▁of ▁bas alt ▁known ▁as ▁M OR B ▁for ▁mid - o cean ▁r idge ▁bas alt , ▁and ▁gab bro ▁below ▁it ▁in ▁the ▁lower ▁ocean ic ▁cr ust . ▁Mid - o cean ▁r idge ▁bas alt ▁is ▁a ▁th ole i it ic ▁bas alt ▁and ▁is ▁low ▁in ▁in compatible ▁elements . ▁H ydro ther mal ▁v ents ▁fue led ▁by ▁mag m atic ▁and ▁vol can ic ▁heat ▁are ▁a ▁common ▁feature ▁at ▁ocean ic ▁spread ing ▁cent ers . ▁ ▁Most ▁ ▁cr ust ▁in ▁the ▁ocean ▁bas ins ▁is ▁less ▁than ▁ 2 0 0 ▁million ▁years ▁old , ▁which ▁is ▁much ▁younger ▁than ▁the ▁ 4 . 5 4 ▁billion ▁year ▁age ▁of ▁the ▁Earth . ▁This ▁fact ▁reflect s ▁the ▁process ▁of ▁l ith os phere ▁rec y cling ▁into ▁the ▁Earth ' s ▁mant le ▁during ▁sub du ction . ▁As ▁the ▁ocean ic ▁cr ust ▁and ▁l ith os phere ▁moves ▁away ▁from ▁the ▁r idge ▁axis , ▁the ▁per id ot ite ▁in ▁the ▁underlying ▁mant le ▁l ith os phere ▁co ols ▁and ▁becomes ▁more ▁rig id . ▁The ▁cr ust ▁and ▁the ▁relatively ▁rig id ▁per id ot ite ▁below ▁it ▁make ▁up ▁the ▁ocean ic ▁l ith os phere , ▁which ▁s its ▁above ▁the ▁less ▁rig id ▁and ▁vis c ous ▁ast hen os phere . ▁ ▁D riv ing ▁mechan isms ▁ ▁Ocean ic ▁l ith os phere ▁is ▁formed ▁at ▁an ▁ocean ic ▁r idge