The explosive eruption was probably caused by high pressure steam which would likely cause steam and other gas pockets (that would later become air pockets) when the molten material rapidly solidified. Rock with air or gas cells is called pumice. Pumice can be light enough that it floats on water. It can also be a good heat insulator. The deposited tephra that formed Kandovan's rock cones consisted of stone-sized debris and ash. You can see the composition if you look closely at the surface of the caves or the cones.
