What would happen if asteroid 99942 Apophis collides with the earth? In the news recently it's been said that in 2029 the asteroid called Apophis, aka 'The God of Chaos', will fly by very close to earth. To quote Geek.com


  In 2029, a massive asteroid called 99942 Apophis will fly past our
  planet at an approximate distance of 19,000 miles within the distance
  of some orbiting spacecraft.


It's said the chances of a collision are about 1 in 100,000, which is small but not not completely infeasible either.

The asteroid has a diameter of around 350 meters with a mass of 4×10^10 kg and an average orbital speed of around 30km/s.

What kind of damage could we expect on earth if it were to collide with us?
If it hits the deep ocean, not much relatively speaking. In the recent past, people wildly exaggerated the capability of an asteroid (at least ones in the size range of Apophis) to cause massive tsunamis; in reality, the ocean efficiently and quickly absorbs the impact. That said, it'd still be terrifying to see such an impact in deep water.

If it hits a city, it would cause violent devastation, immediately killing 10's or 100's of thousands with its initial impact, and millions with the resulting shock wave and debris.