led to the evolution of the eukaryotes.Therefore, oxygen was largely respon-
sible for the evolution of higher forms of life (Table 3).GREENSTONE BELTS
Greenstones are ancient metamorphosed rocks unique to the Archean. Earth’s
crust formed in a burst of creation beginning as early as 4 billion years ago,
just few million years after the formation of the planet itself. The early crust
was composed of a thin layer of basalt embedded with scattered blocks of
granite called “rockbergs.” The granite combined into stable bodies of base-
ment rock, upon which all other rocks were deposited. The basement rocks
became the nuclei of the continents and are presently exposed in broad, low-
lying, domelike structures called shields (Fig. 23).
The Precambrian shields are extensive uplifted areas surrounded by
sediment-covered bedrock called continental platforms. These are broad,
shallow depressions of basement complex (crystalline rock) filled with
nearly flat-lying sedimentary rocks.The best-known areas are the Canadian
Shield,which covers most of eastern Canada and extends down into Wis-
consin and Minnesota in North America, and the Fennoscandian Shield,Figure 22Energy flow
in the biosphere begins
with photosynthesis.
Historical Geology
CO 2ChlorophyllSolar
energy H
2 OO 2PhotosynthesisCNHN