▁spread ing ▁cent ers ▁div iding ▁the ▁axis ▁into ▁segments . ▁One ▁hypothesis ▁for ▁different ▁along - axis ▁depth s ▁is ▁variations ▁in ▁mag ma ▁supply ▁to ▁the ▁spread ing ▁center . ▁Ult ra - slow ▁spread ing ▁rid ges ▁form ▁both ▁mag m atic ▁and ▁am ag m atic ▁( current ly ▁lack ▁vol can ic ▁activity ) ▁r idge ▁segments ▁without ▁transform ▁fault s . ▁ ▁Vol can ism ▁ ▁Mid - o cean ▁rid ges ▁exhib it ▁active ▁vol can ism ▁and ▁se ism icity . ▁The ▁ocean ic ▁cr ust ▁is ▁in ▁a ▁constant ▁state ▁of ▁' ren ew al ' ▁at ▁the ▁mid - o cean ▁rid ges ▁by ▁the ▁processes ▁of ▁se af loor ▁spread ing ▁and ▁plate ▁t ect on ics . ▁New ▁mag ma ▁stead ily ▁emer ges ▁onto ▁the ▁ocean ▁floor ▁and ▁intr udes ▁into ▁the ▁existing ▁ocean ▁cr ust ▁at ▁and ▁near ▁rif ts ▁along ▁the ▁r idge ▁axes . ▁The ▁rocks ▁making ▁up ▁the ▁cr ust ▁below ▁the ▁se af loor ▁are ▁young est ▁along ▁the ▁axis ▁of ▁the ▁r idge ▁and ▁age ▁with ▁increasing ▁distance ▁from ▁that ▁axis . ▁New ▁mag ma ▁of ▁bas alt ▁composition ▁emer ges ▁at ▁and ▁near ▁the ▁axis ▁because ▁of ▁decom pression ▁mel ting ▁in ▁the ▁underlying ▁Earth ' s ▁mant le . ▁The ▁is ent rop ic ▁up well ing ▁solid ▁mant le ▁material ▁exceed s ▁the ▁solid us ▁temperature ▁and ▁mel ts . ▁The ▁cry st all ized ▁mag ma ▁forms ▁new ▁cr ust