O because of this water that lodges in openings in

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oBecause of this,water that lodges in openings in rocks is not pureit containsabundant dissolved salts.This situation is especially common in coastal environments, where rocksare bathed in sea spray that can invade cavities, cracks, and joints--Formation of salt crystals (if water evaporates), from evaporating water, can alsoapplygreat pressure(by crystals making contact either w/ opposing wall or w/ crystalsattached to it)on the walls of pre-existing cavities and widen them, contributing tomechanically break a rock apart, through a process calledcrystal wedgingthermal expansion and contraction in presence of morning dew, due todaily temperature fluctuations of up to 50°C, can also generate fracturesand produce the shattered pebbles we find in deserts-Cleopatra’s Needle, a granite obelisk that is now located in New York’s Central Park,provides an impressive illustration of the effect ofcrystal wedgingon weatheringoThe obelisk stood in Egypt for more than 3500 years, where its delicatelyengraved inscriptions remained well preserved in the warm, dry climateUnfortunately, before it was moved to New York in 1879, it was stored ata site that let salty water invade the column, allowing salt crystals togrow.In New York’s more humid climate, the salt crystals absorbedmoisture and expanded, destroying the inscriptions in the process-Thermal expansion and contractionoMany field geologists believe that the shattered pebbles found in deserts arecreated by repeatedthermal expansionandcontractionassociated with dailytemperature fluctuations of up to 50°CoEach mineral in a rock has a different susceptibility to expansion and contraction,depending on its bonding structurequartz expands three times more than feldspar for the same increase intemperatureWhen granite out- crops (which contain both feldspar and quartz) areheated, the quartz grains push against and may displace the neigh- boringfeldspar grainsBecause silicate rocks, like granites, are poor conductors of heat,this process takes place only within a few centimeters of thesurfaceoAs a resultthe warmed surface rock may expand andpush away from the cooler rock below, resulting in theformation of a crack that is parallel to the surface
With repeated heating and cooling, the surfacelayer may split apart from the main body of rockand crumble.The process continues as the newly exposedrock surface is subjected to the sameconditions of repeated heating and coolingoInitially, the interpretation of these field observations was not supported bysimple laboratory experiments. Re- peated heating and cooling of rocks, thoughtto simulateonearly 250 years of thermal expansion and contraction, failed to result in anymeasurable weathering.

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