The secret ingredient of this layer cake is magma, which flowed from deep below and injected itself between layers of toxic sedimentary rock, formed from the dried-out sea. “Coal was the last thing on the top, but we know coal covered the whole basin,” Elkins-Tanton says.
To cause a mass extinction that unfolded over a mere tens of thousands of years, somehow all that carbon had to suddenly burn off and rapidly warm the whole planet. “There's only a few things that cause global change like that,” Elkins-Tanton says. “One is a giant meteor strike, which—there's no evidence for it. It would have to be a really big one and the evidence would be there. Another one is a nuclear war—pretty sure that did not happen.”
A third option, Elkins-Tanton continues, is “you've got to figure out a way to change the whole atmosphere. And the way to change the whole atmosphere is to drive chemicals up into the stratosphere.” For that, you need an explosive volcanic eruption and, critically for Elkins-Tanton, you need the rocks to prove it.But not all volcanoes are so ornery. For example, at the moment Kilauea isn’t explosive because its magma (what you call the gooey stuff while it’s still underground—it becomes lava when it emerges) is relatively thin and runny. When Kilauea’s magma bubbles to the surface, it releases its gases in an orderly fashion.Mount St. Helens, on the other hand, teemed with relatively thick magma, which better traps gases. As it ascends, the mass of magma suddenly becomes more buoyant, and it expands. And that means a bigger blowout. “If you have enough gases in the magma, instead of it bubbling out like soup, it explodes like a shaken soda bottle,” Elkins-Tanton says. “The carbon dioxide in soda is in solution. It's not in the form of bubbles until you shake it up or open it. And that's the same as releasing pressure as the magma comes closer to the surface, and all the volatiles form bubbles.”
“That's like Pinatubo or Mount St. Helens, but on a much bigger scale,” she continues. “And those things have enough heat and gases that they rise all the way up and puncture through the tropopause into the stratosphere.” The tropopause is a boundary layer between the troposphere—the bit of the atmosphere that we call home—and the stratosphere, which starts about 6 miles up. The troposphere is relatively chaotic, filled with all kinds of clouds, winds, and weather systems, whereas the stratosphere is relatively calm. (Planes fly in this zone to avoid turbulence, in fact.)