Many (me included) have been wondering what is this pyroclastic noise all the big boys are talking about since google knows very little about this. The following paper gave the answer: http://tessendorf.org/papers_files/RIV.pdf It's basicly just about displacing a surface with a summed set of noise layers on different scales like any fractal texture. The only special thing is that the absolute value of the noise is used and the "valley" areas of the displaced surface/volume are left smooth by s