▁ocean ▁rid ges ▁appears ▁to ▁involve ▁only ▁its ▁upper ▁ 4 0 0   km ▁( 2 5 0   mi ), ▁as ▁dedu ced ▁from ▁se ism ic ▁tom ography ▁and ▁from ▁observations ▁of ▁the ▁se ism ic ▁dis continu ity ▁in ▁the ▁upper ▁mant le ▁at ▁about ▁ 4 0 0   km ▁( 2 5 0   mi ). ▁On ▁the ▁other ▁hand , ▁some ▁of ▁the ▁world ' s ▁largest ▁t ect onic ▁pl ates ▁such ▁as ▁the ▁North ▁American ▁Pl ate ▁and ▁South ▁American ▁plate ▁are ▁in ▁motion , ▁yet ▁only ▁are ▁being ▁sub duct ed ▁in ▁restricted ▁locations ▁such ▁as ▁the ▁L esser ▁Ant illes ▁Arc ▁and ▁Scot ia ▁Arc , ▁pointing ▁to ▁action ▁by ▁the ▁r idge ▁push ▁body ▁force ▁on ▁these ▁pl ates . ▁Computer ▁model ing ▁of ▁the ▁pl ates ▁and ▁mant le ▁mot ions ▁suggest ▁that ▁plate ▁motion ▁and ▁mant le ▁con ve ction ▁are ▁not ▁connected , ▁and ▁the ▁main ▁plate ▁driving ▁force ▁is ▁sl ab ▁pull . ▁ ▁Imp act ▁on ▁global ▁sea ▁level ▁▁ ▁In cre ased ▁rates ▁of ▁se af loor ▁spread ing ▁( i . e . ▁the ▁rate ▁of ▁expansion ▁of ▁the ▁mid - o cean ▁r idge ) ▁has ▁caused ▁global ▁( e ust atic ) ▁se ale vel ▁to ▁rise ▁over ▁very ▁long ▁times cal es ▁( mill ions ▁of ▁years ). ▁In cre ased ▁se af loor ▁spread ing ▁means ▁that ▁the ▁mid - o cean ▁r idge ▁will ▁then ▁expand ▁and ▁form ▁a