Although this is written as a generalized history encompassing a large scope of geological events and avoiding extensive use of specialized vocabulary, it is still helpful to be familiar with the following key concepts, or frames of reference:

Geologic Time Scale:

For convenience, the Earth history is divided into four eras. The eras are subdivided into periods, which are further subdivided into epochs. This chart will be useful to the reader: (from Bradford B. Van Diver Roadside Geology of New York; Mountain Press Publishing Company, fifth printing, 1992)

Plate Tectonics:

A theory that became one of the major building blocks of geological sciences in this century, plate tectonics states that crust and rigid upper mantle of the Earth behave as a unit called lithosphere. Lithosphere is divided into approximately 12 major plates, which ride on asthenosphere, a zone of greater plasticity. Tectonic plates can spread apart, ride past each other, or collide, producing ocean floors, causing seismic activity, and building mountain chains. Thus, there are times of continental togetherness, followed by times of continental drifting apart. These diagrams show the position of continents at different times in Earth history relative to each other.

A. About 200 Million Years Ago

B. About 100 Million Years Ago

C. Now

(from Bradford B. Van Diver, Roadside Geology of New York; Mountain Press Publishing Company, fifth edition, 1992)

This page hosted byGeoCities. Get your own Free Home Page

Site© 1996-2004