Geology Reference
In-Depth Information
Chapter 18
Time before Time Began: The
Precambrian
In This Chapter
Forming the earth and its different layers
Interpreting granite-gneiss and greenstone formations of the Archean eon
Building the first supercontinents in the Proterozoic eon
Evolving an atmosphere through early photosynthesis
The Precambrian time period of earth's history is very mysterious. It occurred so long
ago that most of the evidence for the processes that first took place on earth have been
destroyed. Almost no fossils and very few rocks are left from this period. The few rocks
that do remain tell a very interesting story about earth's beginnings as a planet.
In geologic time the Precambrian covers the first 4 billion years of earth's history. Geolo-
gists separate the time period into three eons: the Hadean, the Archean, and the Protero-
zoic. During this long expanse of time very important events took place, such as the cre-
ation of the first continental crust, the accumulation of water on the surface, the forma-
tion of an atmosphere, and the appearance of the earliest life forms.
In this chapter, I describe scientists' theories and hypotheses about how the earth (and
solar system) formed, how the first continents were created, and what geologic processes
(such as plate tectonics and the rock cycle) began billions of years ago and continue to
occur today.
In the Beginning . . . Earth's Creation from
a Nebulous Cloud
