That’s a nice question! The geological periods basically reflect the natural patterns and changes of Earth history. Geologists of Victorian times recognized that successive stratal successions were characterized by different fossil assemblages: former biological dynasties, if you like. Such dynasties could end abruptly, as in the simultaneous demise of the ammonites and belemnites in the sea, and the dinosaurs on land. It was (and remains) sensible to use that biological revolution to mark the boundary between two major time divisions, the Cretaceous and Tertiary periods.

The primary impact of volcanic eruptions on our climate is the cooling effect caused by sulphate aerosols. This is significant but is temporary, lasting only a few years post-eruption.