It was also extremely brief. The snow where it struck would be completely vaporized, yes, but the stuff on the slope above and farther out on the plateau would have melted and poured down onto it. With exposure that brief, the rock itself wouldn't have had the time to get heated up to the full 5600 degrees of the fireball (or even close to that temperature). Like when you stick your hand on something really hot and jerk it away immediately. You get angry red marks but your skin doesn't melt off like it would if you held it there. I'd imagine the rock closest to the explosion would be all nice and glossy now, but I doubt it would actually have melted the entire plateau.