Basalts formed at mid-ocean ridges and hotspots originate in the mantle and are used to provide information on the composition of the mantle. Magma rising to the surface may undergo fractional crystallization in which components with higher melting points settle out first, and the resulting melts can have widely varying water contents; but when little separation has occurred, the water content is between about 0.07–0.6 wt%. (By comparison, basalts in back-arc basins around volcanic arcs have between 1 wt% and 2.9 wt% because of the water coming off the subducting plate.)[1]