Continental margins extend up to hundreds of kilometres from landmasses, which are often mountainous, to continental shelves with water depths typically of a few hundred metres, the outer limit of which is defined by a shelf break usually at about 500 m (Fig. 30.1). Beyond the shelf break, the margin continues into deep water, via a continental slope (of between about 1° and 10°), to a deep-ocean basin of very low gradient at abyssal depths of several thousand metres. In the deep sea, the outer boundaries of an ocean basin are normally defined by tectonic features, such as spreading centres and fracture zones, that rise above the abyssal plain (Fig. 30.1).
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