The Earth has a brittle shell (know as the crust) that surrounds its mantle, molten outer core, and dense metal inner core ( see Looking Beneath the Surface ). This shell is fragmented into many pieces, called plates ( see Earth Like a Puzzle ). Built-up stress produced by the movement of plates causes earthquakes along their boundaries. There are two basic types of plates, continental and oceanic plates, and they are made up of continental and oceanic crust, respectively, along with a portion of the upper mantle. Continental crust is generally older, thicker, and less dense then oceanic crust, and, because of isostasy, floats higher on the mantle than oceanic crust ( see isostasy exercise ). The Earth´s oceans fill low-lying areas, and thus, are located largely above oceanic plates. Where plates are moving apart (divergent plate boundaries), new oceanic crust is formed during seafloor spreading along mid-ocean ridges (e.g., the Atlantic Ocean ; see This Dynamic Planet Illustration ). Ocean crust is recycled into the mantle by a process called subduction in areas known as subduction zones.
Continental margins are the transitional zone between continental and oceanic plates and also are locations where land meets the sea. Active continental margins are those found in subduction zones, where there is enhanced earthquake activity (e.g., Western North America , see image above).
Passive continental margins are those passively moving away from sites of seafloor spreading (e.g., Eastern North America , see image above). Technically, the term continental margin includes the regions know as the continental shelf, slope and rise; however, it often is used more generally to describe the entire continental-oceanic transitional zone. On active margins, coastal mountains, volcanoes, and a narrow continental shelf are common; passive margins normally have a large coastal plain