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The Devistation of a Large Meteoritic Impact

First some definitions:


Asteroid	A relatively small, inactive, rocky body orbiting the Sun.
Comet	A relatively small, at times active, object whose ices can vaporize in sunlight forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust and/or gas.
Meteoroid	A small particle from a comet or asteroid orbiting the Sun.
Meteor	The light phenomena which results when a meteoroid enters the Earth's atmosphere and vaporizes; a shooting star.
Meteorite	A meteoroid that survives its passage through the Earth's atmosphere and lands upon the Earth's surface.




The interaction of small asteroids in the Earth's atmosphere
Depending on the size of the meteroid, what its made up of and the angle it hits the earth's atmosphere, a meteoroid can reach different zones of the atmosphere. Small meteoroids hit the earth all the time, but never reach the ground. The collisions between free atoms and air molecules in the atmosphere produce heat, light and ionization, i.e. a meteor, usually evaporating the meteoroid. See diagram of different kinds of phenomenon Here.

Meteoroid velocities: about 30 km/s (about 30x a bullet). Large meteoriods throw material up into and above the atmosphere, and is highly heated during re-entering, sparking fires. Typical ejecta fall within a diameter or two of the crater. Larger debris will make their own secondary craters.

Meteoroids falling in the ocean would create great tsunamis (enormous waves), destroying coastal lands, heating the ocean and introducing water vapor into the atmosphere.

The lighter material thrown high in the atmosphere would heat up as it re-enters. Dust left in the atmosphere would block sunlight, now cooling the surface. This 'nuclear winter' would kill vegetation which in turn would kill off species dependent on vegetation.

The atmosphere's chemistry could change drastically, creating acid rain. The impact, if large enough, could disturb the Earth's mantle, creating enormous lava flows.