... an asteroid hit the earth?

Last time a large asteroid struck the Earth, 65 million years ago, it caused the extinction of the dinosaurs.

What if a large asteroid hit the earth ?

In the case of a 10-kilometer asteroid, the damage would be catastrophic. The meteorites reach the speed of about 30 km / s. Unlike small meteors that crash every day with our atmosphere, a meteor of this size will not be slowed to much by air friction. When it reaches the surface, the impact will be so strong that no matter if it hit the sea or the land.
Only the impact with the Earth's crust will stop the asteroid. The impact energy vaporize the asteroid and a good part of the Earth's crust, creating a crater of more than 100 km in diameter. Throwing all the rocks that are released into the air.
Some of the debris will go so fast that come out from the atmosphere entering in the Earth’s orbit. Most of them, however, will rain down across the surface of the earth, not only in the impact site, heating the atmosphere will be so hot as to cause forest fires. All that is not repaired underground will burn.
Fines and dust soot of the fires will remain in the atmosphere for about a year, blocking the sunlight. Without it most of the plants, including seaweed, they will die. Many species of animals, including men, if they are not lucky and resourceful, will become extinct, or for the initial disaster or, in subsequent years due to lack of food and generally of a good environment.