Ice melts faster than normal under several conditions including if the temperature is raised significantly. It also melts if the transition point from solid to liquid water, known as the freezing point, is lowered or if the pressure on the ice is increased.
Substances that form a solution with water can alter the freezing point. Sodium chloride, for example, can create such a solution. The addition of salt to ice in a 10 percent solution lowers the transition point from 32 degrees Fahrenheit to 20 degrees. At temperatures above the transition point, the ice melts.
Increasing the pressure on ice causes it to melt. The ice at the bottom layer of a glacier is typically under so much pressure that it is always found as a liquid. This explains how glaciers flow, albeit at a slow rate.