Earth's longest mountain range is hidden underwater and wraps the globe like a baseball seam
A continuous chain of undersea mountains snakes through every ocean for about 65,000 kilometres, several times the length of the Andes. Along its crest the seafloor is being born, as magma wells up between separating plates. How fast the ridges spread even shifts global sea level and the chemistry of seashells.
Mid-ocean ridges form where tectonic plates pull apart. As they separate, mantle rock rises, and the drop in pressure lets it melt. The magma erupts as lava and cools into fresh basalt crust, with gabbro beneath. The crest typically sits about 2,600 metres deep and rises roughly 2,000 metres above the deepest basin floor. Moving away from the axis, the rock cools, thickens and grows denser, so the seafloor sinks, its depth roughly proportional to the square root of its age.
The Mid-Atlantic Ridge was found first, which explains the name, though most spreading centres are not actually mid-ocean. Hints came from soundings taken during the British Challenger expedition in the nineteenth century, analysed by Matthew Fontaine Maury and Charles Wyville Thomson, and echo sounders confirmed the rise in the early twentieth century. Magnetic minerals in the new basalt lock in the direction of Earth's field as they cool; because the field has flipped at known times, the striped pattern of reversals dates the crust and reveals spreading speeds.
Those speeds vary from about 10 to 200 millimetres a year. The North Atlantic widens around 25 millimetres a year and has a rugged crest with rift valleys up to 20 kilometres wide, while the fast East Pacific Rise has no rift valley at all. Because old crust is recycled at subduction zones, most ocean floor is under 200 million years old, a fraction of Earth's 4.54 billion years. Slab pull, the weight of sinking plates, probably drives plates more than ridge push does.
Ridges shape the wider planet too. Faster spreading inflates the ridges, displacing water and raising sea level over millions of years; this partly explains why Cretaceous seas stood 100 to 170 metres higher than today. Hot vents at the ridges strip magnesium from seawater, and the resulting magnesium-to-calcium ratio determines whether marine organisms build their skeletons from calcite or aragonite.
Source: Mid-ocean ridge
