Definition of Archean (Archaean)

Earth’s first geological era for which

there is an extensive rock record, the Archean also preserves

evidence for early primitive life forms. The Archean is the

second of the four major eras of geological time: the Hadean,

Archean, Proterozoic, and Phanerozoic. Some time classification

schemes use an alternative division of early time, in

which the Hadean, Earth’s earliest era, is considered the earliest

part of the Archean. The Archean encompasses the one

and one-half-billion-year long (Ga = giga année, or 109 years)

time interval from the end of the Hadean era to the beginning

of the Proterozoic era. In most classification schemes, it is

divided into three parts, including the Early Archean (4.0–3.5

Ga), the Middle Archean (3.5–3.1 Ga), and the Late Archean,

ranging up to 2.5 billion years ago.

The oldest known rocks on Earth are the 4.0-billionyear-

old Acasta gneisses from northern Canada that span the

Hadean-Archean boundary. Single zircon crystals from the

Jack Hills and Mount Narryer in western Australia have been

dated to be 4.3–4.1 billion years old. The oldest well-documented

and extensive sequence of rocks on Earth is the Isua

belt located in western Greenland, estimated to be 3.8 billion

years old. Life on Earth originated during the Archean, with

the oldest known fossils coming from the 3.5-billion-year-old

Apex chert in western Australia, and possible older traces of

life found in the 3.8-billion-year-old rocks from Greenland.

Archean and reworked Archean rocks form more than 50

percent of the continental crust and are present on every continent.

Most Archean rocks are found in cratons, or as tectonic

blocks in younger orogenic belts. Cratons are low-relief tectonically

stable parts of the continental crust that form the nuclei

of many continents. Shields are the exposed parts of cratons,

other parts of which may be covered by younger platformal

sedimentary sequences. Archean rocks in cratons and shields

are generally divisible into a few basic types. Relatively lowmetamorphic

grade greenstone belts consist of deformed

metavolcanic and metasedimentary rocks. Most Archean plu-

tonic rocks are tonalites, trondhjemites, granodiorites, and

granites that intrude or are in structural contact with strongly

deformed and metamorphosed sedimentary and volcanic rocks

in greenstone belt associations. Together, these rocks form the

granitoid-greenstone association that characterizes many

Archean cratons. Granite-greenstone terranes are common in

parts of the Canadian Shield, South America, South Africa,

and Australia. Low-grade cratonic basins are preserved in

some places, including southern Africa and parts of Canada.

High-grade metamorphic belts are also common in Archean

cratons, and these generally include granitic, metasedimentary,

and metavolcanic gneisses that were deformed and metamorphosed

at middle to deeper crustal levels. Some well-studied

Archean high-grade gneiss terranes include the Lewisian and

North Atlantic Province, the Limpopo Belt of southern Africa,

the Hengshan of North China, and parts of southern India.

The Archean witnessed some of the most dramatic

changes to Earth in the history of the planet. During the

Hadean, the planet was experiencing frequent impacts of

asteroids, some of which were large enough to melt parts of

the outer layers of the Earth and vaporize the atmosphere

and oceans. Any attempts by life to get a foothold on the

planet in the Hadean would have been difficult, and if any

organisms were to survive this early bombardment, they

would have to have been sheltered in some way from these

dramatic changes. Early atmospheres of the Earth were

blown away by asteroid and comet impacts and by strong

solar winds from an early T-Tauri phase of the Sun’s evolution.

Free oxygen was either not present or present in much

lower concentrations, and the atmosphere evolved slowly to a

more oxygenic condition.

The Earth was also producing and losing more heat during

the Archean than in younger times, and the patterns,

styles, and rates of mantle convection and the surface style of

plate tectonics must have reflected these early conditions.

Heat was still left over from early accretion, core formation,

late impacts, and the decay of some short-lived radioactive

isotopes such as 129I. In addition, the main heat-producing

radioactive decay series were generating more heat then than

now, since more of these elements were present in older halflives.

In particular, 235U, 238U, 232Th, 40K were cumulatively

producing two to three times as much heat in the Archean as

at present. Since we know from the presence of rocks that

formed in the Archean that the planet was not molten then,

this heat must have been lost by convection of the mantle. It

is possible that the temperatures and geothermal gradients

were 10–25 percent hotter in the mantle during the Archean,

but most of the extra heat was likely lost by more rapid convection,

and by the formation and cooling of oceanic lithosphere

in greater volumes. The formation and cooling of

oceanic lithosphere is presently the most efficient mechanism

of global heat loss through the crust, and it is likely that the

most efficient mechanism was even more efficient in times of

higher heat production. A highly probable scenario for

removing the additional heat is that there were more ridges,

producing thicker piles of lava, and moving at faster rates in

the Archean as compared with the present. However, there is

currently much debate and uncertainty about the partitioning

of heat loss among these mechanisms, and it is also possible

that changes in mantle viscosity and plate buoyancy would

have led to slower plate movements in the Archean as compared

with the present.

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