▁Earth ' s ▁magnetic ▁field ▁are ▁recorded ▁in ▁those ▁ox ides . ▁The ▁orient ations ▁of ▁the ▁field ▁preserved ▁in ▁the ▁ocean ic ▁cr ust ▁compr ise ▁a ▁record ▁of ▁directions ▁of ▁the ▁Earth ' s ▁magnetic ▁field ▁with ▁time . ▁Because ▁the ▁field ▁has ▁revers ed ▁directions ▁at ▁known ▁intervals ▁throughout ▁its ▁history , ▁the ▁pattern ▁of ▁geom agnet ic ▁revers als ▁in ▁the ▁ocean ▁cr ust ▁can ▁be ▁used ▁as ▁an ▁indicator ▁of ▁age ; ▁given ▁the ▁cr ust al ▁age ▁and ▁distance ▁from ▁the ▁r idge ▁axis , ▁spread ing ▁rates ▁can ▁be ▁calculated . ▁ ▁Sp reading ▁rates ▁range ▁from ▁approximately ▁ 1 0 – 2 0 0   mm / yr . ▁S low - sp reading ▁rid ges ▁such ▁as ▁the ▁Mid - At l antic ▁R idge ▁have ▁spread ▁much ▁less ▁far ▁( show ing ▁a ▁ste eper ▁profile ) ▁than ▁faster ▁rid ges ▁such ▁as ▁the ▁East ▁Pacific ▁R ise ▁( gent le ▁profile ) ▁for ▁the ▁same ▁amount ▁of ▁time ▁and ▁cool ing ▁and ▁consequ ent ▁bath ym et ric ▁deep ening . ▁S low - sp reading ▁rid ges ▁( less ▁than ▁ 4 0   mm / yr ) ▁generally ▁have ▁large ▁r ift ▁valle ys , ▁sometimes ▁as ▁wide ▁as ▁ 1 0 – 2 0   km ▁( 6 . 2 – 1 2 . 4   mi ), ▁and ▁very ▁rug ged ▁terrain ▁at ▁the ▁r idge ▁cr est ▁that ▁can ▁have ▁relief ▁of ▁up ▁to ▁a ▁. ▁By