There are two ideas about how glaciers formed on Earth between about 717 and 630 million years ago – a time known as “Snowball Earth”. The idea that at least two long glaciations happened during which communication between the ocean and the atmosphere was cut off (described in the top half of this image) is more likely based on the evidence we have today. In this scenario, the Earth was ice-free at 670 and 630 million years ago because carbon dioxide built up in the atmosphere.
Credit: Zina Deretsky, National Science Foundation