▁mode led ▁by ▁the ▁cool ing ▁of ▁a ▁l ith os phere ▁plate ▁or ▁mant le ▁half - space . ▁A ▁good ▁approximation ▁is ▁that ▁the ▁depth ▁of ▁the ▁se af loor ▁at ▁a ▁location ▁on ▁a ▁spread ing ▁mid - o cean ▁r idge ▁proportional ▁to ▁the ▁square ▁root ▁of ▁the ▁age ▁of ▁the ▁se af loor . ▁The ▁overall ▁shape ▁of ▁rid ges ▁results ▁from ▁Pr att ▁is ost acy : ▁close ▁to ▁the ▁r idge ▁axis ▁there ▁is ▁hot , ▁low - dens ity ▁mant le ▁supporting ▁the ▁ocean ic ▁cr ust . ▁As ▁the ▁ocean ic ▁plate ▁co ols , ▁away ▁from ▁the ▁r idge ▁axis , ▁the ▁ocean ic ▁mant le ▁l ith os phere ▁( the ▁col der , ▁dens er ▁part ▁of ▁the ▁mant le ▁that , ▁together ▁with ▁the ▁cr ust , ▁compr ises ▁the ▁ocean ic ▁pl ates ) ▁thick ens ▁and ▁the ▁density ▁increases . ▁Thus ▁older ▁se af loor ▁is ▁under lain ▁by ▁dens er ▁material ▁and ▁is ▁deeper . ▁ ▁Sp reading ▁rate ▁is ▁the ▁rate ▁at ▁which ▁an ▁ocean ▁bas in ▁wid ens ▁due ▁to ▁se af loor ▁spread ing . ▁R ates ▁can ▁be ▁computed ▁by ▁mapping ▁of ▁marine ▁magnetic ▁anom al ies ▁that ▁span ▁mid - o cean ▁rid ges . ▁As ▁cry st all ized ▁bas alt ▁extr ud ed ▁at ▁a ▁r idge ▁axis ▁co ols ▁below ▁Cur ie ▁points ▁of ▁appropriate ▁iron - t itan ium ▁ox ides , ▁magnetic ▁field ▁directions ▁parallel ▁to ▁the