Palaeos: Palaeos The Archean Eon
Archean The Archean Eon

The Archean

The Archean Eon of Precambrian Time: 3.8 - 2.5 billion years ago

The Archaen World

The Origin of the Continents

Rocks of the Lower Archean (in geology time is often  referred to vertically, because younger rocks are deposited above older ones) are rare, and include the oldest known terrestrial rocks, about 3.8 billion years old.  In fact, the "age of the oldest preserved rocks on Earth's surface" has been formally proposed as a definition for the base of the Archaean (more recently however, highly metamorphised rocks, and zircon crystals, have been dated from the previous, Hadean, eon).  Most of the oldest rocks are so altered through subsequent metamorphic processes it  is difficult to know under what conditions they were  formed.  The situation is rather brighter with the more numerous rocks of the Meso- and Neoarchean, from 3.2 to 2.5 billion years ago.  These are mostly volcanic in nature, consisting of pillow-like structures identical to those of present-day lavas which have formed underwater.  The implication is that at this time the entire Earth was covered by ocean.  Perhaps the bulk of the continental masses, formed through volcanic outpourings, had yet to appear from beneath the waves.

This general period, from about 3.0 to 2.5  billion years ago, was the period of maximum continent formation.  70% of continental landmasses date from this period (Thus, most of the continents  are extremely ancient).  Modern Earth sciences recognize that the present continents are built around cores of extremely ancient rock, called "shields".  A large part of Australia is a "shield", as is much of Canada, India, Siberia, and Scandinavia. 

The Origin of Life

The appearance of life on Earth was preceded by a period of chemical evolution, whereby the relative simple organic molecules gradually aggregated together to form larger and more complex macro-molecules, and finally the first life itself.  Scientists claim to be able to repeat all these  stages in their laboratories, but doubts have been expressed occasionally.

We do not know when life first appeared on Earth.  According to some sources, the oldest fossil microorganisms are as old as the oldest sedimentary rocks.  If so, we can assume that life has been around as long as conditions have been suitable.  At the time of these first organisms there was probably no free oxygen, as there is now, but rather a "reducing atmosphere" composed of methane, carbon dioxide, hydrogen and water vapor.  

The microorganisms of this period may have used methane or hydrogen rather than oxygen in their metabolism.  They are therefore referred to as  "anaerobic" (non-oxygen-using).  Fermentation is  modern example of anaerobic metabolism.  This type  of metabolism is 30 to 50 times less effective than oxygen-based ("aerobic") metabolism, or respiration.  The first organisms may have been chemoautotrophs, organisms which obtain their carbon from carbon dioxide by oxidizing inorganic compounds.  Later came "heterotrophs," which derive their food from other organisms or organic from matter which they were able to consume, and autotrophs, which create organ carbon compounds from carbon dioxide, using energy from sunlight.  The first autotrophs -- the "plants" of the Archean ecosystems -- were quite similar to modern blue-green algae.

Not all of the single-celled organisms of this time  were solitary.  Beginning perhaps 3 billion years  ago, and much more often from 2.3 billion years ago, blue-green algae would form the basic structure of large mats,  called stromatolites.  Modern-day stromatolites can still be found in sheltered bays in West Australia,  where the water is so salty that creatures that would otherwise eat them are not able to exist.  The fact that such organisms have survived to the present day gives some idea of how slow their evolution is.  The transformation of the biosphere seemed to be as slow as the transformation of the geosphere.

The Geological Time-Scale for the Archean Era




SPAN (Mya)

Notes and Events



2500 - 2300 mya

More or less conventional plate tectonics



2800 - 2500 mya

First large continental shields


3200 - 2800 mya

First widely-accepted fossil evidence of life.  First banded iron formations.


3600 - 3200 mya

First stromatolites? Formation of relatively stable crust units (possibly even earlier [Nutman et al. 2001], but see generally negative review [Sankaram, 2002]).  


~3800 - 3600 mya

Debatable geochemical evidence for life (no longer widely accepted) 


Early Imbrian

3850 - 3800 mya

Late heavy bombardment of Earth-moon system.  Ryder (2001).


Links: Introduction to the Archaean Era - 3.8 to 2.5 billion years ago - UCMP website; Archean Life: several easy pages on the Archean from Prof. Kevin Hefferan (U. Wisc. Stevens Point); Geol 02C: lecture notes from Prof. Bret Bennington of Hofstra.; Peripatus: Archean Era: notes and quotes from our own Chris Clowes (some of which incorporated in these pages); Archean Summary: nice short, unattributed article; Wikipedia has at the time of writing (110909) a still somewhat basic (compared to their coverage of other eras) page.

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page uploaded on Kheper Site on 2 June 1998, on Palaeos Site 9 April 2002
text content by M.Alan Kazlev 1998-2002
last modified ATW060902, edited RFVS111016
checked ATW040119

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