Hum,
i just watched the BBC Horizon programme.
The programme puts forward a very persuading case that the Kebir crater was not the origin of the desert glass, rather it was formed by an air burst of a 100 metre sized asteroid.

Seemingly these airbursts generate temperatures similar to the sun, and blow a huge plume of gas out into space. The damage isn't from the crater, if a crater is formed at all , but from the fireball that will fry huge areas of the surface.

“Attempting to blow up an incoming asteroid, Hollywood style, could well make things worse by increasing the number of devastating airbursts.”

(see link above)

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`Irony` actually does mean `metal like`...