, ▁while ▁the ▁l ith os phere ▁is ▁sub duct ed ▁back ▁into ▁the ▁ast hen os phere ▁at ▁ocean ▁tren ches . ▁Two ▁processes , ▁r idge - push ▁and ▁sl ab ▁pull , ▁are ▁thought ▁to ▁be ▁responsible ▁for ▁spread ing ▁at ▁mid - o cean ▁rid ges . ▁R idge ▁push ▁refers ▁to ▁the ▁grav itation ▁sl iding ▁of ▁the ▁ocean ▁plate ▁that ▁is ▁raised ▁above ▁the ▁hot ter ▁ast hen os phere , ▁thus ▁creating ▁a ▁body ▁force ▁causing ▁sl iding ▁of ▁the ▁plate ▁down s lo pe . ▁In ▁sl ab ▁pull ▁the ▁weight ▁of ▁a ▁t ect onic ▁plate ▁being ▁sub duct ed ▁( p ul led ) ▁below ▁an ▁over lying ▁plate ▁at ▁a ▁sub du ction ▁zone ▁drag s ▁the ▁rest ▁of ▁the ▁plate ▁along ▁behind ▁it . ▁The ▁sl ab ▁pull ▁mechanism ▁is ▁considered ▁to ▁be ▁contrib uting ▁more ▁than ▁the ▁r idge ▁push . ▁ ▁A ▁process ▁previously ▁proposed ▁to ▁contribute ▁to ▁plate ▁motion ▁and ▁the ▁formation ▁of ▁new ▁ocean ic ▁cr ust ▁at ▁mid - o cean ▁rid ges ▁is ▁the ▁" m ant le ▁convey or " ▁due ▁to ▁deep ▁con ve ction ▁( see ▁image ). ▁However , ▁some ▁studies ▁have ▁shown ▁that ▁the ▁upper ▁mant le ▁( as then os phere ) ▁is ▁too ▁pl astic ▁( flex ible ) ▁to ▁generate ▁enough ▁fr iction ▁to ▁pull ▁the ▁t ect onic ▁plate ▁along . ▁Moreover , ▁mant le ▁up well ing ▁that ▁causes ▁mag ma ▁to ▁form ▁beneath ▁the