Question propagation...

From a flist entry: Hey stargate peeps, does anyone know if it has been definitively explained why the Iris in the Stargate does not get vaporized in an initialization of a wormhole? I understand the idea that the placement so close to the formed wormhole prevents total rematerialization and hence stops projectiles and such, but how powerful, canonically, is the opening ka-whoosh? I know we've seen it vaporize people; what about other things?