A planet encased in ice
In the rugged Flinders Ranges of South Australia, layers of ancient rock preserve a radical chapter in Earth's history. These rocks; they contain definitive evidence of a global ice age so severe it nearly froze the entire planet. Known as the Sturtian glaciation, this event began around 717 million years ago and may have lasted for over 50 million years. The rocks here show that massive ice sheets extended all the way to the equator, a finding that helped confirm the "Snowball Earth" hypothesis.
The primary evidence lies in sedimentary rocks called diamictites. These are a chaotic mix of mud, sand, and enormous boulders, which geologists interpret as tillites—the lithified rubble left behind by glaciers. Among the most telling clues are dropstones. These are isolated boulders, some as large as cars, found embedded in finely laminated marine mudstone. They formed when icebergs, breaking off from continental glaciers, floated out to sea. As the icebergs melted, they dropped their rocky cargo onto the fine sediment of the quiet seabed. The presence of dropstones is an sign of glacial activity over an ocean.
Proof from magnetism
The most startling discovery from these South Australian rocks was their location during the Cryogenian Period. Through palaeomagnetic analysis, scientists determined the latitude at which these glacial sediments were deposited. When rocks form, magnetic minerals within them align with Earth's magnetic field, acting as a fossil compass. The magnetic data from the Elatina Formation in the Flinders Ranges shows a nearly horizontal alignment. This indicates the rocks formed at a latitude of less than 10 degrees from the equator. Glaciers at sea level in the tropics can only mean one thing: the planet was extraordinarily cold.
Further evidence comes from the layers directly overlying the glacial deposits. These are thick sequences of carbonate rock, often called "cap carbonates". Their existence points to a dramatic and rapid shift in climate. As the ice finally melted, a super-greenhouse effect, caused by the buildup of volcanic carbon dioxide over millions of years, took hold. This caused a rapid warming of the oceans, which absorbed immense amounts of CO2 from the atmosphere. The result was the swift precipitation of these carbonate layers, capping the glacial deposits and ending the longest and most severe ice age known in the geologic record.