Connie Masters of Balclutha asks :-

I have read that the Earth was once a snowball, even the oceans were covered by 800 metres of ice. Was this so?

Daphne Lee, a geologist at Otago University, responded.

The Earth has experienced several major episodes of continental glaciation during its long history. For instance, during the Late Paleozoic (around 300 million years ago), a huge ice sheet covered much of the Gondwana supercontinent, and around 20,000 years ago in the most recent glacial period, much of North America and Europe, as well as Antarctica, were covered with ice. But while the polar regions were encased in ice, the climate of the rest of the planet ranged from temperate to tropical, as it does today.

We can recognise the existence of such ancient ice sheets, and even the direction of ice flow, from characteristic grooves and striations left on bedrock by rock debris carried at the base of the glaciers, and from glacial sediments (tillites) left after the glaciers melted.

Recently, geologists have recognised puzzling evidence of glaciation on all continents at both high (polar) latitudes and low (tropical) latitudes during the late Precambrian (around 650 million years ago). For this to happen, the Earth's climate over the entire planet must have been extremely cold indeed. This led to the hypothesis that perhaps the Earth's surface was completely frozen over, and that a 800m blanket of ice may have smothered the oceans.

So did the Earth really become a great frozen snowball? Geologists are still divided on the question. However, recent research on marine organisms shows that photosynthesis and life in the oceans went on as usual, which makes it unlikely that the entire surface of the Earth was enveloped in ice.

There is a great deal of research still going on, and there may be more surprises to come.