Getting out of bed on a cold winter morning is always a dreadful activity, but I'm much happier to be stepping on the rug beside my bed than the cold tiles in my bathroom. In reality, the rug and tiles are at the same temperature, just one feels colder and the other warmer because of a property known as thermal conductivity. The low conductivity rug insulates my feet, allowing me to loose heat slowly, giving me the apparent feeling of warmth, whereas the high conductivity tiles suck heat from my feet, making them feel cold. The rocks underneath glaciers do the same to the ice, and may in some cases, cause them to melt.
Rocks have conductivities that range from values below glacial ice to values that are significantly greater. The differences between thermal conductivity of rocks beneath glaciers and ice sheets can cause heat to bend towards the high conductivity material as it flows towards the surface. This lateral heat flow also warps isotherms—an imaginary line that traces out constant temperatures—resulting in high temperatures on one side of a geologic contact and low temperatures on the other. The effect can amount to a few degrees. While this may not sound like much, in regions where the ice is close to the melting point, it doesn't take much of an increase to produce melt.
There are several hundred lakes beneath the ice sheet in Antarctica. Much of it covered with more than two kilometers of ice. At the base of the ice sheet in many of these regions, temperatures are close to the melting point. It has yet to be confirmed that thermal conductivity contrasts beneath the ice are responsible for some of these lakes, but modeling shows it is possible.
Willcocks, S., Hasterok, D., Jennings, S., 2021. Thermal refraction: implications for subglacial heat flux. Journal of Glaciology, Journal of Glaciology 1–10. https://doi.org/10.1017/jog.2021.38