What is the global system of underwater mountain chains where new oceanic crust forms?

A mid-ocean ridge is the global system of underwater mountain chains where new oceanic crust forms.

Mid-ocean ridges occur at divergent plate boundaries, where tectonic plates move away from one another. Magma rises from the mantle into the opening, cools, and solidifies as basaltic oceanic crust. This process is called seafloor spreading.

The global ridge system extends for about 65,000 kilometres and is the longest mountain range on Earth, although most of it lies beneath the oceans. The Mid-Atlantic Ridge is one of its best-known sections; it separates the North American and Eurasian plates in the north and the South American and African plates farther south.

A mid-ocean ridge is not the same as a deep-sea trench. Ridges create crust at spreading centers, whereas trenches commonly mark subduction zones where oceanic lithosphere descends into the mantle. Earthquakes along ridges are usually shallow and are associated with faulting and magma movement.