Between 720 and 640 million years ago, Earth was thought to be entirely frozen, with global temperatures reaching -50°C (-58°F). This hypothesis, known as “Snowball Earth,” is actually quite controversial, as geoscientists cannot agree on whether or not it actually happened based on the available evidence. A new study published in Nature Geoscience has added another piece to the puzzle, revealing that it may have been powerful volcanic eruptions that freed the planet from this cryogenic grip.
During normal ice ages, gradual changes in the Earth’s orbit around the Sun, and the way it “wobbles” on its axis, cause the planet to receive significantly less sunlight than normal. This causes more glaciers to form, which reflect a lot of the incoming solar radiation back into space. This cools the planet further, encouraging the glaciers to keep growing out from the poles towards the equator. Eventually, Earth’s rotation changes back again to its “original” state, and the planet begins to warm, canceling this glaciation. This is known as a runaway feedback loop.
During the Cryogenian period, however, something caused the glaciation to continue and either mostly or entirely cover the entire planet, from the poles to the tropics. This would mean that the entirety of the Earth was a cohesive reflective “snowball,” which, without intervention, would have remained permanently frozen.
Most scientists think that the breakup of Rodinia, an ancient supercontinent, around 750-800 million years ago caused an immense river discharge into the oceans. This ultimately changed the oceans’ chemistry, allowing for a massive withdrawal of carbon dioxide from the atmosphere, which initiated the epic glaciation process.
Enigmatic features called “cap carbonates” – limestone sheets normally formed in the deep ocean – have been found on top of these glacial deposits, marking the end of the global glaciation. These features have multiple theorized formation mechanisms, but this new study by a team at the University of Southampton have arguably come up with the most robust one yet.
Underwater ridge volcanism likely produced both massive changes in ocean chemistry and atmospheric carbon dioxide concentrations. Gary Hincks
During the breakup of Rodinia, tens of thousands of kilometers of mid-ocean ridges were formed over tens of millions of years. This likely permitted a vast amount of extremely hot lava to escape from the depths up into the ice-capped ocean. This ridge volcanism produced hyaloclastites, a volcanic glass formed by rapid cooling that’s unstable under changing environmental conditions.
Using simulations, the team worked out how quickly this hyaloclastite would degrade into calcium, magnesium, phosphorous, and carbonates. Ultimately, they calculated that it was just enough to lay the groundwork for the chemistry that would eventually form those mysterious cap carbonates, right at the end of Snowball Earth.
Significantly, volcanism releases massive amounts of carbon dioxide, a powerful greenhouse gas. Therefore, this cap carbonates formation theory also suggests that extensive volcanism beneath the ocean, alongside that occurring on land, deposited huge amounts of carbon dioxide into the ocean and atmosphere, starting the warming process that led to the demise of the Cryogenian.
Although there is plenty of evidence suggesting that this incredible event did occur, many scientists debate it, noting that sedimentary features dating back to this time show wave patterns – the type that can only form when there are liquid, surface oceans on the planet.
