In The Time of Dinosaurs

<How big a boom will this thing make when it hits?> Rachel asked.
<That depends on the speed of the comet and its size,> Ax said. <The Mercora have observed the comet. They say it is approximately five of your miles across. It is approaching at a speed of fifteen miles per second.>
<Per second?> Marco asked.
<Yes. When it hits it will release as much energy as, say, a million of the nuclear weapons on that submarine.>
<Excuse me? A million nukes?>
<Well, assuming the "nukes" are reasonably well-made examples of primitive nuclear -
technology. I am being very approximate,> Ax said. <There will be shock waves. One shock wave will go forward into and through the earth. It will compress the rock beneath it, which will release all the carbon
dioxide trapped there. There will be a huge fireball from the exploding gases and from the vaporized comet itself. Everything within a hundred miles or so, every animal, every plant, everything, will be incinerated.
There will be a huge crater, maybe ten, twenty miles deep. The second shock wave will bounce back from the impact. It will blow massive quantities of burning rock all the way out into space. These burning rocks will fall across a wide area. As they reenter the atmosphere they will probably cause a massive heat wave, so hot that trees and grass will catch fire and burn. Any living thing out in the open will be cooked alive as ->
<Enough!> Cassie cried.