The theory of plate tectonics is nowadays more or less universally accepted by geologists, and I have mentioned the basic idea briefly at the beginning of this class. The basic thought is, that instead of being permanent fixtures of the earth's surface, the continents and ocean basins undergo continuous change. Both are parts of lithospheric plates that move against each other, and in the process new crust is created at midoceanic ridges (spreading centers), and old crust is consumed at convergent plate boundaries (subduction zones). Even before the theory of plate tectonics, there were a variety of geologic observations that suggested that the continents were on the move, but because nobody had a good idea what the underlying driving mechanisms might be, the idea languished in obscurity for the first half of the 20th century. For now we will take plate tectonics as a theory with a broad observational data base in its support, and will assume that it essentially works as outlined in Chapter 3.
PLATE MARGINS
Alfred Wegener, the pioneer of continental drift, thought that the continents as plates move through the oceanic crust, implying thus that the shorelines of the continents are the margins of the continental plates. However, even though that may be initially a reasonable assumption (the shorelines being major geographic features), continental margins need not necessarily be plate margins. Today scientists have a fairly good understanding of how the plates move and how such movements relate to earthquake and volcanic activity. Most movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident. There are basically three different types of plate boundaries (divergent, convergent, transform), and a fourth type (boundary zones) is sometimes designated when it is difficult to define a clear boundary:
Divergent boundaries -- where new crust is generated as the plates pull away from each other.
Convergent boundaries -- where crust is destroyed as one plate dives under another.
Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.
