Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma. Much evidence suggests that Basaltic magmas result from dry partial melting of mantle. Basalts make up most of oceanic crust and only mantle underlies crust. Basalts contain minerals like olivine, pyroxene and plagioclase, none of which contain water.
Basalts erupt non-explosively, indicating a low gas content and therefore low water content.
In the laboratory we can determine the melting behavior of garnet peridotite. Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form.