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 CO 2 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 CO 2 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 CO 2 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.

Remnant zircons can record the characteristics of the igneous rocks in which they formed, telling researchers about the volcanoes that produced them. Since the volcanoes along plate tectonic subduction zones, like those around the Pacific Ring of Fire today, are the most common source of the kinds of igneous rocks that host zircons, remnant zircons can tell us where those volcanoes once were, even if they have since been trapped in sedimentary rocks.

The researchers carefully compiled analyses of zircons in sedimentary rocks around the world into one dataset. The measured ages of those zircons indicate times when continental arcs of volcanoes were active along subduction zones. When the volcanoes are active, they’re not just forming new volcanic rocks—they’re also releasing CO 2 and thereby influencing the climate.

The results showed low continental arc activity during the great Cryogenian ice ages and a pronounced spike in activity in the Cambrian period, followed by a dropoff. That is, volcanic activity rose during the warming periods when CO 2 was building up in the atmosphere, and it fell during the cold periods.

This work follows a paper published last year that proposed a similar connection around the time of the Cretaceous. It argued that as plate tectonics caused more continental arcs to form, that volcanic activity could liberate CO 2 from carbonate rocks along the edges of the continents. Contrary to previous ideas, this suggests that continental arc activity is a more important contributor of atmospheric CO 2 than the underwater volcanism at the mid-ocean ridges.

As the Cambrian period came to an end, the supercontinent known as Gondwana (which would later become the southern half of Pangaea) was being mashed together. As the ocean basins between pieces of Gondwana were squeezed shut, subduction zones would have ceased, and continental arc activity would have died down.

The researchers point to the Himalayas as a smaller-scale example of this phenomenon. The Indian subcontinent was a neighbor to Australia when Pangaea came together. As Pangaea split up, India rocketed northward, subducting the oceanic crust in front of it and creating an arc of volcanoes on its leading edge. When India collided with Eurasia, however, there was no oceanic crust left to subduct, and volcanoes on both sides of what had been an ocean would have run out of fuel.

That on its own would have lowered the amount of carbon dioxide being belched into the atmosphere. But the compression between the colliding continental plates also forced up an imposing mountain range, and mountain ranges erode rapidly, with the breakdown of silicate minerals pulling CO 2 out of the atmosphere. The result was a significant drop in atmospheric CO 2 .

The assembly of Gondwana would have been one of many such collisions taking place at the same time. Relative to the cold Cryogenian period, continental arcs kicked up as the Gondwana continents began steaming toward each other—pumping out CO 2 and warming the planet into the Cambrian. When they collided, that volcanic activity stopped and erosion could have helped cool the planet down.

So it’s quite possible that the ebb and flow of these continental arcs had a role in the volatile climate during that time, which would make the climatic extremes inescapable consequences of plate tectonics. Those extremes left their marks on the biosphere. Multicellular animals first appear in the fossil record during the brutal Cryogenian ice ages, and they took off when the world thawed. The peak of the Cambrian warmth, however, saw several mass extinction events. And when things cooled off, life thrived and diversified again.

Seeing these tectonic and climatic changes as the villains in this play might be too simplistic, however, as some researchers think they may also have been responsible for the evolution of calcium carbonate skeletons—an innovation that played a key role in the Cambrian explosion of life.

Geology, 2014. DOI: 10.1130/G34962.1 (About DOIs).