Snow can be a bish! Especially when the solid ground below it is 20' down. Thus they run tires with a non-aggressive tread, massive width (and height) and very little air pressure. Those guys will run 6psi WITHOUT beadlocks, and about 1psi with.
They do similar here in the winter. As in true snow wheeling (42" tires or bigger required). It becomes a pretty crazy battle between keeping the rig light enough, having it stay together, and choosing a tire that doesn't just dig you to the ground, but still allows some traction.
Anyways, Iceland is pretty sweet.