Decompression melting occurs because

Answer 1

Think of it this way... a peridotite sample buried deep in the mantle is very hot, but also under a great deal of pressure due to the weight of rock above it. If that peridotite were to be brought up near the surface where it would be under far less pressure while still remaining quite hot, it would begin melting (ie, partial melting). This is exactly what happens at mid-ocean ridges and oceanic hotspots, where upwelling currents in the mantle bring hot peridotite mantle rock up to shallower depths (< 100 km) where they begin melting and contribute to the basalts that erupt at these settings.