Some proposed they're related to periods of intense volcanic activity, others that they were caused by continental drift and periods of mountain building. (drtimball.com)
In fact, it may be more closely linked to unusually intense volcanic activity and interactions between sea ice and ocean currents during the period. (carbonbrief.org)
It has survived asteroid strikes, axis shifts, periods of intense volcanic activity and periods with CO2 levels far higher than they are today. (illconsidered.blogspot.ru)