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Lecture 1 - Archean Notes
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Notes on the Archean
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The Archean is the second eon in Earth’s history, sometimes older texts will call it the Archaeozoic, and is the secondof the four eons in the Earth’s geological history and starts 4bya and ends 2.5bya.
At this time the Earth’s surface was mainly water, with very little continental crust, with ocean being many times
deeper than it is today. The oldest continental crust still present on the Earth dates back to the Archean.
The word Archean is Greek and means beginning or origin, unlike the time periods in Earth’s history the Eon is
measure chronometrically not stratigraphically, both boundaries have been officially recognized by ICS.
The heat flow around this time was almost x3 higher than it is today, and twice the level it was by the time the
Proterozoic began, this was caused by planetary accretion, the creation of the core and decay of the radioactive
elements still present at the surface.
In contrast to the Hadean there are actually many regions around the world that have exposed Archean aged rocks,
many can be seen in Greenland, Siberia, Montana and Wyoming, as well areas of the Baltic, Scotland, India and
western Australia. The most common rock type is Granitic, such as the great melt sheets of granite, diorite and
layered intrusions. Archean are almost exclusively metamorphized, formerly deep-water sea beds, comprised of
graywackes and mudstones.
Greenstone Belts are a very common feature of Archean era rocks, they were formed from large volcanic eruptions,
and where often apart of volcanic island arcs.
Plate tectonics began during the Hadean, but slowed down during the Archaean, this is most likely because the
mantle increased in viscosity caused by outgassing. There was most likely quite a lot of continental crust, but it
would have been covered more or less by the Archean ocean, furthermore because the crust recycling the rocks,
there is actually little information on what continents actually existed at the time.
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