Mid-oceanic ridge

Also called: Mid-ocean ridge · Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents"; Class 11, Ch 12 "Water (Oceans)"

Meaning

A mid-oceanic ridge (also called a mid-ocean ridge) is a linked chain of underwater mountains that runs through all the oceans. It is the longest mountain system on earth. NCERT gives its length as over 70,000 km, and NOAA gives nearly 65,000 km [2]. New ocean floor is made along it, where two plates move apart. It matters because the rocks on either side of it gave the key proof for sea-floor spreading (Hess, 1960s) and then for plate tectonics (1967).

Explanation

How it forms: cause → process → result

• Cause: two plates pull apart.

• The ridges lie along divergent plate boundaries, where two plates move away from each other [2].

• Process: the mantle melts and magma rises.

• Below the ridge rift zones, part of the upper mantle rock melts. This is called partial melting of the upper mantle (only some of the rock turns liquid) [2].
• This makes large amounts of magma, which rises into the gap between the plates.

• It erupts as basalt, a dark volcanic rock [2].

• Result: new ocean floor.

• The basalt cools and becomes new ocean crust.
• This is why mid-oceanic ridges are the most volcanically active places on earth.

Parts of the ridge

• Central rift: a valley along the top (crest) of the ridge. It has intense volcanism. Here the plates pull apart and magma comes up.
• Fractionated plateau: a broken-up, raised zone on each side of the rift.
• Flank zones: the outer slopes on both sides. They fall away to the abyssal plains along the length of the ridge.
• Shape and size:
• It wraps around the globe like the seams on a baseball [2].
• More than 90% of it lies under water, in the deep ocean [2].

Why the ridge proved sea-floor spreading

• Mirror-image rocks.
• Rocks at equal distances on both sides of the crest are very alike in what they are made of (constituents) and in age.

• This means new crust forms at the crest and moves away equally on both sides.

• Young ocean crust.

• Ocean crust is made at the ridges and destroyed at the trenches, so it never gets very old.

• So ocean-crust rocks are much younger than continental rocks.

• Answer to Wegener's critics.

• People had asked what force could move continents. Sea-floor spreading at the ridges helped answer that question.

Earthquakes along the ridge

• When earthquake points are plotted on a world map, one narrow belt follows the mid-oceanic ridges closely.
• Earthquakes here are shallow.
• The plates move apart near the surface.
• So the focus (the point inside the earth where the earthquake starts) is not deep.

Where it occurs

• Mid-Atlantic Ridge: runs north–south down the middle of the Atlantic Ocean [2].
• Indian Ocean ridges: the ridge belt continues from the Atlantic into the Indian Ocean.
• South of India the belt splits:
• one branch goes into East Africa;

• the other joins a line running from Myanmar to New Guinea.

• Place in the ocean floor: it is one of the three divisions of the ocean floor, along with continental margins and abyssal plains. It lies beyond the abyssal plains, which sit between the continental margins and the ridges.

• India's link: India is not on a ridge. Its main earthquake risk comes from the other belt, the Alpine-Himalayan belt, which reaches the Andaman-Nicobar Islands [6].

Don't confuse with

• Deep-oceanic trench: a trench is a boundary where plates converge (move together). One plate sinks there, ocean crust is destroyed and earthquakes are deep-seated. At a ridge the plates diverge, crust is created and earthquakes are shallow. Trenches belong to the continental margins. The Mariana Trench (Challenger Deep, about 10,935 m) is an example [3][4].
• Abyssal plain: the flattest surface on earth. It is made flat by fine sediment settling on the sea bed, not by volcanism. It lies between the continental margins and the ridge.
• Himalayas / Alps (continental fold mountains): these mountains rose where plates pushed together, and they stand on land. The mid-oceanic ridge is longer, mostly under water and built where plates move apart.
• Ring of Fire (Circum-Pacific belt): a convergent belt about 40,000 km long with about 452 volcanoes (75% of earth's) and deep earthquakes [5]. The ridge belt is divergent and has shallow earthquakes.

Prelims Hooks

• Longest mountain system on earth: the mid-oceanic ridge. NOAA gives nearly 65,000 km, with more than 90% under water [2]. NCERT gives over 70,000 km.
• Order from the top of the ridge outwards: central rift (intense volcanism) → fractionated plateau → flank zones.
• Order from coast to deep sea: continental shelf → continental slope → continental rise → abyssal plain → mid-oceanic ridge.
• Process–landform pair: divergent boundary + partial melting of the upper mantle → basalt eruption → new ocean floor at the ridge [2].
• Trap: ridge earthquakes are shallow. Alpine-Himalayan and Pacific-rim earthquakes are deep-seated. Do not swap them.
• Trap: rocks at equal distances on both sides of the ridge crest match in age and make-up, and ocean crust is younger than continental crust, not older.

Mains Points

• The ridge gave the proof for plate tectonics (GS-I).
• Post-war sonar surveys found volcanic ridges, young ocean crust and rocks of matching age on both sides of the crest.
• These findings led to sea-floor spreading in the 1960s and then to plate tectonics in 1967.

• They answered the main objection to Wegener's continental drift: what force moves the continents.

• Plate boundaries control the world's hazard pattern (GS-I / GS-III).

• Divergent ridges have shallow earthquakes and quiet basalt eruptions.
• Convergent belts like the Pacific rim and the Himalayas have deep, very damaging earthquakes and explosive volcanoes.

• This difference is the basis of hazard zoning across the world. India lies on the convergent belt, so it focuses on seismic zonation (the Himalayan arc is now in Zone VI [7]) and tsunami warning (INCOIS).

• Ridges are still poorly mapped (GS-III).

• Only about 26% of the seafloor had been mapped in high resolution as of June 2024 [3].
• This gap limits deep-sea mineral exploration, hazard assessment and our understanding of ocean circulation.

Related concepts

• Ocean floor mapping

Read more

• Mapping the ocean floor: margins, plains, ridges and the belts of earthquakes and volcanoes