n. A person who believes that life on Earth can or will be mostly destroyed by a massive volcanic eruption.
The difference between my interest and that of eruptionists is that they think the big bang will happen tomorrow because it didn’t happen today. And running bison, combined with a couple of low-grade earthquakes in recent weeks, served as proof that ash umbrellas would be a good investment.
—Ken Newton, “Amid the mysteries of Elwood,” St. Joseph News-Press, April 05, 2014
Leckie told the LA Times: "Those bison were running for the sake of running. There was nothing chasing them. There was no mudslide. They were just running."

Two weeks later, an earthquake swarm hit the Yellowstone area.

"That's when things really exploded," he told the Times. "Eruptionists and conspiracists pirated the video and misrepresented it, suggesting that it was shot after the earthquakes."
—Katy Muldoon, “Yellowstone bison stampede fact check: They didn't predict diddly,” Oregon Live, April 04, 2014
2013 (earliest)
The debate will continue, with 'eruptionists' on one side and 'impactors' on the other, and the in-fight between 'asteroid admirers' and 'comet convertors' will probably go on long after the pub closes.
—Dougal Jerram, “Did a Comet, Asteroid or a Giant Volcano Kill Off the Dinosaurs?,” Huffington Post, March 25, 2013
This word is quite old, with lots of citations to be had from the 18th and early 19th century. However, those older cites seem to be using the term as a synonym for what back then would have been called a vulcanologist, but what we now call a volcanologist, an expert on volcanoes and volcanic activity.