From icehouse to hothouse
These rocks in northern Namibia are a physical record of the most extreme climate change in Earth's history. They show the abrupt end of the Marinoan glaciation, an ice age that began around 650 million years ago and lasted for at least 4 million years. During this "Snowball Earth" event, ice sheets extended to the tropics, and possibly covered the entire planet. The evidence for this deep freeze lies in a rock unit called the Ghaub Formation, a type of deposit known as a diamictite. It is a chaotic mixture of boulders, pebbles, and mud that was dropped by melting glaciers and icebergs into the sea.
Directly on top of this glacial deposit, with no significant time gap, lies a thick layer of carbonate rock. This is the thaw layer. Known as the Maieberg Formation, this "cap carbonate" marks a sudden and violent shift in the global climate. The leading theory for the rapid meltdown involves volcanoes. Over millions of years of ice cover, volcanic outgassing pumped enormous quantities of carbon dioxide into the atmosphere. Atmospheric CO2 levels may have reached 350 times modern levels, creating an intense greenhouse effect that finally broke the planet's icy grip. The ensuing hothouse world would have melted the glaciers in perhaps as little as a thousand years.
A world in chemical shock
The transition from a frozen planet to a warm one created bizarre ocean chemistry, which is recorded in the Maieberg Formation. As the ice melted, torrential rains with high levels of dissolved CO2 fell on the bare continents. This acid rain caused rapid weathering of rock, washing huge amounts of calcium and other minerals into the oceans. The resulting seawater became supersaturated with carbonate, causing it to precipitate out of the water and settle on the seafloor. This process formed the cap carbonate layer seen today.
In Namibia, the base of the Maieberg Formation is a specific member called the Keilberg dolostone. This gray or pinkish rock has an average thickness of about 38 meters, though it can be much thicker in some areas. Its very existence, sitting directly atop glacial debris, shows the rapid climatic swing. Paleomagnetic data from these rocks suggest that at the time of their formation, this part of Namibia was located at a latitude of about 33 degrees—far from the poles, confirming that the glaciation was a low-latitude event. This unique geological pairing—glacial deposits immediately capped by warm-water carbonates—makes the rock sequences of the Otavi Group in Namibia important for understanding the Snowball Earth hypothesis.