Typically , the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones . For a typical distance oceanic lithosphere must travel before being subducted , the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones ; for shorter or longer distances , the subduction zone ( and therefore also the mean ) thickness becomes smaller or larger , respectively .