A plausible end-of-the-world scenario you've probably never thought of

You've seen the world end with fire, earthquakes, and WTF in movies like 2012 and The Core. But to find out about truly horrific, plausible scenarios you need to ask a scientist. We talked to Cal Tech's Joe Kirschvink, a geobiologist who studies the ancient history of life — and mass death. He gave us a nightmare…