Minor Ocean Relief Features

At a Glance

• Mid-oceanic ridge Longest mountain chain, about 65,000 km
• Seamount Submerged volcano rising over 1,000 m
• Guyot Flat-topped seamount
• Submarine canyon Deep valley cut in the shelf and slope
• Atoll Ring-shaped coral island with a lagoon
• Exam link UPSC Prelims, Geography: Ocean Relief, Oceans and Seas

Where Does It Fit?

This is the second topic of the category on ocean relief, oceans and seas. The previous topic described the major divisions of the ocean floor, and this one covers the smaller features that rise from or cut into it. The next topic begins the study of the individual oceans with the Pacific. Previous topic: Major Ocean Relief Features. Next topic: The Pacific Ocean.

What Are Mid-Oceanic Ridges?

In short: mid-oceanic ridges are the underwater mountain chains where new ocean floor is made.

A mid-oceanic ridge is a chain of submarine mountains in the middle of an ocean, formed where two plates are moving apart and magma rises to fill the gap, as in Plate Tectonics and Types of Plate Boundaries. The whole system winds around the globe like the seam of a baseball for about 65,000 km, which makes it the longest mountain range on Earth, with peaks that rise 2,000 to 3,000 m above the floor. The crest has a rift valley along its axis, with frequent earthquakes and volcanic activity. The crustal rocks are youngest at the ridge and become older away from it, which is evidence of sea-floor spreading, as in Seafloor Spreading and Paleomagnetism.

Fast-spreading ridges are broad and smooth, while slow-spreading ridges are steep, with a deep rift valley. At the ridges, sea water seeps into the crust, is heated, and comes out through hydrothermal vents, which are chimneys that spout hot mineral-rich water, called black smokers when the water is black with metal sulphides. They were first seen in 1977 near the Galápagos Islands. Their ecosystems of tube worms and bacteria do not depend on sunlight, but on chemosynthesis, and the deposits hold copper, zinc and gold, which has stimulated interest in deep-sea mining. India has been allotted an exploration area for polymetallic sulphides on the Central Indian Ridge by the International Seabed Authority, as in Ocean Deposits and Mineral Resources.

What Are Seamounts, Guyots and Oceanic Plateaus?

• Seamount: an isolated volcanic mountain that rises from the sea floor by more than about 1,000 m but does not reach the surface. There may be hundreds of thousands. They concentrate fish and corals and are important for fisheries and biodiversity. Chains of seamounts, such as the Hawaiian-Emperor chain, form when a plate moves over a hotspot.
• Guyot: a flat-topped seamount whose top was cut flat by waves when it was at the sea level, before it subsided below the surface. The name is from the Swiss-American geologist Arnold Guyot. They are common in the Pacific.
• Oceanic plateau: a large, raised, flat area of the sea floor, often made of huge volumes of lava, such as the Kerguelen Plateau in the southern Indian Ocean and the Ontong Java Plateau in the Pacific.
• Island arc: a curved chain of volcanic islands above a subduction zone, such as Japan, the Aleutians, and the Andaman and Nicobar chain. It lies alongside a trench, as in Major Ocean Relief Features.

What Are Submarine Canyons and Fracture Zones?

Submarine canyons are deep, steep-sided V-shaped valleys that cut the continental shelf and slope, sometimes to depths of a few kilometres. They are cut by turbidity currents, which are avalanches of sediment-laden water that flow down the slope, and by rivers when the sea level was low. Many are the continuation of river valleys, such as the Hudson Canyon off New York, the Congo Canyon and the Swatch of No Ground, a deep canyon in the Bay of Bengal south of the Ganga-Brahmaputra delta. They carry sediment to the deep sea and form the fans on the continental rise.

Fracture zones are long, linear cracks offset from the ridge where the plate moves unevenly, called transform faults, as in Plate Tectonics and Types of Plate Boundaries. The sill is a ridge on the sea floor that separates the basins or the sea from the ocean, such as the Gibraltar sill, which controls the exchange of water between the Mediterranean and the Atlantic.

How Are Atolls Formed?

An atoll is a ring-shaped coral reef with a central lagoon, surrounding a sunken volcanic island. Charles Darwin explained the formation in the 1830s. A volcanic island is first fringed by a coral reef. As the island slowly sinks, the corals grow upward to remain in the light, and the reef becomes a barrier reef with a lagoon between it and the shrinking island. Finally, the island disappears below the sea, and only the ring of coral, the atoll, remains. Drilling at Eniwetok atoll confirmed his theory by finding volcanic rock under more than a kilometre of coral. The Lakshadweep islands and the Maldives are atolls, built on the Chagos-Laccadive Ridge, as in Coral Reef Types and Their Development. As an analytical point, atolls are rarely more than 2 to 3 m above sea level, so for the Maldives and Lakshadweep, the rise of the sea level is an existential question, which is why the study of the sea floor is also a study of the future of island nations.

Why Is This Topic Important?

• Facts Feature definitions asked
• Link Plate tectonics evidence
• Resources Vents and seabed minerals

What Came Next?

The next topic begins the study of the individual oceans with the largest, the Pacific. Next topic: The Pacific Ocean.

Key People and Terms

• Mid-oceanic ridge Underwater mountain chain at a divergent boundary
• Seamount Submerged volcano
• Guyot Flat-topped seamount
• Atoll Coral ring on a sunken island
• Turbidity current Sediment-laden flow down the slope
• Hydrothermal vent Seafloor hot spring

Exam Corner

Mains Angle

Questions link relief to tectonics and resources. These points add depth.

• Tectonics Ridges as evidence of spreading
• Resources Polymetallic sulphides
• Risk Atolls and sea-level rise