3. Impact Theory:
In 2001, Geologists with NASA discovered evidence of an asteroid impact at the end of the Permian period. While most evidence has been erased over the last 250 million years, researchers now believe that this space rock was about the size of Mount Everest when it collided with Earth.
Still, relatively few geologists believe this actually caused mass extinction. It certainly wouldn't have helped, but likely wasn't the direct cause. It may have promoted the following event.
4. Volcanoes. The Siberian Traps
250 million years ago, right at the Permian-Triassic boundary, the Siberian Traps were formed. These were massive lava flows created by a plume eruption. Magma rose through the earth’s crust onto the surface, exploding with hundreds of thousands of cubic kilometers of lava.
This is incredibly rare, only happening eight times in the last 250 million years, but the result is a massive outpouring of magma in violent eruptions. The event that created the Siberian Traps lasted between 1 and 2 million years.