Skip to 0 minutes and 8 secondsEarth's climate is incredibly dynamic, constantly moving through spectacular seasons of winter, spring, summer, and autumn at high latitudes between the tropics and polar regions and dry or rainy at low latitudes near the equator. The changing of the seasons follows such a regular cycle that we can confidently and accurately predict when the changes will occur. But there exist puzzling geologic evidence that shows there has been periods when Earth hasn't followed this regular, predictable cycle. This evidence shows that there have been periods in Earth's history when the planet has frozen over completely and been trapped in this extreme icy state with tens to hundreds of millions of years. These periods are commonly known as a Snowball Earth ice age.

Skip to 0 minutes and 59 secondsAt least three Snowball Earth ice ages have whitened our world from pole to pole. The first evidence emerges between 2,400 and 2,100 million years ago, when Earth went into the longest and most severe winter in its 4,500 million year long history. This ice age was so severe it brought permanent kilometre thick polar ice sheets to the equator. The youngest of this Snowball Earth climates occurred as recently as 715 to 635 million years ago. Evidence of Snowball Earth is found in ancient sediments known as glacial till or tillites and dropstones. Tillites are the broken rock fragments of various sizes formed by moving ice. Some of the first of these were discovered in Scotland in 1871.

Skip to 1 minute and 53 secondsSince then they have been found all over Africa, Brazil, Canada, and Australia. Dropstones are pebbles and boulders carried by icebergs and deposited on the soft mud of the ocean floor out at sea. The sediments have been dated and by using complex scientific methods shown to have formed at the equator. Interestingly, the first recorded Snowball Earth coincides with the first permanent rise of free oxygen in atmosphere. Although we don't know for certain what caused ice sheets to appear on that surface on such an enormous scale, one thought is that the rise of oxygen destroyed the methane gas reservoir that was abundant in the Earth's atmosphere. It was this gas that was responsible for keeping Earth's early surface warm.

Skip to 2 minutes and 43 secondsThe destruction of the methane rich atmosphere by oxygen reacting with methane to form carbon dioxide and water would have resulted in a dramatic cooling of Earth's surface. It is also thought that as ice sheets began expanding and moving from the poles towards the equator, the whiteness of ice increased cooling by reflecting up to 90% of the sun's heat back to space. This is known scientifically as the albedo effect. And it is this process that drives modern cutting edge technologies whereby white roofs cool down buildings by reflecting the sun's heat away. Hopefully, we can now begin to understand why Snowball Earths were such extreme events, particularly when we contrast them to the climatic conditions we experience today.

Skip to 3 minutes and 33 secondsHowever, Snowball Earths weren't simply an example of extreme events that Earth has experienced. They played a crucial role in Earth's transition to a habitable planet and helped to alter the conditions for life.

Snowball Earth: the big freeze

In this video, Dr Ernest Chi Fru explains how on at least three occasions Earth was completely frozen over and covered in ice.

It is hard for us to envisage this and to grasp how Earth went from this extreme icy state to our current climate.

Evidence for Snowball Earth

Evidence has been found in the rock fragments (tillites) carried by moving ice, and pebbles and boulders (dropstones) carried and deposited by icebergs.

This evidence has been found by geologists across the world.

Theory of Snowball Earth

Whilst evidence has been found, the theory of Snowball Earth is still a relatively new scientific hypothesis, and large uncertainties on its causes remain.

This makes it an exciting research topic in Earth sciences today, attracting the attention of many top researchers.

Due to their sheer scale Snowball Earth ice ages were extreme events, but what possible link could they have to the emergence of life?


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Extreme Geological Events

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