l>CHAPTER 5: WEATHERING, EROSION, AND SOILWEATHERING--the disintegration and decomposition of rock at ornear surface.MASS WASTING--the transfer of rock material downslope under theinfluence of gravity.EROSION--the incorporation and transportation of material by amobile agent: WATER, WIND, or ICE.Weathering falls into two catagories: MECHANICAL WEATHERING (physical weathering process)andCHEMICAL WEATHERING.

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MECHANICAL WEATHERING - "making big pieces into smallerpieces"

Types of Mechanical Weathering:FROST WEDGING--ice forms in cracks, splits rocks. Expanding ice at a temperature of 22 oF ( -7.6oC) can exert a pressure of 4.3 million pounds per square foot (2,100 kg/cm2) (atmospheric pressure at sealevel ~ 15 lb/ft2)UNLOADING--removal of rock overburden causes rocks that wereunder pressure toexpand, creating joints, cracks in a rockthat have not had appreciable movement of rock along the cracks. The process of expansion by unloading leads to "sheeting" that forms exfoliationdomes
(such as in Yosemite National Park, CA). Half Dome, Exfoliation Dome, Yosemite, CASheeting in mines due to the release of pressure, by tunneling or quarrying,can cause rock on the sides of shaft to dangerously explode off the walls in rock blasts.THERMAL EXPANSION--daily heating cycle causes 30o C variation;this causes expansion pressure on the surface of rocks that createsfractures.ORGANIC ACTIVITY--plants (roots), burrowing animals, humans.Roots expand after a rainstorm. Salt wedging--precipitating salts create a pressure as the salt crystals form. This forcecan break rocks apart. This is most important near the coasts where ocean spray is constantlybeing deposited on rock surfaces RESULTS OF MECHANICAL WEATHERING--Mechanical weathering acts to increase the surface area availableto chemical attach. Forexample, a fresh block of granite with a volume of 1 cubic meterhas six sides with a total of6 square meters of surface area. The surface area within a cubicmeter of sand approaches asquare kilometer! For each cubic meter of sand on the beachabout 10 cubic meters of clayare deposited offshore. Clay particles are "flat"- similar tochips of mica, and have up to 100times the surface area as an equal volume of sand. The greater the surface area, the greater the area to be worked on by chemical weathering.

CHEMICAL WEATHERING--involves chemical transformation into 1or more new materials.

H2O + CO2 = H2CO3 (carbonic acid) from atmosphere and plantacids dissolve mostrocks through time. H2CO3 + CaCO3 (calcite) = Ca+2 + HCO3-, a reversibleprocess (land and oceans).Calcite is the primary mineral in limestone. Calcite also acts aa cement in sediments toform sedimentary rocks.Quartz (SiO2 ) dissolves and reprecipitates as silica, whichacts as a cement in sedimentsto form sedimentary rocks.Feldspar weathers to become clay (reversible undermetamorphic conditions).Mafic Minerals become clay, silica, and iron oxide minerals(limonite/hematite).Cations (such as K+, Na+, Ca++ and anions (such as Cl-,HSO4- ,HCO3-) stay insolution and add to the saltiness of the ocean, precipitate assalts, or are incorporated intoclays in sediments.


Rock exposures, such as on cliffs or barren hilltops, havecharacteristic appearances due toSpheroidal Weathering (the "rounding of rough edges") that occurs because chemical attackis more effective on corners (it comes from 3 sides of the corner) compared to faces (chemical attack is only from one direction) andDifferential Weathering (some rocks and rock layers resist erosion better thanothers) creating a rough weathered surface. Hoodoos and arches are the result ofdifferential erosion and the beauty of these features has inspired us to create twonational parks Arches and Bryce Canyon National Parks, both in Utah.As chemical and mechanical weathering proceed, rock materialaccumulates as aREGOLITH layer of rock and mineral fragments produced byweathering. If water isavailable, plant and animal activity affect the material, anddead organic matter accumulates.This results in the FORMATION OF SOIL. SOIL--mineral matter (~45%), HUMUS--organic matter (~5%), air &water (~50%).CONTROLS OF SOIL FORMATIONPARENT MATERIAL--the original mineral matter can be either:RESIDUAL(regolith that accumulates "in place") or TRANSPORTED mineralmaterial by water, wind,or ice.TIME--important to all geologic processes; longerexposure = more weathering.CLIMATE--water and temperature affect weathering processes.Hot & humid climatesproduce deep weathered soils lacking essential nutrients. Cold &dry climates results in veryslow weathering which increases chances of wind erosion beforesoil can form.PLANTS & ANIMALS: microscopic organisms do most of the work;the acculation oforganic mater forms HUMUS; roots, worms, rodents, and insectsrework the regolith andhumus to form soil. SLOPE - soils tend to be thickest on flat surfaces (due tomass wasting & erosion). Sunny slopes tend to be warmer and dry faster.SOIL PROFILEO horizon--"fresh" organic matter that accumulates onthe ground surface.

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A horizon--consists of mixed organic and rockfragments: water washesout fines (by a process called ELUVIATION); mineral nutrientsdissolved away by processcalled LEACHING.B horizon--zone of "accumulation" of fines; forms a"hardpan"C horizon--(below "solum" or true soil) is a zone ofweathering bedrock above the actual unweathered bedrock--still considered aregolithTHREE GENERAL KINDS OF SOILSPEDALFER--soils rich in aluminum-rich clays and iron; midlattitudes, with annualprecipitation of at least ~25 inches (East Coast region).PEDOCAL--soils in dry regions (due to high evaporationrates). Enriched with calcium carbonate, caliche (Western United States).LATERITIC--soils in wet, more tropical climates. Allcalcite and silica is leached from the soil, leaving red (iron) and aluminum (aluminum can be enriched enough to formthe ore mineral Bauxite from which we obtain aluminum metal) (South).Click here toreturn to the Class Website