91Ó°ÊÓ

The question enquires about the criteria/percentage which justifies mass extinction.

Mass extinctions, defined as the extinction of at least a majority of all species in a relatively short period of time, have occurred only just a few times in our history of the earth. The most recent mass extinction occurred around 250 million years ago when up to 95 percent of all species became extinct.

An asteroid impact- Although, it can be extremely severe but asteroid impacts are known to wipe out the species residing in the areas near the impact.

A boundary between geological periods- Geologist divides time here between the Precambrian period and the existing day into three long units of time known as eras.

A loss of 50 percent of species- As mentioned above mass extinctions occur at a percentage which is up to more than 90%