Summary

The most striking division of the earth's surface is into continental and oceanic areas underlain by crust of different density and thickness.

The major features of the sea floor are formed by tectonic processes of plate divergence, convergence, and transform faulting. Volcanic activity is the dominant process on the surface of the sea floor and the final sculpting is the emplacement of volcanic features over the tectonic patterns.

The continental margins covered by present sea level are on continental crust. The morphology of these surfaces results from tectonics, sediment deposition and a history of sea level changes.

Plate tectonics does not have to be an answer to all problems, but is accepted as the major tectonic process shaping the earth. Some speculations follow, which are that -- speculations. The lithosphere - asthenosphere and mantle convection are accepted models -- the argument is from this point of departure. If mantle convection and a rising column -- divergence occurs -- this could drive a spreading sea floor plate tectonics system -- it could also lead to uplift and epirogenic movements, block faulting, etc., without rifting and sea floor generation. An uplifted oceanic rise can be driven as described in the plate tectonics model, or may slide by gravity tectonics. The structural patterns are not completely answered and at its present stage, plate tectonics is a working hypothesis. It may be the answer, or part of the answer working in conjunction with other processes. The best approach is an open mind.

Life is intimately tied to the environment, so interactions should exist between a tectonic history of mobile continents and the diversity and extinction of species. The equilibrium mode is a basic concept in biogeography. Life starts in an isolated area initially devoid of life and the number of species increases with time. As new species arrive and evolve from older species, diversity increases and exceeds the rate of extinction of older species. As the populations expand and fill available niches, the area saturates and an equilibrium comes into operation between extinction and origin of new species.

The total number of species possible is limited by habitable area. For a given area, the number of species possible is reasonably predictable. This pattern will respond to changes in area. As the area increases beyond 1000 km2, a limiting possibility of increase is reached so that continents will have provinces limited in size regardless of area. Beyond a limiting size, the total number of species will not increase as area increases; there is a limited number of species possible for any particular land mass (or isolated province). If two continents that each have the limit of species possible (but no faunal overlap) collide, we have double the number of species. Even though the area is adequate -- only half of the species can be supported and the rate of extinction of species is high until limiting equilibrium is again achieved.

If a continent is rifted apart, we still have more than the limiting area in each segment and species diversity can increase on each segment until saturation is again reached -- approximately doubling the number of species. As the continents move, climate conditions change at a rate that evolutionary trends can keep pace, so that changes in species will represent the journey of the plate. A plot of species diversity against geological time shows a correlation between continent drift patterns and species abundances. Since the magnetic field provides the earth with a shield against much of the cosmic ray input, there is much speculation about what effect the reduction of the field during reversals has on extinctions and mutations of species. The general opinion is that there has been negligible effect on evolutionary patterns.

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