Snowball Earth

Scientists have a fascinating theory called Snowball Earth. This idea suggests that Earth was once almost entirely frozen. During these times, the planet's surface had very little liquid water. Most of the oceans and land would have been covered in ice.

This may have happened during the Cryogenian Period. This period included two major times of ice, called the Sturtian and Marinoan episodes. Some researchers believe this freezing helped trigger the evolution of bigger living things. This idea helps explain strange rocks found in warm parts of the world.

How does a whole planet freeze over? It works through a thing called an ice-albedo feedback loop. Ice is very bright and reflects sunlight away from the planet. This reflection makes the Earth even colder. As it gets colder, even more ice begins to grow. This creates a loop where more ice leads to more cooling. Eventually, the ice could spread all the way to the equator.

Some scientists wonder if it was a full snowball or a slushball. A slushball might have had a thin band of water near the middle.

People have been finding clues about old ice for a long time. In 1871, J. Thomson found old glacier material in Scotland. Later, similar findings appeared in Australia and India. In 1891, Hans Reusch reported finding a moraine in Norway.

For a long time, people did not understand these finds. They did not yet know about continental drift. This is the idea that continents move over time. Once scientists understood this, they realized old ice could be in warm places. This happened because the land was in a different spot long ago.

Many scientists helped build this theory with math and rocks. In 1964, W. Brian Harland showed that ice rocks were once in the tropics. In the 1960s, Mikhail Budyko used a computer model to study climate. He found that ice could cover the whole world. In 1992, Joseph Kirschvink gave the idea the name "Snowball Earth." He also suggested how the ice might melt. He thought volcanoes could release CO2 gas to warm the planet.

This gas creates a greenhouse effect to trap heat.

We can see evidence of this in the rocks today. Scientists use palaeomagnetism to study where rocks were made. This is a way to read the Earth's old magnetic field. When rocks form, tiny minerals point toward the magnetic poles. By measuring them, we can guess the old latitude of the land.

We also look for dropstones, which are rocks dropped into seafloor mud by ice.

These clues help us piece together Earth's very cold history. Even with these clues, some scientists still debate the details.