The Cryogenian Period, from approximately 720 to 635 million years ago, is a chapter in Earth’s deep history defined by the most severe ice ages ever recorded. As the second period of the Neoproterozoic Era, it saw the planet undergo extreme climate changes, shifting to a state of widespread glaciation. These conditions created a world almost entirely covered in ice and set the stage for major evolutionary developments that would follow.
The Snowball Earth Hypothesis
The “Snowball Earth” hypothesis proposes that the planet’s surface became almost entirely frozen during the Cryogenian. The trigger for this deep freeze was linked to geological changes, specifically the breakup of the supercontinent Rodinia. The rifting of Rodinia exposed vast amounts of fresh volcanic rock, which led to accelerated weathering of silicate rocks. This process draws large quantities of the greenhouse gas carbon dioxide (CO2) from the atmosphere.
As atmospheric CO2 levels fell, global temperatures dropped, causing ice sheets to expand from the poles. This initiated an “ice-albedo feedback loop.” Ice is highly reflective, so it bounced more of the sun’s energy back into space, which caused further cooling and allowed more ice to form. This runaway effect is thought to have plunged the Earth into a global deep freeze.
This process is believed to have happened at least twice during the Cryogenian. The first and longest of these events was the Sturtian glaciation, which lasted for about 50 to 60 million years. Following a brief interglacial period, the planet was plunged into a second ice age, the Marinoan glaciation. These events represent the most significant glaciations in Earth’s history.
Geological Evidence for a Frozen Planet
Geological evidence for a nearly frozen planet is preserved in the rock record. A primary piece of evidence comes from glacial deposits known as tillites, which are rocks formed from debris left by glaciers. Geologists have found Cryogenian-aged tillites in locations that paleomagnetic data show were at the equator, indicating that ice sheets extended to the tropics.
Another form of evidence is the presence of “cap carbonates.” These are thick layers of limestone and dolostone that directly overlay the glacial deposits. Their existence points to a rapid shift in climate. As the ice melted, the CO2-rich atmosphere created acid rain that weathered the land, washing calcium and bicarbonate into the oceans and leading to the rapid precipitation of these carbonate rocks.
The reappearance of banded iron formations (BIFs) also provides support. BIFs, absent from the rock record for over a billion years, require an anoxic (oxygen-poor) ocean to form. A global ice sheet would have sealed the oceans from the atmosphere, cutting off the oxygen supply and allowing iron from undersea volcanic vents to accumulate. When the ice melted, oxygen mixed with the iron-rich water, causing the iron to precipitate and form these layered deposits.
Life’s Struggle for Survival
How life endured a global deep freeze is a subject of scientific inquiry. The life at the time was simple, consisting of microbes and early eukaryotes like algae. The survival of these organisms depended on “refugia,” which were small pockets of habitability where conditions were less severe. Evidence suggests the oceans were not completely frozen solid, allowing some life to persist.
One potential refuge was near hydrothermal vents on the seafloor, which released heat and chemicals. These created localized environments warm enough for microbial communities to thrive independent of sunlight. Another possibility is that life persisted in sunlit environments on the ice itself. Small pools of meltwater, known as cryoconite holes, could have formed on glacier surfaces, providing a niche for photosynthetic organisms.
Recent studies suggest that open-water conditions existed in mid-latitude oceans, creating larger habitable zones. These areas of “slushy” water or seasonal ice, particularly near the equator, could have provided a sanctuary for photosynthetic algae that required sunlight. The discovery of fossilized seaweed from the late Marinoan glaciation supports the idea that open-water refuges sustained multicellular life through the ice age.
The Great Thaw and Its Aftermath
The end of the Snowball Earth events was as significant as their onset. The mechanism that broke the planet out of its deep freeze was the persistent action of volcanoes. Throughout the millions of years of glaciation, volcanic activity continued, erupting through the ice sheets and steadily releasing CO2 and other greenhouse gases into the atmosphere.
With most of the planet’s surface covered in ice, rock weathering, which removes CO2 from the atmosphere, was almost completely shut down. This allowed volcanic gases to accumulate over millions of years to levels far exceeding those of today. The concentration of CO2 eventually became so high that it triggered a greenhouse effect, causing global temperatures to rise rapidly and leading to a planetary thaw.
The consequences of this thaw reshaped the planet’s biology. As glaciers melted, they washed nutrient-rich rock flour and minerals into the oceans. This influx of nutrients is thought to have fueled a boom in oceanic life, particularly for surviving algae and other photosynthetic organisms. This biological surge increased oxygen levels and spurred the evolution of the first complex, multicellular animals, leading to the Ediacaran Biota and paving the way for the subsequent Cambrian Explosion.