The end Cretaceous mass extinction was caused by to factor, one of which actually triggered the other one. The two factors that contributed to the mass extinction were the asteroid impact and the flood volcanism.
The evidence for the asteroid impact are:
- crater at Yucatan
- chemical elements in the rock layers with space origin
The evidence for the flood volcanism are:
- the rock composition at several places around the planet
- the enormous amounts of pyroclastic material in the rock layers
Both hypothesis are correct, as both events occurred. The asteroid impact created such an explosion that it killed off everything near it, and on top of it there was a very hot wave that traveled around the globe, as well as propelling enormous amounts of particles into the atmosphere that blocked the sun. Also, the impact triggered the volcanoes around the globe, so massive volcanic eruptions started to occur, which just strengthened the devastating impact, and contributed even more to the extinction.