Geology Reference
In-Depth Information
Addressing Archean Rocks
Soon after earth's formation and differentiation, the lithosphere of the earth was separ-
ated into plates that began moving around the surface (similar to what scientists ob-
serve today). The oldest continental crust rocks are from the Archean eon, approxim-
ately 4 billion years ago. They provide a picture of the geologic processes that were oc-
curring at that time. In this section I describe some of the Archean rock formations,
most of which were created about 2.5 billion years ago, and I explain how they provide
evidence for plate tectonics so early in earth's history.
Creating continents
As the heat generated by the formation of the earth and the radioactive decay of ele-
ments in its core escaped through volcanoes on the seafloor, rock materials that consti-
tute the cores of our modern continents began to form. (For details on radioactive de-
cay, see Chapter 16.)
The oldest rocks on a continent constitute its core, or
craton.
A craton is made of a
Pre-
cambrian shield
of ancient rocks that are visible on the surface, or exposed, and a sur-
rounding
platform
of ancient rocks that have been covered with more recent rocks. Fig-
ure 18-1 illustrates the location of each continent's craton.
Figure 18-1:
Cratons of the
modern contin-
ents.
