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. 

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Answer the collision of tectonic plates;