In a series of CGI shows called 'Walking with Dinosaurs' there is a segment where there is a mass extinction due to low-level super volcanic eruptions, nothing like what is talked up. A super volcanic eruption doesn't have to be a naked singularity of explosion rivaling an asteroid strike. In the Pacific region of the Earth, we could be in the beginning, or more probably, in the middle a longer lasting low level of super volcanic eruption right now. A titanic eruption, or eruptions, just more horizontally widespread instead of more vertically orientated in less area, underneath the crust of the Earth that is shifting the magnetic poles of the Earth) we are hardly noticing except for the climate changes it may be causing on the levels of the Earth where we live.
Each Ice Age of the last couple of million has been a crash, a falling over a cliff, so to speak, into one rather than anything like a long decline -- longer than a couple of hundred years -- to one. Ten to thirty thousand years of consistent warming, heating up! then bam! Ice Age. A relatively long-lasting buildup of a widespread low level super volcanic eruption, and/or one simply lifting in material and material heat over a large plain of substrata, could do that. It could be very periodic, especially if it is connected to some periodic activity of our star . . . that activity tied to some periodic activity even greater in our region of the galaxy we periodically travel through.
Of course, to some people tyrannically orientated in bent of mind -- and to too many gullible people -- the Earth has no untoward activity except as it is tied to activity of Mankind. That is a periodic belief, widely mindlessly savage and horrifying in its results, recorded throughout the recorded history of Mankind.