Minor relief features

Besides the major divisions, the ocean floor has smaller but important features:

• Mid-oceanic ridge: two chains of mountains separated by a large depression (rift). Peaks can reach about 2,500 m, and some rise above the sea: Iceland is part of the Mid-Atlantic Ridge.
• Seamount: a volcanic mountain with a pointed summit that rises from the sea floor but does not reach the surface; often 3,000-4,500 m tall. The Emperor seamount chain, extending from the Hawaiian Islands in the Pacific, is a good example.
• Submarine canyons: deep valleys, some as big as the Grand Canyon, cutting across continental shelves and slopes, often from the mouths of large rivers. The Hudson Canyon is the best known.
• Guyot: a flat-topped seamount, showing evidence of gradual subsidence in stages. More than 10,000 seamounts and guyots are estimated in the Pacific alone.
• Atoll: a low island in tropical oceans, made of coral reefs around a central depression. The enclosed water may be part of the sea (a lagoon) or fresh, brackish or very salty.

Example (new): the Lakshadweep islands off India's west coast include atolls: rings of coral reef and low sandy islands around shallow lagoons, rising from the Indian Ocean floor.