ABSTRACT
Active margins occur where tectonic plates converge (to form sub - duction zones in which one plate sinks below another) or where they slide by one another (transform zones) (Figure 1). They are common around the rim of the Pacific Ocean, which gives them the alternative name of Pacific-type margins. The coast of the Pacific northwest of North America and the west coast of South America are active margins with a subduction zone; central and southern California is an active margin with a transform zone. Tectonic activity (earthquakes, volcanoes, uplift, and the formation of new igneous rock) is characteristic of active margins. Passive margins form where two continental plates split (Figure 1). They are not the sites of plate boundaries, and although continental crust abuts oceanic crust, they are part of the same tectonic plate and subduction does not take place. The east coast of the Americas and the west coast of Africa and Europe are examples of passive (or Atlantic-type) continental margins. Tectonic activity at passive margins is negligible as no plate collision or subduction occurs.
