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)
Over millions of years, there are times with many eruptions and reduced mountain building, and CO2 builds up in the atmosphere. (thiniceclimate.org)
The African rift zone and mountain building in West Africa generated new habitats for the evolving frogs, Wake noted. (sciencedaily.com)