Volcanic ash simply lands on a volcanic mountain when it's active. This would make it easier to slide down with a metal shield.
A wooden shield would be better for an aquatic area, since it floats and metal shields don't. (Don't argue with the logic: Link can't sink in water unless he WEARS the Iron Boots -- So his weight on a wooden shield is negligible.)
For a snowy area, I'm not sure if a metal shield would be better or a wooden one. Wooden shields are lighter, so you wouldn't fall into pits or deep snow (think, like a snowshoe). However, they splinter easy, especially on big landings. That's why a metal shield would be good.
(Apparently, they haven't yet discovered plastic in Hyrule, even though their first legend includes robots...)