After-effects of an impact

The formation of craters is surely the most evident sign of an impact, but the possible after-effects are numerous and very dangerous. First of all, the destructive power of an impact is not limited to the region of the crater. A first estimate says that an object of about 70m can destroy average cities. Metropolis only need an impact with a 150-200 m to be wiped off.
The first after effect to be a cause of destruction is the shock wave generated by the impact. This shock wave moves very rapidly and generates on its path earthquakes and fires .
Another danger comes from the climatic after-effects. These are basically due to the release of dust (if the impact is on the ground) or vaporized sea-water (if the impact is in water). This phenomenon, called the chimney effect, releases dust directly in the upper layers of the Earth atmosphere, with two main consequences: the atmosphere becomes very opaque, making a global fall of temperature, and chemical changes of the atmosphere composition take place.


The chimney effect : what happens, from a climatic point of view?

What is the chimney effect? Before the real impact takes place, a comet or asteroid tunnels through the  atmosphere creating what can be seen as a real, temporary chimney. This tunnel is a good way for debris to be drawn from the impact site to the upper atmosphere. This has been observed in the case of comet SL9 on Jupiter, with the formation of giant plumes following the impact. In this way, an asteroid impact is much more efficient at sending dust into the upper atmosphere than a volcanic explosion, and the climatic disruption is probably much greater.
Let's suppose an impact with a 500 yard asteroid. In this case, this event would pulverize an amount of rock which is about 1000 times the volume of the asteroid itself, and send much of it directly into the upper atmosphere, where it would circulate around the globe .
In particular, this huge quantity of dust released, makes the atmosphere much more opaque and for very long periods, generating a temporary cooling of the Earth. At high energies (for example the case of the 5 Km asteroid which was responsible for the dinosaurs' extinction in the Chicuxclub event) scientists think that the impact would produce such a quantity of dust that darkness wouldn't make photosynthesis possible. The release of dust can also lead to important chemical changes of the atmosphere composition.

What about the oceans?

Image courtesy of SANDIA
With much less dust released into the atmosphere, an ocean impact will have very different, and perhaps less damaging, effects than a  land impact. Scientists are only beginning to understand this effect in the case of an impact into Earth's oceans (see tsunami ).
An ocean impact by the same asteroid mentioned above (a 500-meter-diameter asteroid) will vaporize about 83 cubic kilometers of water. At first sight, this value appears to be insignificant, since it only represents less than one tenth of the total amount of water that evaporates from the world's oceans every day.

However, an ocean impact, would send the water vapor high into the atmosphere (compared to the lower atmosphere layers reached by evaporation). These upper layers (the stratosphere) are normally extremely dry and the effects of a sudden injection of a large quantity of water vapor are simply unknown. Other effects of concern are greenhouse warming and ozone depletion. In fact, unlike evaporation, an ocean impact would send salt into the air and this element may affect upper atmosphere ozone levels.
If an asteroid struck thick ice formations, such as Antarctica or the extensive ice sheets of the last Ice Age, the result would likely be similar to a water impact.