Snowball Earth

The Snowball Earth is a geohistorical idea. It says that in some very cold times in Earth's past, almost all of the planet's surface was covered in ice. Very little liquid water could be seen.

This idea is mostly linked to a time called the Cryogenian Period. This time had two big periods of freezing: the Sturtian (around 717–660 million years ago) and the Marinoan (around 650–635 million years ago).

People who support this idea say it helps explain some layers of rock. These rocks look like they were formed by glaciers. But they seem to have been in places that were once near the tropics. Others are not sure about the idea. They think it would be very hard for Earth to freeze completely and then start melting again.

There are still many mysteries. One mystery is whether Earth was a full "snowball" or more like a "slushball" with a narrow strip of open water near the equator. These freezing times are thought to have happened before many new kinds of life with many cells appeared suddenly. This happened in what we call the Avalon and Cambrian explosions. The last big Snowball Earth event might have helped trigger the development of these complex living things.

History

First evidence for ancient glaciation

Long before scientists thought Earth might have frozen completely, they found clues that it was once covered in ice. In 1871, a scientist named J. Thomson discovered rocks in Scotland that showed signs of ancient glaciers. Similar discoveries were made later in Australia and India. In 1891, another scientist named Hans Reusch found more evidence in Norway. These discoveries were hard to understand at the time because people did not yet believe continents could move.

Global glaciation proposed

An Australian geologist named Douglas Mawson studied rocks in South Australia and thought maybe Earth had frozen over. However, he believed Australia and other continents stayed in the same place, which we now know is not true. Later, when scientists learned that continents move, it became clearer why these icy rocks were found in places far from the poles.

In 1964, a scientist named W. Brian Harland found evidence that glaciers had reached tropical areas, suggesting an extremely cold time when Earth might have been covered in ice. In the 1960s, another scientist named Mikhail Budyko created a model showing that if enough ice covered Earth, it could stay that way forever. But his model also suggested this might never actually happen.

The idea of a "snowball Earth" was first used by Joseph Kirschvink in 1992. He suggested that certain rocks formed during this time and that volcanic activity could eventually melt the ice. Other scientists, like Paul F. Hoffman, added more evidence from rocks in Namibia. In 2010, more evidence showed that a large part of Earth was indeed covered in ice during a very cold period long ago.

Evidence

The snowball Earth hypothesis tries to explain why we find signs of glaciers far from the poles. According to models, once ice reaches within 25° to 30° of the equator, it would grow quickly to cover the whole planet. This means that if we find glacier signs near the equator, Earth might have been almost entirely frozen.

To check if this idea is true, we need to see three important things:

1. That a rock layer shows signs that only glaciers could make.
2. That this layer was near the equator when it formed.
3. That glaciers were active in many places at the same time, with no other similar layers from that time.

The third point is hard to prove. Before the Ediacaran period, we lack the usual markers to match rocks from different places. The best we can do is guess the rocks' age using radiometric dating, which is not always very exact.

The first two points are often debated. Many glacier signs can also form in other ways, and figuring out where land was even 200 million years ago is very difficult.

Palaeomagnetism

The snowball Earth idea started because people found what looked like glacier marks near the equator. Since tectonic plates move slowly, it's hard to know where these rocks were when they formed. Palaeomagnetism helps us estimate the latitude where rocks were made, by looking at how magnetic minerals aligned with Earth's magnetic field. This has shown that some glacier signs were within 10 degrees of the equator, but this is still being questioned. Some think these signs might not mean the whole planet was frozen, but just parts of it.

Skeptics say the magnetic data might be wrong if Earth's magnetic field was different long ago. They also point out that it's hard to tell if the magnetic signal we see is original or changed later by events like mountain building.

Low-latitude glacial deposits

Rocks formed by glaciers have special features that help us recognize them. However, many of these features can also form in other ways. As of 2007, there was only one strong example of glacier rocks near the equator, but even this is debated. Still, more evidence is building that there were glaciers near the equator during the Sturtian glaciation. Signs include dropstones, varves, glacial striations, and diamictites.

Open-water deposits

Some layers from the snowball Earth time look like they needed open water to form. This includes thick bands of glacier rocks separated by thin layers of non-glacier rocks, showing that glaciers melted and grew again many times. There are also signs of waves, far-travelled ice-rafted debris, and possible plant life, suggesting that large parts of the ocean were not frozen. This makes a completely frozen Earth less likely.

Carbon isotope ratios

There are two types of carbon in seawater: carbon-12 and the rarer carbon-13. Living things, like plants, usually use more carbon-12, so ocean water with life has less carbon-13. During the snowball Earth time, we see big changes in the amount of carbon-13 compared to carbon-12. By studying these changes, scientists have identified four or maybe five glacial events in the late Neoproterozoic.

Banded iron formations

Banded iron formations are layers of iron oxide and chert. They form when iron in the ocean meets oxygen and turns into rust. These formations are usually very old and linked to when Earth's air began to have oxygen. After this time, we only see them again with Cryogenian glacial deposits. Supporters of the snowball Earth idea say these formations appeared because the ocean was sealed by ice, limiting oxygen. Opponents say they might have formed in isolated lakes.

