The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth's Past Mass Extinctions

by Peter Brannen

As the planet slowly teeters, the amount of sunlight reaching different parts of the planet changes. For locales near the tropics, the effect can be switching from a monsoon climate to a drier one. As a result, lakes get deeper and shallower in roughly 20,000-year intervals, over and over and over again. The rocks when the lakes are shallow—red mudstone, with animal footprints and tree roots—are very different from when the lakes are deep—black, thinly laminated, with exquisitely preserved fish fossils. “The lake sediments are like a rain gauge that’s color-coded,” Olsen said. Sedimentary rocks laid down in these rift valley lakes of the End-Triassic are a veritable seersucker of red and black, testifying to the planet’s regular wobble. Olsen determined that the first, most devastating wave of extinction happened within just one of these layers, perhaps in fewer than 20,000 years—a geological instant.