What is a continental margin?

    Continental margins are of two main types, those formed during rifting of a continental segment of a plate (which leads to the formation of a new ocean basin) and those formed at the convergence of two plates (marked by a submarine trench and a subduction or Benioff zone).

    Rifted margins (sometimes called passive margins) always form in pairs either side of the oceanic spreading centre that appears after continental break-up. Rifted margins are tectonically active for just a few million years during their formation but soon become inactive after the continent breaks in two and as the oceanic spreading centre retreats from the margins. The structure of the crust and upper mantle under a rifted margin varies according to the amount of mantle melt, and subsequent volcanic activity, generated during rifting. Broadly, any rifted margin occupies the transitional region (of variable width and structure) between 30-35 km thick continental crust and 7 km thick oceanic crust. During rifting, lateral offsets in segments of rifted margin are occupied by active transform faults (which initially are continent-continent shear zones) that eventually become inactive transform continental margins after continental separation.

Some examples of pairs of rifted margins are:

1. Somali Republic and South Yemen either side of the Gulf of Aden, Indian Ocean
2. The eastern edge of the Grand Banks of Newfoundland and the western edge of the Iberian peninsula, North Atlantic Ocean.
3. The western side of southern Africa and the eastern side of South America, South Atlantic Ocean.
4. Western Australia and the east coast of India, Indian Ocean.

    Convergent margins occur where an oceanic segment of a plate is being subducted beneath a second plate. A convergent margin is indicated at the surface by a deep oceanic trench (a linear deep which is hundreds of kilometres long) and at depth by a zone of earthquakes that dips beneath the non-subducting plate to depths of a few hundred kilometres or more. When two oceanic segments of plates interact in this way the non-subducting plate is characterised by a chain of volcanic islands (an island arc).

Some examples of convergent margins are:

1. The west coast of South America (offshore the coasts of Peru and Chile) in the south-east Pacific Ocean.
2. The Aleutian island arc in the north Pacific Ocean.
3. The Lesser Antilles island arc in the West Indies, central Atlantic Ocean.

The importance of continental margins.

    Continental margins include the sites of some of the world's largest accumulations of sediments and these sediments may provide the source of significant reserves of oil and gas.

    The large relief and potential for the accumulation of shallow gas and gas hydrates on some margins, plus the fact that most convergent margins are seismically active, means that many margins are prone to slope failure. Such failures can generate dangerous tsunamis or 'tidal waves' that can cause great damage to coastal areas.

    The relief of continental margins often perturbs the flow of ocean currents and can even contribute to the upwelling of cold, nutrient-rich, deep waters which encourages increased biological productivity. Therefore, some continental margins are associated with important commercial fisheries.

    Thus, a clearer knowledge of the structure, composition and evolution of continental margins will not only improve scientific understanding of lithospheric processes but will also be crucial to the future wealth and well-being of coastal nations. Previous investigations have left many unanswered questions, primarily because of the large spatial scale of continental margin structures and the significant resources required for their study.