Phenomenon:FrostWedgingThissimpledemonstrationcanbeusedasaphenomenonatthebeginningofaunitonphysical(mechanical)weatheringofrocks.ThiseasydemonstrationcanbeusedtoactivatestudentthinkingabouthowphysicalprocessesmaychangerocksattheEarth’ssurface.Frostwedgingisaphysicalprocesswhererockscanbebrokenintosmallerpiecesandiscausedbyrepeatedcyclesoffreezingandthawing.Waterhastheuniquepropertyofexpanding(~9%)whenitfreezes.Theexpansionoficeiswhyicefloatsinliquidwater.AttheEarth’ssurface,liquidwaterworksitswayintocracksinrock,anduponfreezing,expandsandenlargestheseopenings.Aftermanyfreeze-thawcycles,therockisbrokenintoangularfragmentsasseeninthetopfigure.Frostwedgingisanespeciallyefficientprocessinmountainousregionswherethereisalargeseasonaland/ordailychangeintemperature.Inthisclassroomactivity,plasterofParisisamodelforrockandasmallwaterballoonisamodelforwaterwithinafractureinarock.MixwaterandplasterofParis(accordingtothelabeldirections)andhalffilla12oz.plasticcupwiththeplaster.Putasmallwaterballoon(~1inchdiameter)inthecenterofthecupandfilltherestofthecupwithplaster.Makesurethattheplastercompletelyencapsulatesthewaterballoonandthatitisnotexposedtothesideofthecup.Aftertheplastersets,removeitfromthecup.Asanexperimentalcontrol,itisusefultoprepareanothercupofplasterwithoutawaterballoon.

Aftertheplasterhasdried(oneormoredays),putbothpiecesofplasterintoafreezer.Asthewaterballoonfreezes,itexpandsandcrackstheplaster.Inthefigure,thepieceofplasterontheleftdoesnotcontainawaterballoonanddidnotfracturewhenputinthefreezer.Ontheright,theplastercontainsawaterballoonthatfracturedwhenitwasfrozen.Usingacontrolinthisactivity,demonstratesthattheplasterfracturesbecauseoftheexpansionofthewaterballoonwheniffreezesandnotjustbecausetheplasterwasputinthefreezer.

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