2 Weathering: the disintegration, or breakdown of rock material
3 Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces
4 Chemical Weathering: breakdown as a result of chemical reactions CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-
5 Mechanical Weathering Physical breakuppressure releasewater: freeze - thaw cyclescrystallization of salt in cracksthermal expansion and contractionplant rootsfriction and impactAll this increases the total surface area exposed to weathering processes.
6 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.
7 Mechanical Weathering Many igneous rocks, particularly granite, are formed at great depth.When they come to the surface, they expand.Creates a morphology with exfoliation slabs – slabs sheet off in onionlike layersStone Mountain, GeorgiaExpansion of rocks pose a risk in deep mines – explode off wallsThese fractures, or joints can enhance frost wedging and other erosive processes
11 Stone Mountain, Georgia, showing the product of exfoliation due to unloading Summit of Half Dome, Yosemite, shows exfoliationStone Mountain, GA
12 Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints
13 Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.Effects of freeze thaw on granite. Mt. San Jacinto,southern California, Transverse Ranges. About 8,500 feet.
14 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.WeatheringRain may enhance this process
18 Role of Physical Weathering Reduces rock material to smaller fragments that are easier to transport2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering
20 Rates of weatheringJoints in a rock are a pathway for water – they can enhance mechanical weatheringWater flowing through joints attacks the rock, rounding corners.As minerals are converted to clay, the rock expands, breaking outer layers offCan enhance mechanical weathering
21 Chemical WeatheringDefinition: transformation/decomposition of one mineral into another through chemical changesAgents of chemical weathering:WaterOxygenCO2Living organismsAcid RainMineral breakdowncarbonate dissolvesprimary minerals --> secondary minerals (mostly clays)Net loss of elements retained in the soil.
22 Water is the main operator: Chemical WeatheringWater is the main operator:DissolutionMany ionic and organic compounds dissolve in waterSilica, K, Na, Mg, Ca, Cl, CO3, SO4Acid ReactionsCO2 dissolves in rain water and creates carbonic acidCarbonic acid easily weathers limestone and marble
23 DissolutionH2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-water + carbon dioxide + calcitedissolve into calcium ionand bicarbonate ionBiological activity in soilsgenerates substantial CO2Bicarbonate is the dominantion in surface runoff.
24 Chemical Weathering Oxidation Living Organisms Oxygen dissolved in water promotes oxidation of sulfides, ferrous oxides (rust), native metalsLiving OrganismsPlant material makes H+ ions availableLichens
25 Chemical WeatheringHydration: attachment of water molecules to crystalline structure of a rock, causing expansion and weaknessHydrolysis: combination of hydrogen and oxygen in water with rock to form new substances
26 Solution: process by which rock is dissolved in water Chemical WeatheringSolution: process by which rock is dissolved in waterIs strongly influenced by pH and temperatureWhen water becomes saturated, chemicals may precipitate out forming evaporite deposits.Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.
27 Resistance to Weathering First toCrystallizeFast WeatheringBowen’sReactionSeriesGoldrichStabilitySeriesLast toCrystallizeSlow Weathering
32 Karst TopographyA type of landscape in rainy regions where there is limestone near the surface, characterized by caves, sinkholes, and disappearing streams.Created by chemical weathering of limestone
33 ‘Karst’ landforms develop in areas underlain with limestone This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July.‘Karst’ landforms develop in areas underlain with limestoneWhen Lake Jackson’s sinks rupture and the lakewater drains out, the process is referred to as a drawdown. It has occurred many times during the last century, in 1900, 1907, 1909, 1932, 1935, 1936, 1957, 1982, and again in 1999.
34 ErosionThe process by which water, ice, wind or gravity moves fragments of rock and soil.
38 Mass MovementsLandslides, mudslides, slump and creep
39 Rates of weathering Weathering Climate Temperature and moisture characteristicsChemical weatheringMost effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weatheringLeast effective in polar regions (water is locked up as ice) and arid regions (little water)Mechanical weatheringEnhanced where there are frequent freeze-thaw cycles
40 Mechanical and Chemical Weathering Fracturing, disintegration caused by mechanical weathering exposes more surface area.Greater surface area, means more places for chemical action to occur.