Hess mistakenly thought that the oceanic crust consisted of altered  peridotite, the material of the mantle itself. 
It is now known, however,  that it is made up of basaltic rocks, <tag "515674">derived</> indirectly from peridotite,  and erupted by volcanic action along the ridge itself, building up the long  submarine mountain range and giving rise to the relatively few volcanoes  which poke their heads above sea level.   