Cap carbonate rocks

After glacial layers, we often find thick layers of limestone or dolomite called cap carbonates. These have unusual chemistry and structures, like large ripples. One idea is that when the ice melted, lots of CO₂ dissolved in water, forming acid rain that washed minerals into the ocean, creating these layers. However, there are problems with this idea, such as why some places lack these cap carbonates.

Changing acidity

Boron isotopes suggest that the oceans became more acidic before and after the Marinoan glaciation. This might show a buildup of CO₂ in the air, but it doesn't necessarily mean the whole Earth was frozen.

Space dust

Iridium, a rare element mostly found deep in Earth, can be found on the surface from space dust. During a snowball Earth, iridium would collect on the ice and show up when the ice melted. Finding iridium at the base of cap carbonate layers has been used to suggest the glaciation lasted at least 3 million years, but this doesn't prove the whole planet was frozen.

Cyclic climate fluctuations

Studies of chemical weathering show patterns that match cycles of cold and warm periods, similar to ice ages today. Glacial rocks in Scotland show layers of glacier and shallow ocean sediments, suggesting the snowball Earth might have had warm periods too.

Mechanisms

A Snowball Earth event starts when Earth gets colder, causing more snow and ice to form. This ice and snow send sunlight back into space, making Earth even colder—a process called positive feedback. Over time, this can cause Earth to become almost completely covered in ice.

Several things could make Earth colder, such as changes in gases in the air, volcanic eruptions, or shifts in Earth’s orbit. During the Cryogenian period, more oxygen in the air lowered methane levels, which also helped cool the planet. Once ice covers much of Earth, it sends even more sunlight away, continuing the cycle of cooling.

When the ice starts to melt, volcanoes release gases that warm the planet again. This melting can happen fast, in just a few thousand years. As ice melts, darker land and water absorb more sunlight, helping to warm the planet faster. This cycle may have happened many times, with Earth freezing and then warming again over millions of years.

Scientific dispute

Scientists question the Snowball Earth idea because they find clues that the ice melted and came back again many times. They see signs in rocks and chemicals that show both icy and warm times close together. Also, some computer models have trouble freezing the whole world, and the amount of a gas called CO₂ needed to melt the ice seems very high.

Some believe the big freezes happened when continents split apart, creating high lands where glaciers could form. Others think Earth’s tilt or how it spun might have caused ice to appear in places that aren’t usually cold. Another idea is that Earth’s turning changed in ways that made old ice clues look like they were from the equator, without the whole planet being frozen.

Survival of life through frozen periods

Scientists wonder how life could survive if the whole Earth were covered in ice. Some think that life might have hidden in special places. For example, tiny sea creatures could live deep in the ocean near hot spots on the sea floor. Others believe life could hide under the thick ice, in small pockets of liquid water, or even as frozen seeds waiting for better times. There might also have been small areas of open water where wind or heat from the Earth kept the ice thin enough for sunlight to reach.

Even with these hiding places, we would expect to see big changes in the types of animals and plants living on Earth. But the fossils we have do not show these big changes. Some simple animals, like sponges, managed to survive these icy times. Later, many more kinds of animals appeared and thrived.

Implications

The idea that Earth was almost completely covered in ice has important effects on how life developed on our planet. When the ice covered Earth, it would have made it hard for many plants and animals that need sunlight to survive.

After the ice melted, the water left behind was saltier. This could have changed how the oceans worked.

Scientists think that when the ice melted, it made warm layers of water that mixed with colder water below. This mixing might have helped create conditions where new kinds of life could grow and change quickly. Some scientists believe that the freezing and warming times helped life evolve faster during a time called the Cryogenian Period. This was when many new kinds of animals and other living things began to appear.

Occurrence and timing

Palaeoproterozoic

Main article: Huronian glaciation

The Snowball Earth idea tries to explain old icy layers found in rocks in Canada. Some scientists think these rocks show ice was near places that are usually warm today, but others disagree. In South Africa, there are also old icy rock layers that might have formed near the equator. Around 2.25 billion years ago, a big change in Earth's air may have removed a gas that kept the planet warm. Because the sun was weaker back then, Earth could have frozen over completely between 2.5 and 2.2 billion years ago.

Neoproterozoic

Main articles: Sturtian glaciation, Marinoan glaciation, and Baykonurian glaciation

During a later time called the Neoproterozoic, Earth had several big cold periods. The Marinoan was the most powerful, and the Sturtian was also very widespread. One shorter cold period, the Gaskiers glaciation, was intense but did not freeze the whole planet. There is some debate about another possible cold time called the Kaigas event. Some scientists think it might just be part of the Sturtian period, and it was probably not as big as the other two big icy times. Evidence now shows Earth may have frozen many times during this period, which makes the Snowball Earth idea more complicated.