Mid-ocean ridge

Lore & Background

The first discovered mid-ocean ridge was the Mid-Atlantic Ridge, which is a spreading center that bisects the North and South Atlantic basins. Most mid-ocean ridges of the world are connected and form a global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. The continuous mountain range is 65000 km long.

A Global Mountain Range Beneath the Waves

This dwarfs the Andes, the longest mountain chain on any continent, by several times. The ridges thread through every ocean basin on the planet, and their combined trace across the seafloor has been compared to the stitching seam of a baseball.

Although the name mid-ocean ridge originated with the Mid-Atlantic Ridge, the first such feature ever identified, which bisects both the North and South Atlantic basins, most spreading centers actually sit off-center within their host oceans. Nevertheless, the traditional label persists for all of them. These features are not isolated; they are stitched together by plate tectonic boundaries, forming a single interconnected global system known simply as the Ocean Ridge.

The Forge of New Oceanic Crust

At the heart of every mid-ocean ridge lies a continuous process of crustal creation driven by plate separation. As tectonic plates pull apart at a divergent boundary, the underlying mantle material rises to fill the growing gap. This isentropic upwelling causes the solid mantle rock to exceed its solidus temperature, triggering decompression melting without any external heat source. The resulting magma collects along the linear weakness between the separating plates and eventually erupts as lava onto the seafloor.

Upon cooling, this lava solidifies into basaltic rock known as mid-ocean ridge basalt, or MORB, a tholeiitic composition notably low in incompatible elements. Beneath this upper basaltic layer, slower-cooling magma crystallizes into gabbro, forming the lower portion of the oceanic crust. Together, these layers constitute the new oceanic lithosphere that steadily pushes outward from the ridge axis. The entire cycle of upwelling, melting, eruption, and solidification operates without pause, ensuring that the ocean floor is in a perpetual state of renewal at every spreading center on the planet.

Spreading Rates and the Shape of the Ridge

The speed at which an ocean basin widens profoundly shapes the topography of its mid-ocean ridge. Spreading rates span a broad range, from roughly 10 to 200 millimeters per year, and this rate governs whether a ridge develops a deep rift valley or a smooth, gentle crest. The Mid-Atlantic Ridge, spreading at about 25 mm/yr, exemplifies this steep, deeply incised profile.

The fastest recorded rate, over 200 mm/yr, occurred during the Miocene on the East Pacific Rise. At the other extreme, ultraslow ridges below 20 mm/yr, like the Gakkel Ridge in the Arctic and the Southwest Indian Ridge, form both magmatic and amagmatic segments. The depth of seafloor away from the axis follows a predictable pattern: it is proportional to the square root of the lithosphere's age, a relationship explained by the progressive cooling and densification of the plate under Pratt isostasy.

Volcanism, Heat, and the Youth of the Seafloor

Mid-ocean ridges exhibit active volcanism and seismicity. New magma steadily emerges onto the ocean floor and intrudes into the existing ocean crust at and near rifts along the ridge axes. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. Most crust in the ocean basins is less than 200 million years old, which is much younger than the 4.54 billion year age of Earth.

Hydrothermal vents fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers. A feature of the elevated ridges is their relatively high heat flow values, of about 1–10 μcal/cm2s, or roughly 0.04–0.4 W/m2. Scientists can reconstruct the history of these ridges by studying marine magnetic anomalies: as crystallized basalt extruded at a ridge axis cools below Curie points of appropriate iron-titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides. Because the field has reversed directions at known intervals throughout its history, the pattern of geomagnetic reversals in the ocean crust can be used as an indicator of age; given the crustal age and distance from the ridge axis, spreading rates can be calculated.

Reader's Guide

Spreading rates range from approximately 10–200 mm/yr. Slow-spreading ridges such as the Mid-Atlantic Ridge have large rift valleys and rugged terrain; fast-spreading ridges such as the East Pacific Rise lack rift valleys.

The spreading rate of the North Atlantic Ocean is ~25 mm/yr, while in the Pacific region it is 80–145 mm/yr. The highest known rate is over 200 mm/yr in the Miocene on the East Pacific Rise. Ridges that spread at rates <20 mm/yr are referred to as ultraslow spreading ridges.