller pieces which increases the surface area available for chemical weathering
• Climate: > freezing and thawing can cause mechanical weathering of rock by ice wedging
> chemical weathering increases in warm, humid climates; the constant moisture is highly destructive to exposed surfaces
> weathering is slow in hot, dry climates and very cold climates
weathering_rates_of_weathering_notes.notebook
February 06, 2014
• Human activities:
> mining exposes rock to strong acids and other chemical processes
> construction removes soil, exposing rock
> recreational activities (hiking or riding ATVs) can speed up weathering • Plant and animal activities:
> rock disturbed by plants and animals weathers more rapidly than undisturbed rock
> some biological wastes of animals can cause chemical weathering
– caves with large populations of bats, have large quantities of bat guano (speeding up chemical weathering), which attracts small insects(speeding up mechanical weathering)