Why are there no back-arc basins around the eastern Pacific margin?
There are many back-arc basins (system of island arc and back-arc seas) on the western margin of the Pacific Ocean, but we do not find any back-arc basins on the eastern of the Pacific margin even though volcanism related to subducting slabs has been operating on both sides of the Pacific. It seems that subduction of oceanic plates contributes to volcanism not only through the supply of slabderived fluids necessary for upwelling of magma diapirs, but also by inducing convention in the mantle wedge so as to maintain the hot mantle layer within the wedge. Dehydration from the hydrous layer of old descending slabs at depths of 100 to 300 km is important for the genesis of volcanic chains and back-arc spreading. In contrast, dehydration from the hydrous layer of young descending slabs acts quite differently on the origin of volcanism. These differences are shown by the examination of high pressure and high temperature phase changes in MORB (mid -oceanic ridge basalt) + H2O reactions.