It doesn't matter how compressible it is, its temperature still rises from pressure. That is why ice is so slippery, pressure (from a tire, foot, or ice skate) melts a thin layer of water on the surface which acts as a lubricant (which is also why ice stops being slippery when its temperature gets low enough that normal pressures can't raise it enough to melt).
Answer is thermal energy from tectonic processes. Cite: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi...