Plate Tectonics

This chapter describes how rifting may lead to continental break-up and the formation of a new ocean bound by passive continental margins. Passive continental margins define the transition from continental crust of normal thickness to oceanic crust. They are results of continental rifting that has caused the crust to break and give space to a new ocean. The break-up splits the rift into two, often unequal, parts that become tectonically inactive (passive) as the ocean starts to spread and widen. It is shown how some passive margins are magmatic with large amounts of volcanic activity, while other margins are magma-poor. Some margins are narrow, while others are wide and strongly thinned. The chapter also discusses hyperextended margins, where continental crust has been extended to the point that mantle rocks are exposed at sea bottom. This chapter presents these different settings and discusses how passive margin types are related to the final stages of rifting or initial stages of break-up. The type of magmatism involved is discussed, as are the depositional patterns that characterize the different kinds of passive margins. A number of natural examples are presented in this chapter, particularly from the south and north Atlantic margins.