The Cyclical Opening and Closing of Ocean Basins
Tectonics are processes which define deformation within the earth and describe the forces which produce deformation. The Plate tectonics theory explains that earth's lithosphere (outer rigid shell) is composed of several dozen "plates", or pieces, which float on a ductile mantle. The theory describes how these plates may rift into pieces, diverging apart and forming a new ocean basin, followed by motion reversal, convergence back together, plate collision, and subsequent mountain building. This cycle of opening and closing an ocean basin is called a Wilson cycle.
Plate tectonics is one of the great unifying theories in geology. Every part of the earth's crust and every kind of rock can be related to the plate tectonic conditions that existed at the time they formed. The keys to understanding plate tectonics and the Wilson cycle are knowing that 1) beginning with a parent igneous rock of mafic/ultramafic composition, all the other rocks on the earth can be generated and 2) each and every rock forms only under a specific set of tectonic conditions.
Most geologic activity occurs at the three kinds of plate boundaries:
There are two models that summarize earth evolutionary processes:
The Wilson cycle begins with a hypothetical geologically (tectonically) quiet continent. The model is divided into nine stages, but the stages are arbitrary and do not exist naturally. Within the earth is an ongoing series of processes and it is more important to understand these processes, how they are related, and how one process leads to the next process. Also note that the Wilson cycle is a simple, ideal model. In reality, the earth has many continents, which migrate across its spherical surface in very complex ways.
Contributed by Lynn Fichter