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I actually watched a Discovery special on this recently.
The Earth has been hit several times by "mega asteroids" before. They can apparently tell by the kind of rock found in different areas as well as the shape that it is sitting. They actually showed how they proved that the Gulf of Mexico is the largest impact crater on the planet.
That video showed the firestorm part a bit different though. According to the show I saw there would be a massive burn off in the last thousands of miles before and once the meteor entered the Earth's atmosphere. The pieces that fly off as a result would incinerate in the atmosphere and basically act like a series of nuclear explosions big enough to level even the biggest sprawling cities on the planet.
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