If one could manage to get your ashes into lava, they would just vaporize. Getting that kind of access is not easy though. I don't think the National Park Service would allow it. They require that any dispersal of cremains be done away from where others can see it.
I wasn't thinking maybe Mt St Helen's but the Forest Service seems to have a blanket prohibition against it.
Neither burial nor scattering of ashes are an appropriate use of the forests and are not authorized on the lands of the National Forest System. The placing of remains creates a permanent occupancy of the land and the placing of headstones or monuments documenting the location are similarly permanent. Placing of statues, flowers and other items of remembrance would be considered a monument.