Why doesn't the iris disintegrate with the activation of the stargate?

The placement of a barrier a few microns above the event horizon of a wormhole prevents the creation of an unstable vortex, or 'swoosh' effect. The vortex itself isn't necessary for the wormhole to be created; the Nox and future Cassandra in '1969' are shown to be able to activate the stargate without the 'swoosh'. It's not just the iris either: a wormhole formed when the gate was buried in '100 days', because a hard naquadah crust was formed over the gate, whilst not inside it, allowing a wormhole to connect but the MALP to be crushed.(Like the iris does to a few unlucky Jaffa every now and then).

Atlantis' shield is another example of a barrier that allows the gate to operate but no vortex to form.

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