English: Schematic diagram showing the physical processes within the Earth’s upper mantle that lead to the generation of magma. A to D are different plate tectonic settings. The graphs show the geotherm (temperature curve inside the Earth, red) and the solidus (temperature where rock starts to melt, green). When the two curves cross each other, magma is generated by partial melting.
A) the curves do not cross - no magma is generated
B) at mid-ocean ridges magma generation occurs at quite shallow depths due to high temperatures and very thin lithosphere
C) over mantle plumes magma generation occurs at larger depths due to even higher temperatures but thicker lithosphere
D) over subducting slabs magma generation occurs at larger depths due to lowering of melting temperature of the rock by fluids released from the slab