These are small conical piles of ash or soot, perhaps from prior volcanic eruptions. The best explanation I heard was that the ash originally would have fallen out fairly evenly over the snow, but some kind of iterative process of differential melting would have concentrated the ash over the course of many seasons. This might have occurred by the ash lowering the albedo, so that where the ash was more concentrated, the snow melted quicker in the summer, further concentrating the ash in hollows or pits in the snow. The ash piles would be left in relief as the snow melted by warm air around them during warmer summer days. This process might be repeated after winter snowfall and summer melt over many years. We saw a similar phenomenon on the Solheimsjokull glacier that we hiked earlier in the week, and that was the guide's explanation.