Flat-topped submarine mountain or seamount, the summit of which lies 1000-2000 m below the ocean surface. The flat top may be a result of marine and/or subaerial erosion.
An underwater mountain range developed at a section of oceanic crust where magma rises up through a cracking and widening ridge. Some magma cools below the crust, some of it forces into fractures, and much flows out to form new crust, which is then pushed away from the ridge. As new crust is created by the extrusion of lava each side of the ridge, it takes up the prevailing magnetic polarity of the earth, which reverses from time to time. As a result, symmetrical bands of crust, with alternating polarity, develop on either side of the ridge. These magnetic patterns are used to calculate the rate of the sea-floor spreading resulting from the lava flow. The term mid-oceanic ridge properly refers to the ridge at the centre of the Atlantic Ocean, which comes to the surface at points such as Tristan da Cunha and Ascension Island. Other ridges, such as the Pacific-Antarctic ridge, are not truly at the centre of the ocean.
The creation of new crust as magma rises up at a constructive plate margin. The magma pushes the plates apart creating new oceanic crust and pushing away the far end of the plate.
An isolated steep-sided mountain up to 1000 m tall on the sea floor, which does not break the water surface. Most are conical in shape and volcanic in origin, with summits 1000-2000 m below the sea surface. A flat-topped seamount is a guyot.