Water is carried down within the oceanic crust, then
it is released at a depth of about 100 to 125 km, where it travels into the hot (but not yet molten) mantle rocks
Water lowers the melting temperature of the mantle rock, thus even though we do not heat nor decompress it, we are still able to partially melt the mantle rock.
The basaltic magma produced by this process moves up, but before it reaches the Earth's surface, some other process must occur to produce the final magma which has a higher weight percent of SiO2.(Recall "rocks")
