The "Krypton-portion" that Argo City rested on was composed of anti-kryptonite. the ground in the city was covered by lead to protect the citizens. Maybe there was only enough lead to shield the city proper, & the dome was there to keep them from leaving the city. Or the dome was part of an experiment that enabled the city to survive Krypton's destruction.
Originally in Action Comics #252, Argo City was blasted into space with a surrounding “air bubble” but no dome. This was changed in later retellings of Supergirl’s origin. However, I always thought the visual depiction of the Argo City asteroid surface barely extending beyond the dome to be unconvincing (and let’s not even consider how the inhabitants survived the blast acceleration).
However, the first story arc of Superman/Batman implied that the new Kara Zor-El’s home was located on a chunk about the size of Australia. This huge size would make it more feasible for a city to survive intact, shielded from the explosion. Maybe the incredible mass would even dampen some of the acceleration, resulting in fewer G-forces. Finally, such a large chunk of heavy Krypton would conceivably retain at least a trace atmosphere, though such an “air bubble” might soon dissipate in the vacuum of space.
What if the original Kara Zor-El’s home asteroid “lifted off” in the same manner, but was later “trimmed down”? We know in the final issue of DC Comics Presents that the Argo City Asteroid was only the size of Metropolis’ Central Park, which seems absurdly small – but what if that was the largest area that could be enclosed by a dome? Perhaps there was a pre-existing domed site within Argo City, or one was hastily constructed post-explosion to contain the dwindling atmosphere.
It would have been expedient to concentrate the survivors in one place, and abandon the remainder of the surface. Then the rest of the asteroid was reshaped, either by remote-controlled explosives or reprogrammed excavation robots. Why? There are a few good reasons.
Probably the fractured crust was already beginning to break up into smaller fragments. Shedding all this unstable material would be deemed wise, since nobody wants any debris coming loose from the “leading edge” and striking the dome.
Perhaps Zor-El’s propulsive system would work more efficiently by pushing less mass, and the irregular contours made the asteroid difficult to maneuver. “Streamlining” this rock would make it easier to position the thrusters, since the center of mass would be calculated faster.
Most important was the danger of X-Kryptonite. Even the lead lining of Argo City would have been overwhelmed by a continent of this lethal material, and you also had to worry about radiation from outlying areas seeping through the dome. Thus, removing everything just beyond the dome and leaving only enough below the city to maintain structural stability was essential. Alas, even this amount was eventually deadly for Argo City. :cry: