Plate tectonics set the thermostat for early animal life

California's Mt. Shasta, part of the Cascade Range, a continental volcanic arc.

NASA Earth Observatory

Earth’s climate didn’t exactly roll out the red carpet for the first multicellular life. Prior to the Cambrian explosion of animal life, there was a time known as the Cryogenian Period, which included two stretches where ice may have gripped the entire planet. The Cambrian, however, turned into a “hothouse”—atmospheric CO₂ likely hasn’t been that high since. Following this period, the climate cooled off, though it came nowhere near the previous deep freeze.

While some indications of temperature and atmospheric CO₂ have been preserved from deep antiquity, it has been hard to say why they changed at that time. University of Texas at Austin geologist Ryan McKenzie and several collaborators set out to see if they could pull together evidence of volcanic activity over that period, since it's a major source of CO₂ on geologic timescales. Doing so, however, required tallying up many needles found in many haystacks.

The mineral zircon is usually found as tiny crystals within igneous rocks like granite and its volcanic twin rhyolite. Zircon is, in many ways, a geologist’s best friend. It forms a closed, crystalline cage that traps radioactive uranium and the lead it decays into, allowing a crystal’s age to be accurately determined via radiometric dating. It’s also a remarkably tough mineral, surviving erosion that destroys many a lesser crystal. The oldest piece of the Earth ever dated was a miniscule 4.4-billion-year-old zircon grain—found in a sedimentary rock that formed "only" about 3 billion years ago.