Since igneous rocks form at high temperatures, andunderpressure conditions ranging from one to numerous atmospheres.However,the conditions at the Earth"s surface space somewhat various than theconditionsat which many rocks and minerals form. Therefore, the materialsareno longer at equilibrium when they room exposed come surfaceconditions.Under these conditions, over there is a propensity for every ordered solution toseek lower levels of energy or order. This is every done throughweathering.

You are watching: This can occur mechanically or chemically

Weathering - the fragmentation anddecompositionof absent at or near the surface of the earth. It affects the rocksin place and also no carry is involved. This distinguishesweatheringfrom erosion.

Mechanical/physical weathering - physicaldisintegrationof a rock into smaller fragments, each v the very same properties as theoriginal. Occurs largely by temperature and pressure changes.

Chemical weathering - process by i beg your pardon theinternalstructure the a mineral is transformed by the enhancement or remove ofelements.Change in phase (mineral type) and composition are because of the action ofchemical agents. Chemical weathering is dependent on availablesurfacefor reaction temperature and presence that chemically activefluids.Smaller fragment sizes weather by chemical means more swiftly thanlargeparticles early out to rise of surface ar area. Look in ~ the diagrambelow and you will check out that as the particles acquire smaller, the totalsurfacearea obtainable for chemical weathering increases.


Erosion - the incorporation andtransportationof weathering products by a mobile certified dealer such as wind, water, ice.

All 3 processes might act independently, yet willmoreoften than not, occur simultaneously. Different circumstance willhave one weathering process an ext important than another. Theprocessesmay also act in concert v one another.

Types of mechanically Weathering:
Frost Wedging - water broadens when itfreezes.This photograph shows the separation, personal, instance layers in ~ the sedimentary rockbreaking apart through repeated cycles that freeze-thaw. A similarprocess happens as soon as the rock is continuously wetted and also dried together saltcrystalsdissolve indigenous the rock then thrive when that is dried. Both processescan an outcome in the rocks being heaved - so what was when a nice regularpattern of bricks set in a pavement will certainly eventually end up being a chaoticjumbleof bricks oriented every i beg your pardon way. Thermal growth andContraction- heating causes rock to expand, cooling outcomes in contraction;differentminerals expand and also contract at different rates. This phenomenawilllook very similar to frost wedging and salt decision growth, yet willtypicallyhappen in climates that undergo too much diurnal temperature changes.
Mechanical Exfoliation - rock breaksapart inlayers that room parallel come the earth"s surface; together rock is uncovered,it increases (due come the reduced confining pressure) resulting inexfoliation.The photograph is indigenous G. K. Gilbert (1903) in Sequoia NationalPark.The granite boulder is shaped by exfoliation; the boulder is about 40feetin diameter, and the be separate fragment relaxing on the is about 10 feetthick. Exfoliation is really common whenever plutonic igneous rocksare exposed. Due to the fact that the plutonic rocks cool at depth under greatpressure,they essentially de-pressurizes as soon as the overburden is removed.Thiscauses sheets of rock to peel turn off subparallel to the earth"s surface,orwhatever is the least pressurized surface.
In this picture from Yosemite nationwide Park,the exfoliationsheets room subparallel to the sink walls.
Abrasion - physical grinding of rockfragments.Here, the photo shows part pits that have actually been eroded into the absent bysandblasting. In addition to the physical weathering (thesandblasting),chemical weathering has actually taken place as the rock mirrors some indications ofsolutionweathering together well.
Another photo which mirrors the powerfuleffect ofwind generated abrasion is the dual Arch from arches NationalPark.The edge of the arches have weathered follow me joints, preexistingtectonicallycontrolled upright surfaces in the rock. Then mechanicalabrasiontook over and carved the end the arches.

Types of chemical Weathering:


H2O + CO2+ CaCO3 --> Ca+2+ 2HCO3- water + carbon dioxide + calcite dissolve intocalciumion and also bicarbonate ionDissolution is an extremely common in areas that have a greatdealof limestone. Acidic waters (from contamination or natural) dissolvelimestone permitting for extr water to obtain entrance. Cancausesinkholes and also karst features and also dissolution of law andgravestones.

