Glacial deposits found at what was then the equator, proving that about 717 million years ago Earth froze almost completely. The Snowball Earth hypothesis once seemed outlandish, but these rocks showed that glaciers really did reach the tropics.
State Government Photographer, CC0
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.
💡Fun Facts
The term "Snowball Earth" was first proposed in its modern sense by Caltech geologist Joseph Kirschvink in 1992.
The Sturtian glaciation, which began 717 million years ago, is believed to have lasted for about 57 million years.
Following the thaw, the planet swung to the other extreme, with surface temperatures in a super-greenhouse climate possibly reaching scorching levels.
Some scientists theorize that the extreme environmental stress of the Snowball Earth events spurred the subsequent evolution of complex, multicellular life.
The geological sites are within Ikara-Flinders Ranges National Park, which is accessible year-round. Some trails may close during summer months (December-March) due to extreme heat. The Wilpena Pound Visitor Centre is open daily from 8am to 5pm.
Admission
A vehicle entry fee is required for the park, which can be purchased online. A day pass starts around $12.50 AUD.
Accessibility
The terrain is rugged. Main access roads are sealed, but many scenic drives and access to specific geological sites like Brachina Gorge are on unsealed roads, where a 4WD vehicle is recommended. Walking trails vary in difficulty.
