The snowball Earth hypothesis, often called the Sturtian glaciation, states that, at some point, Earth was entirely or almost entirely frozen over. By analyzing sedimentary deposits, scientists have concluded that this event has actually been a reoccurring phenomenon but the largest, and thus primary snowball Earth event, occurred nearly 750 million years ago during the Cryogenian period.
Discovering snowball Earth
The theory that Earth may have been completely iced over has actually been around for quite a while. Its earliest roots tie back to Brian Harland of Cambridge University in 1962; he was the first to hypothesize a glacial explosion that would cover Earth’s entire surface. Little was thought of Harland's hypothesis until 1992, when Joseph Kirschvink, a biomagnetist and paleomagnetist at the California Institute of Technology, came to the same conclusion as Harland when analyzing modern data, that this point in Earth's history would be coined “snowball Earth.”
After Kirschvink hypothesized such an event, scientists set out to test it. Francis Macdonald, an Earth scientist at Harvard University, and his team attempted to prove the hypothesis by researching glacial sediment in Canada.
During the Cryogenian period, Earth’s continents were just starting to separate and spread apart to what they are today. This means that 700 million years ago, Canada was actually located on the equator, the hottest point on Earth. Macdonald believed if there were traces of ice in the hottest part of Earth at the time, then it is likely that ice also covered the rest of the planet in some form as well. The team was able to prove just that when they used uranium-lead mass spectrometry to analyze volcanic rocks that were sandwiched between glacial sediment. In doing so, they confirmed that the rocks and sediment were deposited 716 million years ago, exactly as snowball Earth was believed to be taking shape. This research proves that there was some form of ice or glacial matter on or near the equator and thus supports Kirschvink’s hypothesis.
Snowball Earth formation
In normal ice ages, glaciers branch out from either pole and eventually retract. But in the event of snowball Earth, instead of retracting, the glaciers kept stretching closer and closer to the equator. As the ice grew, its shiny white surface reflected heat back into space (an effect known as albedo). This caused conditions to get even colder, dropping to a frigid -20 degrees at the equator. The increase of light reflection and decrease of temperature only resulted in more ice formation and caused the cycle that formed snowball Earth to continue. Consequently, glaciers grew to sizes of continents and stayed this way for about 10 million years before beginning to thaw.
The most popular theory as to how snowball Earth thawed is that the carbon released from volcanoes was enough to melt the massive amounts of ice. Even if Earth was completely frozen, volcanoes would be nearly unaffected and would continue to produce CO2. The temperatures would also be so cold that water would not be able to evaporate and thus, there would be no rainfall to rinse CO2 out of the atmosphere. This would only further contribute to the warming already started by volcanic carbon release.
Snowball Earth thaw has also been credited to something that is commonly overlooked: clouds. In order for a planet-sized glacier to defrost, CO2 levels would need to have reached around 20% of atmospheric volume. The problem with this is that levels were only between 1-10% during the time of snowball Earth. But when a cloud's ability to trap CO2 emissions and heat account is taken into account, the levels become closer to what was originally needed; nearly 100 times lower than the 20% previously projected.
Implications on life forms
It is important to remember that life during snowball Earth is not like life as we know it today. 750 million years ago, life consisted of single-celled eukaryotic organisms living mostly in or near water. Even so, a complete freeze of Earth's surface would have been detrimental to life and result in mass extinction of species without adaptation mechanisms. Fortunately, life continued to thrive, and some speculate that just the opposite happened and life exploded.
In analyzing fossil records from 500 million years ago, scientist have coined the term "Cambrian Explosion," or the sudden influx of multicellular species that lead to the evolution of more complex organisms like mammals. Some speculate that this explosion of life happened because of the excess of oxygen present during the icy time, while others believe that, unlike the flexibility of plant cells, the harsh conditions forced the specificity required of animal cells. Some even believe that the Cambrian Explosion caused snowball Earth: the influx of animals in the oceans caused carbon dioxide to settle in the bottom of the ocean resulting in a lack of warming.
There is also a theory that meltwater ponds on the surface of the ice near active volcanoes may have provided refugee for early life forms. Others theorize that life could have survived in deep sea hydrothermal vents and hot springs that would have stayed too warm to completely freeze over. In either case, the only group that could have survived in such intense and extreme conditions are extremophiles. Proof of this comes during examination of fossils, specifically red algae. Although these fossils are smaller than a single human hair, they tell a great deal about how they survived snowball Earth. The fossils show that the algae were rigid and bumpy and that might have been why they were able to survive such tough conditions. While red algae is believed to have been resilient, there would have had to been pockets of the Earth that remained unfrozen in order for it to survive. It could have been immensely cold, but it is unlikely that the algae could have survived if all oceans on Earth were frozen.
Scientists are mostly in agreement that Earth went into a deep freeze at some point during the Cryogenian period, but some question the extent of the freeze, calling it more of a “slushball’ or “mudball” rather than a snowball. Many scientists use the fact that a thaw of a completely frozen Earth would have been nearly impossible to discredit the idea of snowball Earth. To prove this, they analyze the popular thawing theory of increased CO2 levels from volcanic activity that many claim to be the cause of the thaw. But this would mean CO2 levels would have needed to be hundreds of times higher than they are today,  but there is no evidence of this dramatic increase.
Another theory is that instead of a slushball or snowball, Earth may have been covered in large, continent-sized glaciers in which tropical oceans never completely froze over. This hypothesis would also explain how there was never a mass extinction and how life could have survived such conditions. As mentioned in the previous section, there were actually very few extinctions during the time. This is why many scientists believe that Earth was never completely frozen. In this scenario, life could have flourished in the non-frozen parts of Earth and it would make it easier for the Earth to warm, and thus cause the subsequent thawing that would follow.
Other closely related articles in this wiki include:
Relevant online sources to this wiki article include:
- The Snowball Earth Theory of Global Glaciation 
- Snowball Earth, Ancient Earth Habitats.
- Snowball Earth Research