meteorite impacts characterized the unusual geology of the Archean. Since
greenstone belts are geologically unique to the Archean, their absence from the
geologic record around 2.5 billion years ago marks the end of the eon.
ARCHEAN CRATONS
Plate tectonics has played a prominent role in shaping the planet practically
from the very beginning. Continents were set adrift from the time they orig-
inated, within a few hundred million years after the formation of the planet.
This is revealed by the presence of 4-billion-year-old Acasta gneiss, a meta-
morphosed granite, in the Slave Province of Canada’s Northwest Territories.
It is suggested that the formation of the crust was well underway by this time.
The Slave Province contains the oldest terrestrial crust known on Earth and
represents an ancient core of a continent known as a craton.
The discovery leaves little doubt that at least small patches of continen-
tal crust existed on Earth’s surface during the first half-billion years of its his-
tory. The 3.8-billion-year-old metamorphosed marine sediments of the Isua
Formation in a remote mountainous area in southwest Greenland (Fig. 27)
provide evidence of a saltwater ocean. Earth’s oldest sedimentary rocks also
Figure 27The location
of the Isua Formation in
southwestern Greenland,
which contains some of
the oldest rocks on Earth.ARCHEAN ALGAEArea of oldest rocksGreenlandIcelandBaffin
IslandBaffin
BayGreenland
SeaDenmarkStr
D ait
av
is
Str
ait