Mid-ocean ridge

A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. This uplifting of the ocean floor occurs when convection currents beneath the ocean bed force magma up where two tectonic plates meet at a divergent boundary. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world, whose total length is ~60,000 km.

Description

Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor through a gap in the earth's crust called a rift. In most instances, they accumulate and form new crust and submarine volcanoes.

The rocks making up the sea floor are younger near the centre of the ridge and age with increasing distance from its centre. This is evidence that new magma constantly emerges through a rift. The area around the mid-ocean ridge is dominated by volcanic basalts.

The sea floor is made up of rocks generally much younger than the Earth itself. This suggests that the sea floor is in a constant state of 'renewal'. Moving away from the mid-ocean ridge, ocean depth progressively increases until it reaches ocean trenches.

Formation Processes

There are two processes which are thought to be responsible for the spreading seen at mid-ocean ridges, and there is some uncertainty as to which is the most dominant. Slab-pull and ridge-push are currently the most popular processes. Ridge-push occurs when the weight of the ridge pushes the rest of the tectonic plate away from the ridge, often towards a subduction zone. At the subduction zone, "slab-pull" comes into effect. This is simply the weight of the tectonic plate being subducted (pulled) below the overlying plate dragging the rest of the plate along behind it.

The other process thought to contribute to the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" (see image). However, there have been some studies which have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along.

The rate at which the mid-ocean ridge creates new material is known as the spreading rate, and is generally measured in mm/yr. The common subdivisions of spreading rate are fast, medium and slow, whose values are generally >100 mm/yr, ~60 mm/yr, and <20 mm/yr respectively. The spreading rate of the north Atlantic Ocean is 10 mm/yr, while in the Pacific region, it is 40-60 mm/yr.

The mid-ocean ridge systems form new oceanic material. As this new material cools (from magma to rock) particles in the rock align themselves to the Earth's magnetic field. By studying the orientation of these fixed particles in the oceanic crust, it has been possible to see that the Earth's magnetic field has suddenly flipped throughout its history.

Discovery

Because a mid-ocean ridge is submerged at very deep depths in the middle of the ocean, its existence was not even known until the 1950s, when it was discovered through surveys of the ocean floor conducted by research ships.

More specifically, the Vema, a ship of the Lamont-Doherty Geological Observatory of Columbia University, traversed the Atlantic Ocean, recorded data about the ocean floor from the ocean surface. A team lead by Marie Tharp and Bruce Heezen analyzed the data and concluded that there was an enormous mountain chain running along the middle of the Atlantic. The mountain range was named the Mid-Atlantic Ridge; it remains the most famous part of the mid-ocean ridge.

At first, it was thought to be a phenomenon specific to the Atlantic Ocean, because nothing like such a massively-long undersea mountain chain had ever been discovered before. However, as surveys of the ocean floor continued to be conducted around the world, it was discovered that every ocean contained parts of the mid-ocean ridge.

Impact

Alfred Wegener had proposed the theory of continental drift in 1912. However, the theory had been dismissed by geologists because there was no mechanism to explain how continents could plow through ocean crust, and the theory became largely forgotten.

Following the discovery of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. In the 1960s, geologists began to propose the mechanism of sea floor spreading. Plate tectonics was a direct consequence of sea floor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major paradigm shift in geological thinking.

Description

Formation Processes

Discovery

Impact

List of oceanic ridges

See also

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Description

Formation Processes

Discovery

Impact

List of oceanic ridges

See also