Pongola Glaciation: Earth's Earliest Known Ice Age

The Pongola glaciation is the oldest known ice age in Earth’s history, dated to about 2.9 billion years ago in the Mesoarchean era. Its evidence lies in ancient rocks of the Pongola Supergroup in South Africa and Eswatini. It shows that even under a fainter young Sun and an atmosphere without oxygen, Earth’s climate could cool enough for glaciers to form.

Background: climate of the Archean

During the Archean Eon, from about 4.0 to 2.5 billion years ago, the Sun shone at only around 70 to 80 per cent of its current brightness. Without help, Earth’s oceans should have frozen. Yet the rock record shows liquid water for most of the eon, which scientists explain by a strong greenhouse effect from carbon dioxide and methane.

Because the Archean was generally warm, any sign of ice is significant. It shows that the balance of greenhouse gases and other factors could shift enough to plunge parts of the planet into glacial conditions.

The Pongola Supergroup

The Pongola Supergroup is a sequence of volcanic and sedimentary rocks laid down on the Kaapvaal Craton, one of the oldest surviving pieces of continental crust. It is named after the Pongola area and river of northern KwaZulu-Natal and neighbouring Eswatini, where the rocks are exposed.

It has two main parts. The lower Nsuze Group consists largely of volcanic rocks, dated to about 2.98 billion years ago. The upper Mozaan Group is mostly sedimentary, made of sandstones, shales and iron-rich layers deposited in a shallow sea, and is a little younger. The glacial deposits occur within the Mozaan Group.

The evidence for ice

What geologists found

Glacial deposits in the Mozaan Group were described by the geologists Victor von Brunn and David Gold in the early 1990s. In 1998 Grant Young, von Brunn, Gold and W. E. L. Minter published a detailed study in the Journal of Geology, presenting it as the oldest reported glaciation on Earth.

The main evidence is diamictite, a poorly sorted rock containing pebbles and boulders of many sizes scattered through fine mud. Glaciers produce such deposits when they melt and drop their load of debris. Some of the stones in the Mozaan diamictites carry scratches, called striations, of the kind made when rock fragments are dragged along under moving ice.

Dropstones and chemistry

The researchers also found dropstones: isolated stones that sank into finely layered muds, bending the layers beneath them. These are typically dropped from icebergs or floating ice. Combined with the diamictites, they point strongly to glaciers reaching the sea.

The team also studied the chemistry of the shales associated with the glacial layers, using it to track changes in weathering and climate. Chemical weathering of rock draws carbon dioxide out of the atmosphere, so evidence of strong weathering fits the idea that a fall in greenhouse gases helped trigger the cooling.

What caused the glaciation?

The cause is uncertain, and several factors may have worked together. Chemical weathering of newly formed continental crust removes carbon dioxide from the air and stores it in rocks and the ocean. As continents grew during the Archean, this process could have weakened the greenhouse effect.

Methane may also have been important. If early oxygen-producing microbes released small amounts of oxygen, it could have destroyed some atmospheric methane, a powerful greenhouse gas. This idea links the Pongola cooling to the early history of photosynthesis, though it remains speculative.

The position of the Kaapvaal Craton may also have mattered. If it lay at high latitude, or the glaciers were in mountains, ice could have formed without the whole planet freezing.

Was it global?

There is no firm evidence that the Pongola glaciation was a worldwide event. Similar deposits of roughly the same age occur in the nearby Witwatersrand Basin, also on the Kaapvaal Craton, but convincing glacial deposits of the same age are scarce elsewhere. Most geologists therefore see it as a regional glaciation.

This sets it apart from later, larger ice ages such as the Huronian glaciation of about 2.45 to 2.2 billion years ago and the Snowball Earth glaciations of the Cryogenian period, when ice may have reached the tropics.

Timeline

• c. 2.98 billion years ago: Volcanic rocks of the Nsuze Group are erupted on the Kaapvaal Craton.
• c. 2.95-2.85 billion years ago: Sediments of the Mozaan Group are laid down in a shallow sea.
• c. 2.9 billion years ago: Glaciers deposit diamictites and dropstones in the Mozaan Group.
• Early 1990s: Victor von Brunn and David Gold describe the glacial deposits.
• 1998: Young, von Brunn, Gold and Minter publish the evidence as Earth’s oldest reported glaciation.

Why it matters

The Pongola glaciation is the first known sign that Earth’s climate could swing from warm to icy. It helps scientists test ideas about the faint young Sun, early greenhouse gases and the role of life in shaping climate, and it shows that Earth’s thermostat has never been fixed.