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Although the Archean spans almost half of Earth’s history, its rocks rep-
resent less than 20 percent of the total area exposed at the surface. Further-
more, all known Precambrian rocks have suffered some heating episode and
metamorphism. Unlike most ancient rocks in the 3.5- to 3.8-billion-year
range throughout the world, only a few such as those of the North Pole
sequence have a history of low metamorphic temperatures.Therefore, rocks of
this region have remained relatively cool throughout geologic history.
Rocks subjected to the intense heat of Earth’s interior have lost all traces
of fossilized life. Even in mildly metamorphosed rocks, the existence of micro-
fossils, which are the preserved cell walls of unicellular microorganism, is often
difficult to prove. Most of these apparent fossils are simple spheres with few
surface features.They are composed of inorganic carbon compounds squeezed
into spheroids by the growth of mineral grains deposited around them. How-
ever, some spheres were linked in pairs or in chains, and others were in groups
of four and unlikely created simply by inorganic processes.
Associated with the North Pole rocks were cherts, extremely hard
siliceous rocks containing microfilaments, which are small, threadlike struc-
tures of possible bacterial origin. Similar cherts with microfossils of filamen-
tous (threadlike) bacteria are found at eastern Transvaal, South Africa, and date
between 3.2 and 3.3 billion years old. They also exist in 2-billion-year-old
chert from the Gunflint Iron Formation on the north shore of Lake Superior
in North America. Most Precambrian cherts appears to be chemical sediments
precipitated from silica-rich water in deep oceans.The abundance of chert in
the Archean might serve as evidence that most of the crust was deeply sub-
merged. However, cherts in the North Pole region appear to have a shallow-
water origin.
Chert-forming silica leached out of volcanic rocks that erupted into
shallow seas. The silica-rich water circulated through porous sediments, dis-
solving the original minerals and precipitating silica in their place.Microor-
ganisms buried in the sediments were thus encased in one of nature’s hardest
substances, enabling the microfossils to survive the rigors of time. Modern sea-
water is deficient in silica because organisms such as sponges and diatoms (Fig.
17) extract it to build their skeletons. Generally, only the silica spicules that
make up the skeletons of sponges remain as fossils. Diatoms exhibit beautiful
glasslike silica laceworks within their cell walls. Massive deposits of diatoma-
ceous earth, also called diatomite, composed of diatom cell walls are a tribute
to the great success of these organisms following the Precambrian.
The North Pole stromatolites are distinctly layered accumulations of cal-
cium carbonate with a rounded, cabbagelike appearance.The size and shape of
the Archean-age microfossils and the formof the stromatolites suggest these
microorganisms were either oxygen-releasing or sulfur-oxidizing photosyn-
thetic species dependent on sunlight for their growth. Sulfur-metabolizing bac-
27ARCHEAN ALGAE