The Proterozoic

Between 2.7 and 1.8 billion years, widespread glaciations were a feature of climate. There were at least three discrete glaciations, concluded from tillites in Ontario and Wyoming. Findings in South Africa suggest that the Earth was covered in ice up to the equator. The condition has been dubbed “Snowball Earth”.

The glacial rocks are covered with either volcanic magma or carbonates (limestone, marble). It is not clear why the glaciation ended. Possible explanations include a catastrophic volcanic eruption, overturning of stagnant ocean water or a cosmic impact. Something has increased the content of greenhouse gases in the atmosphere and thus allowed for the ice caps to melt. The planet remained ice-free (including poles) for the following 1 billion years.

The amount of oxygen rose from 1% to 10 % mostly due to the activity of cyanobacteria.

There were at least two glaciation periods between 850 Mio and 570 Mio years. The ice coverage could have extended to fairly low latitudes, which would mean a repetition of the “Snowball Earth” scenario. Computer models indicate that some equatorial oceans may have remained ice-free. These events may play an important role in the survival of early life and the subsequent evolution of species.