ABSTRACT

The outer shell of the solid Earth – the lithosphere –is not a single, unbroken shell of rock; it is a set of snugly tailored plates (Figure 5.2). At present there are seven large plates, all with an area over 100 million km2. They are the African, North American, South American, Antarctic, AustralianIndian, Eurasian, and Pacific plates. Two dozen or so smaller plates have areas in the range 1-10 million km2. They include the Nazca, Cocos, Philippine, Caribbean, Arabian, Somali, Juan de Fuca, Caroline, Bismarck, and Scotia plates, and a host of microplates or platelets. In places, as along the western edge of the American con - tinents, continental margins coincide with plate boundaries and are active margins. Where continental margins lie inside plates, they are passive margins. The breakup of Pangaea created many passive margins, including the east coast of South America and the west coast of Africa. Passive margins are sometimes designated rifted margins where plate motion has been divergent, and sheared margins where plate motion has been transformed, that is, where adjacent crustal blocks have moved in opposite directions. The distinction between active and passive margins is crucial to interpreting some large-scale features of the toposphere.