Reading Comprehension (RC) | The basic theory of plate tectonics recognizes two ways

The basic theory of plate tectonics recognizes two ways continental margins can grow seaward. Where two plates move away from a midocean rift that separates them, the continental margins on those plates are said to be passive. Such continental margins grow slowly from the accumulation of riverborne sediments and of the carbonate skeletons of marine organisms. Since most sequences of such accretions, or miogeoclinal deposits, are unreformed, passive margins are not associated with mountain building.

Along active margins continents tend to grow much faster. At an active margin an oceanic plate plunges under a continental plate, fragments of which then adhere to the continental margin. The process is met with extensive volcanism and mountain building. A classic example is the Andes of the west coast of South America.

In the original plate-tectonic model western North America was described as being initially passive and then active. It was assumed that the continent grew to a limited extent along this margin as oceanic rocks accreted in places such as the Coast Ranges of California. The model was successful in explaining such disparate features as the Franciscan rocks of the California Coast Ranges, created by subduction, and the granite rocks of the Sierra Nevada that originated in volcanoes.

The basic plate-tectonic reconstruction of the geologic history of western North America remains unchanged in the light of microplate tectonics, but the details are radically changed. It is now clear that much more crust was added to North America in the Mesozoic era than can be accounted for by volcanism and by the simple accretion of sediments. Further, some adjacent terranes are not genetically related, as would be expected from simple plate tectonics, but have almost certainly travelled great distances from entirely different parts of the world.