As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases. Since rocks mixtures of minerals, they behave somewhat differently. 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.