Large scale volcanism has played a critical role in the long-term habitability of Earth. Contrary to widely held belief, volcanism rather than impactors have had the greatest influence on, and bear most of the responsibility for, large scale mass extinction events throughout Earth’s history. We examine the timing of Large Igneous Provinces (LIPs) through Earth’s history to estimate the likelihood of nearly simultaneous events that could drive a planet into an extreme moist or runaway greenhouse, quenching subductive plate tectonics. This would end volatile cycling and may have caused the heat-death of Venus. With a conservative estimate of the rate of simultaneous LIPs, in a random history statistically the same as Earth’s, pairs and triplets of LIPs closer in time than 0.1-1 Myrs are likely. This simultaneity threshold is significant to the extent that it is less than the time over which the environmental effects persist.