Oxidation (rust)

4Fe+2 +3O2--> 2Fe2O3 ferrous steel + oxygen integrate to kind ferric ironoxide(hematite)

Will happen to all iron-bearing silicates tovarying degrees.Common reaction minerals are hematite, limonite, and goethite.

Hydrolysis2KAlSi3O8+ 3H20 --> Al2Si2O5(OH)4+ 4SiO2 + 2K(OH) potassium feldspar in acidic water hydrolyses tokaolinite+ quartz + potassium hydroxide

Silicate mineral (unstable at the earth"ssurface) weatherto kind clay mineral such together kaolinite (stable in ~ the earth"ssurface).Feldspars typically weather to create clay minerals.

Factors that affect chemical weatheringClimateLiving organisms bioturbation acid production and mineraldecompositionTimeMineral ingredient Goldich Dissolution series (Bowen"s Reaction Series)

Chemical Weathering products Clays steels ores rounding of boulders (chemicalexfoliation)

Soils and also Soil Formation

Dependence of weathering form on the meantemperatureand annual rainfall.

Weathering prices depend ~ above the ingredient of therock,temperature range and rainfall amount. Weathering producessoils.Soils may or may not remain in place, and any soil might be a combinationof residual and transported material.

Residual soil: continues to be in place; has not beentransported(gruss).

Transported soil: Transported through wind orwaterand deposited.

A finish soil profile will have the followingcomponents:

O horizon: necessary debris and leaflitteron the surface.

A horizon: Topsoil - leaching, water movementdown, Organicand Mineral product transported downward.

B horizon: Subsoil - build-up of dissolvedmaterialand good clays, hardpan.

C horizon: Partially altered parent rock material.

Bedrock: Unweathered parent rock material.

These horizons space not current in all soil profiles.In locations of fast erosion, B & C might be current or C only. In someareas no soil profile will develop at all.

Factors in soil Formation

Climate: The higher the rainfall amount,the morerapid the rate of erosion and also leaching. Laterites form in humidclimateswhere just Al2O3(Bauxite) and Fe(OH)3 remain. Topography: The steeper the surface slope,the morelikely any eroded material is to it is in transported the end of the system. parent Material: Granites are an ext resistantto weatheringthan gabbros. Sandstones are much more resistant come weathering thanlimestonesin humid climates, yet limestones are more resistant 보다 sandstones inarid climates. Plant and also Animal activity: Plant and also animalactivityproduces humic acids that are an effective erosion agents. Plants canphysically erode as well as chemically erode. Tree stabilizesoilprofiles, animals (including man) tend to destabilize the soil profile,increasing erosion. Time: Reaction prices are slow, the much longer arock unithas to be exposed, the an ext likely it is to be weathered.These determinants can it is in remembered through the acronym ClORPT -Climate,Organicactivity (plants and animals), Relief (topography),Parentmaterial, and Time.

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Mineral stability

Sediments are the by-product of weathering.Sedimentsare corpuscle of minerals, some of them altered from the original rock,some simply diminished in sized, and some new minerals through reaction.The stability of minerals can be predicted making use of the Bowen"s reactionseries,however, in the case of the weathering series this is recognized as theGoldichDissolution Series:


MgPyroxeneCalcic Plagioclase

Mg-CaPyroxeneCalcic-Alkalic Plagioclase

AmphiboleAlkalic-Calcic Plagioclase

Biotite Alkalic Plagioclase

Potassium Feldspar



Minerals crystallize native a melt in ~ differenttemperaturesduring the migration and emplacement of the magma. Those mineralsthat crystallize at greater temperatures will be the least stable in ~ thesurface. From this the is noticeable that quartz will be the moststablemineral in the weathering environment, and also will be a dominantconstituentof sediments and sedimentary rocks.