Archean Eon: Earth's Early Continents and the First Life

The Archean Eon is the second of the four great divisions of Earth’s history, lasting from about 4.0 billion to 2.5 billion years ago. During this immense span, around a third of the planet’s existence, Earth’s crust cooled and stabilised, the first lasting continents formed, and simple microbial life spread through oceans that contained almost no free oxygen. The rocks that survive from the Archean are rare, but they hold the earliest clear record of a living planet.

Where the Archean fits in Earth’s history

Geologists divide Earth’s 4.5-billion-year history into four eons. The Hadean covers the planet’s first few hundred million years after its formation, a period from which almost no rock survives. The Archean follows, then the Proterozoic, and finally the Phanerozoic, the eon of visible animal life that began about 539 million years ago.

The Archean is split into four eras: the Eoarchean (about 4.0 to 3.6 billion years ago), Paleoarchean (3.6 to 3.2), Mesoarchean (3.2 to 2.8) and Neoarchean (2.8 to 2.5). Unlike later boundaries, which are defined by layers of rock and fossils, these divisions are set by fixed numbers, because Archean rocks are too scattered and altered to provide reference points.

The name comes from the Greek arkhe, “beginning”. The American geologist James Dwight Dana used the term in 1872 for the most ancient rocks he knew. Its start is often placed close to the age of the Acasta Gneiss in Canada, around 4.0 billion years old and among the oldest known intact rocks.

A hotter, younger planet

Archean Earth was very different from today. The planet’s interior was hotter, because more heat remained from its formation and because radioactive elements had not yet decayed as far. This extra heat produced unusual lavas called komatiites, rich in magnesium and erupting at temperatures that some estimates put at around 1,600°C, far hotter than any lava today.

The Sun, by contrast, was fainter. Models of stellar evolution suggest it shone at roughly 70 to 80 per cent of its present brightness. Yet rocks laid down in water show that Earth had liquid oceans for most of the eon. This puzzle, known as the faint young Sun paradox, is usually explained by a stronger greenhouse effect from carbon dioxide and methane.

The early Archean also came soon after a period of heavy impacts. Some scientists argue for a spike in collisions around 4.1 to 3.8 billion years ago, the Late Heavy Bombardment, although whether it was a distinct event is debated. Layers of impact spherules in South Africa and Australia show that large asteroids continued to strike Earth well into the eon.

The first continents

Most Archean crust has been destroyed, but fragments survive as cratons: old, thick and stable blocks that form the cores of modern continents. Among the best known are the Kaapvaal Craton in southern Africa, the Pilbara and Yilgarn cratons in Western Australia, the Superior and Slave cratons in Canada and the North Atlantic Craton of Greenland.

Two kinds of rock dominate these regions. Greenstone belts are sequences of volcanic and sedimentary rock, often turned green by metamorphism, that include pillow lavas erupted under water. Around them lie large bodies of light-coloured granitic rock known as TTG (tonalite, trondhjemite and granodiorite), which formed when older crust melted and which built up the first continental masses.

How these continents grew is one of the central questions of Earth science. Some geologists think a form of plate tectonics was operating early in the Archean; others argue that the crust behaved differently, with hot blobs of material rising and sinking vertically, and that modern-style plate tectonics began only around 3.2 to 2.5 billion years ago. By the Neoarchean, several cratons may have joined into larger land masses, such as the proposed Kenorland supercontinent.

An atmosphere without oxygen

The Archean atmosphere contained almost no free oxygen. It was dominated by nitrogen and carbon dioxide, with smaller amounts of methane. Evidence for this comes from minerals such as pyrite and uraninite, which are destroyed by oxygen but survive in Archean river gravels, and from unusual patterns in sulphur isotopes that can only form when oxygen is very scarce.

The oceans were also low in oxygen and rich in dissolved iron. This iron later precipitated to form banded iron formations, striped rocks that are among the most important sources of iron ore today.

Late in the eon there are hints of change. Some rocks from about 2.5 billion years ago, such as the Mount McRae Shale of Western Australia, record brief “whiffs” of oxygen, suggesting that oxygen-producing microbes were already active before oxygen finally built up in the atmosphere during the Proterozoic.

The first life

The Archean contains the oldest widely accepted evidence of life. Layered mounds called stromatolites, built by microbial mats trapping sediment, occur in the Dresser Formation of Western Australia at about 3.48 billion years old and in the Strelley Pool Formation at about 3.43 billion years old. Older claims, such as 3.7-billion-year-old structures from the Isua belt in Greenland and possible microfossils from Quebec, remain disputed.

All Archean life was microscopic. It consisted of bacteria and archaea, single-celled organisms without a nucleus. Many lived by chemistry rather than sunlight, for example by producing methane or using sulphur compounds. Photosynthesis appeared early, and many scientists think cyanobacteria, which release oxygen, had evolved by around 3.0 to 2.7 billion years ago, although the timing is contested.

The broader story of how life began is covered in Origin of Life on Earth.

Climate and ice

Despite the faint Sun, the Archean was mostly warm. The oldest known evidence of glaciation, the Pongola glaciation of about 2.9 billion years ago, comes from southern Africa. It shows that the balance of greenhouse gases could shift enough to allow ice to form, at least regionally.

Archean riches

Archean rocks are economically important. The Witwatersrand Basin of South Africa, formed between about 2.9 and 2.7 billion years ago, has produced more gold than any other region on Earth. Greenstone belts in Western Australia, Canada and elsewhere host large gold and nickel deposits, and Archean iron formations supply iron ore.

By the numbers

Timeline

• c. 4.0 billion years ago: Formation of the Acasta Gneiss, among the oldest known intact rocks; conventional start of the Archean.
• c. 3.8-3.7 billion years ago: Rocks of the Isua belt in Greenland form, including some of the oldest sediments.
• c. 3.48 billion years ago: Stromatolites form in the Dresser Formation of Western Australia.
• c. 3.43 billion years ago: A stromatolite reef develops in the Strelley Pool Formation.
• c. 3.47-3.24 billion years ago: Large asteroid impacts leave spherule layers in South Africa’s Barberton Greenstone Belt.
• c. 2.9 billion years ago: The Pongola glaciation, the oldest known ice age.
• c. 2.7 billion years ago: Widespread crust formation; possible assembly of Kenorland.
• c. 2.5 billion years ago: End of the Archean; first signs of oxygen appear.

Why it matters

The Archean is when Earth became recognisably a living planet with continents and oceans. Its rocks preserve the earliest evidence of life and the chemical conditions in which it arose, and they help scientists judge whether similar worlds elsewhere, such as early Mars, might once have supported life.