Archean

The Archean Eon constitutes the second major division of Earth’s geological history. It directly follows the Hadean Eon and precedes both the Proterozoic and the Phanerozoic. Representing an immense span of early terrestrial time, the Archean marks a formative stage in the development of the planet’s crust, atmosphere and earliest life. This eon encompasses a period beginning approximately 4,031 million years ago, extending to around 2,500 million years ago, a timeframe defined chronometrically rather than by rock strata. The beginning of the eon closely overlaps with the latter stages of the Late Heavy Bombardment, while it ends shortly before the onset of the Huronian glaciation.

Etymology and Development of Classification

The term Archean originates from the Greek word meaning ‘beginning’ or ‘origin’, reflecting its position early in Earth’s history. The classification of the eon has undergone significant revision over time. Before the recognition of the Hadean Eon, the Archean was believed to span from Earth’s formation to about 2,500 million years ago. It was once grouped within the wider Precambrian, which had traditionally been regarded as an azoic or lifeless interval until microfossil discoveries demonstrated biological activity in deposits formerly thought to be barren.
Modern classification is chronometric. The lower boundary, set at 4,031.3 million years ago, corresponds to the age of the oldest intact terrestrial rock formations. Material preceding this age is limited to mineral grains and extraterrestrial samples, as no coherent rock formations survived from the Hadean.

Geological Characteristics

The Archean Earth differed markedly from the modern planet. Its internal heat budget was almost three times higher at the start of the eon and remained about double the present-day level at its end. This heat derived partly from the remnants of planetary accretion and partly from the heightened decay of radioactive isotopes. As a consequence, mantle temperatures were substantially hotter, influencing volcanic activity, crustal stability and early tectonic processes.
Although a handful of Hadean mineral grains survive, the earliest coherent rock formations exposed today are Archean. These occur across several cratonic regions, including Greenland, Siberia, the Canadian Shield, the Wyoming and Minnesota River Valley areas, the Baltic Shield, Scotland, India, Brazil, western Australia and southern Africa. The dominant lithologies comprise granitic and granodioritic rocks, extensive melt sheets and intrusive complexes, including anorthosites, layered intrusions and distinctive sanukitoids. Metamorphosed deep-water sediments such as greywackes, mudstones and banded iron formations are frequent, reflecting widespread submarine deposition.
Volcanism in the Archean was intense. Lava types uncommon today, including komatiites, were produced during vigorous volcanic eruptions. The rarity of carbonate rocks suggests that the oceans were more acidic than in later eons due to elevated carbon dioxide levels. Greenstone belts—characteristic sequences of metamorphosed volcanic and sedimentary rocks—are among the most recognisable Archean geological formations. These belts record the development of volcanic island arcs and associated deep-sea sedimentation within forearc basins and often mark sutures between protocontinental fragments.
The emergence and behaviour of plate tectonics remain subjects of active research. Plate tectonic processes likely originated during the Hadean and persisted into the Archean, but may have operated differently due to higher mantle temperatures and lower lithospheric strength. As mantle water was progressively outgassed, viscosity increased, potentially slowing tectonic motion. Continental crust formed in significant quantities, yet most of it remained submerged beneath deeper-than-modern oceans. Only toward the end of the eon did substantial continental landmasses begin to rise above sea level.
Archean continental configuration is difficult to reconstruct owing to extensive recycling and metamorphism. Competing hypotheses suggest the existence of early landmasses such as Ur, dating to around 3,100 Ma, or Vaalbara, as early as 3,600 Ma. However, only about 8 per cent of present-day continental crust is composed of Archean material; the remainder has been reworked or subducted. By the Neoarchean, tectonic activities resembled modern systems more closely, though slab detachment events were more frequent due to hotter and weaker plates. Geological evidence indicates the presence of sedimentary basins, volcanic arcs, intracontinental rifts, continental collisions and global-scale orogenic episodes.
Asteroid impacts were prevalent during the early Archean, with spherule layers indicating continued bombardment. Large impactors, some comparable in size to the Chicxulub impactor, may have struck Earth approximately every 15 million years. These collisions significantly influenced atmospheric composition, acting as sinks for oxygen and contributing to fluctuations in redox conditions.

Atmospheric and Oceanic Conditions

The Archean atmosphere differed profoundly from today’s. It is widely regarded as a reducing atmosphere dominated by methane, with negligible free oxygen. Estimates suggest oxygen concentrations were less than 0.0001 per cent of present atmospheric levels, and possibly even lower. Despite this global anoxia, short-lived oxygenation events occurred at several intervals, around 2,980–2,960 Ma, 2,700 Ma and 2,501 Ma. Some researchers interpret these pulses as potential precursors to the Great Oxidation Event, though the majority opinion places the major rise of atmospheric oxygen in the Palaeoproterozoic. In certain shallow marine settings, localised oxygen ‘oases’ appear to have existed during the Mesoarchean.
Archean oceans were also reducing and lacked a persistent redoxcline, a feature common in younger, more oxygenated oceans. Sulphate concentrations were extremely low because of minimal atmospheric oxygen, and sulphide formation resulted mainly from reduction of organic sulphite or mineralisation of reduced sulphur species. The oceans were enriched in heavier oxygen isotopes compared with present-day values, although these declined through the later Archean as emerging continental landmasses underwent weathering, altering seawater chemistry.
Solar luminosity during the Archean was only 75–80 per cent of its modern value, presenting the so-called faint young Sun paradox. Despite this reduced luminosity, geological evidence indicates the presence of liquid water by about 4,000 Ma, as shown by highly deformed gneisses derived from sedimentary precursors. Greenhouse gases, in greater abundance than today, likely maintained temperatures consistent with stable oceans and active hydrological cycles.

Early Life and Biological Developments

The Archean Eon marks the first clear appearance of life on Earth. Microbial communities formed shallow-water mats known as stromatolites, which represent some of the earliest fossil evidence. Life during this eon consisted exclusively of prokaryotes—bacteria and archaea—which were adapted to reducing environmental conditions and often utilised anaerobic metabolic pathways.
Photosynthetic organisms, particularly early cyanobacteria, emerged during the mid- to late Archean. Although these microbes released oxygen as a by-product of photosynthesis, the gas was rapidly consumed by reduced compounds in the oceans and crust. Over time, increasing photosynthetic activity contributed to gradual oxygenation, ultimately setting the stage for the Great Oxidation Event after the end of the Archean.
Biogeochemical cycling during this period was shaped by the absence of free oxygen. Sulphur and nitrogen cycles differed markedly from their modern forms, with widespread abiotic denitrification and limited availability of oxidised nitrogen species. Organic carbon burial rates, however, appear to have been similar to those observed today, demonstrating that biological productivity, though microbial, was substantial.

Significance of the Archean

The Archean Eon represents a foundational chapter in Earth’s history. It witnessed the stabilisation of the first extensive continental crust, the formation of ancient cratonic nuclei, and the development of tectonic processes that gradually began to resemble modern plate dynamics. Its reducing atmosphere and active volcanic conditions shaped early ocean chemistry, while widespread impacts and extreme thermal regimes influenced environmental stability.