Answer:
The higher the pressure on anything, the hotter it will get. The hotter a rock gets, the more possibility that it will have to melt.
So you're out the base of the crust, with any kilometers of crust above you, and that's a lot of pressure. Then take into account that you have thousands of degrees of heat coming from the mantle-core boundary, and directly from radioactive decay within the mantle, which only adds to the temperature.
The net result is like melting a stick of butter. It's very easy to do without a lot of energy -- since Earth is a massive ball of thermal energy, then well... it's easy to do.
