▁Th erm al ▁contra ction ▁theory ▁The ▁thermal ▁contra ction ▁theory ▁pos its ▁that ▁during ▁the ▁winter ▁months , ▁thermal ▁contra ction ▁cra cks ▁form ▁only ▁a ▁few ▁cm ▁wide ▁and ▁a ▁couple ▁of ▁metres ▁deep ▁because ▁of ▁the ▁extreme ▁cold ▁weather . ▁Over ▁the ▁next ▁few ▁months , ▁the ▁snow ▁mel ts ▁and ▁the ▁remaining ▁water ▁fills ▁the ▁cra cks ▁and ▁the ▁per ma f rost ▁below ▁the ▁surface ▁free zes ▁it . ▁These ▁tiny ▁cra cks ▁turn ▁into ▁per ma f rost . ▁Once ▁the ▁summer ▁months ▁arrive , ▁the ▁per ma f rost ▁exp ands ; ▁horizontal ▁compression ▁produces ▁u pt urn ing ▁of ▁the ▁fro zen ▁sed iment ▁by ▁pl astic ▁de formation . ▁The ▁next ▁winter ▁the ▁cold ▁ref ree zes ▁and ▁cra cks ▁the ▁already ▁forming ▁ice ▁w edge , ▁opening ▁a ▁way ▁for ▁the ▁mel ting ▁snow ▁of ▁spring ▁to ▁fill ▁the ▁empty ▁crack . ▁The ▁mean ▁annual ▁air ▁temperature ▁thought ▁needed ▁to ▁form ▁ice ▁wed ges ▁is ▁− 6 ° ▁to ▁− 8   ° C ▁or ▁col der . ▁ ▁Form s ▁There ▁are ▁three ▁different ▁forms ▁of ▁ice ▁wed ges : ▁active , ▁in active ▁and ▁cast s . ▁All ▁three ▁forms ▁are ▁pre val ent ▁today ▁and ▁can ▁be ▁found ▁in ▁different ▁parts ▁of ▁the ▁world . ▁ ▁Active ▁Active ▁ice ▁wed ges ▁are ▁those ▁that ▁are ▁still ▁evol ving ▁and ▁growing . ▁During ▁each ▁year , ▁a ▁layer ▁of ▁ice ▁will ▁be ▁added ▁if ▁crack ing ▁occurs , ▁but ▁crack ing