Mira came to a few conclusions as to why her shovel disintegrated slower than the piece of ice she threw in the fog. One is that her shovel was much denser than the piece of ice she threw. Now, this is probably true even if it's not the main reason, but with how powerful the fog is, it would still disintegrate in an instant, but one instant will be a bit longer than the other.
Another reason is the ground might not be as toxic as the fog. However, Mira has some serious doubts about that. The ground is literally a purplish-black color, almost like it's been contaminated for years. Not to mention that the ground seems to be the source of the fog or at least tied to the source. If anything, the ground should be several times more toxic than the fog.
