Describe two mechanisms by which mantle rock can melt without an increase in temperature

Mantle rock can melt without an increase in temperature if there is a decrease in pressure exerted by overlaying rock layers. This is called decompression melting. Rocks can also melt in the mantle due to dehydration. A subducting slab carries with it water as it is pulled into the mantle. The gradual release of the water contained in the slab is the mechanism that causes melting. 

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.