Historical Geology 102
Archean Eon


Archean Eon

4.6-2.5 billion years ago.
Archean aged rocks are relatively common in contiental cratons.
Two basic groups of rocks are found:
1) Granulites
    - highly metamorphosed granites, tonalites, gabbros and gneiss (continental crustal rocks)
2) Greenstones
    - metamorphosed volcanic and sedimentary rocks
    - contain minerals such as chlorite and actinolite which give them a greenish color.

Greenstone Belts

Typically found in belts which have a synclinal form.
Show a sequential transition from ultramafic volcanics at the bottom to felsic volcanics and capped by sediments.
Ultramafic Volcanics
- surface temperatures of 1600 degrees C or higher.
Mafic Volcanics
- pillow lavas are common
- indicate eruption of lava underwater.
Sedimentary Rocks
- composed dominantly of graywackes ("dirty sandstones"), conglomerates and sandstones.
- deposited in shallow water deltas, tidal flats, shallow marine shelf environments.

Archean Tectonics

Two models have been developed to explain the formation of greenstones and granulites.

Back Arc Basin Model

"Mini-continents" formed during the Hadean.
These plates consisted of three parts:
1) active volcanic arc
2) back arc area of older continental crustal material
3) oceanic crust
Convection beneath the back arc area results in extension and the development of a back arc basin.
Volcanic deposits and sediments collect in this basin.
Collision of this "mini-continent" with a similar "mini-continent" results in metamorphism of the volcanic and sedimentary deposits and gentle deformation into a broad syncline.

Micro-Plate Collision Model

"Mini-continents" formed during the Hadean.
These plates consisted of two parts:
1) active volcanic arc
2) oceanic crust
No back arc basin or back arc continental material.
Volcanic and sedimentary deposits accumulate on the fore arc and back arc areas of the volcanoes.
Collision occurs with similar volcanic arcs.
Greenbelts form from the collision of the back arc and fore arc areas.

Archean Plate Tectonics

Possibly occurring at a faster rate than today.
Higher internal temperature of the Earth.
Higher temperature - more vigorous convection.
Possibly faster plate motion; more collisions.

The Beginning of Life

Oldest evidence of life - 3.5 b.y. old fossils from Australia.
Life is believed to have developed 3.8-3.6 b.y. ago.
First life on Earth - non-photosynthetic bacteria.

Earliest Life

Life first developed in the oceans due to:
1) nutrients and chemicals necessary for life are found in abundance.
2) currents mix this material easily.
3) water provides protection from ultraviolet radiation.
Earliest organisms were anaerobic - did not require oxygen (O2)
Amino acids to RNA and DNA?
First cell membrain?

Fossils of the Archean

Most fossils from the Archean are either stromatolites and single cells.
Stromatolites are layered sedimentary structures which formed from mat-like colonies of cyanobacteria (blue-green algae).
The cyanobacteria would trap sediment and calcium carbonate - forming a dome-like layered structure.
Oldest stromatolites are 3.5 b.y. old.
Photosynthesis developed by at least 3.0 b.y. ago - possibly as early as 3.5 b.y. ago.