What process creates new oceanic crust as tectonic plates separate?

Seafloor spreading is the process that creates new oceanic crust as tectonic plates separate.

At a mid-ocean ridge, mantle material rises and melts because pressure decreases. Magma reaches the seafloor, cools and solidifies, adding basaltic crust to both sides of the ridge. Continuous spreading gradually moves older crust away from the ridge.

Harry H. Hess proposed the seafloor-spreading hypothesis in 1960, helping explain how continents could move even though the ocean floor itself had once seemed permanent. Evidence came from magnetic stripes on the seafloor: iron-bearing minerals record reversals of Earth’s magnetic field as lava cools. Matching stripes on opposite sides of a ridge show that crust formed successively.

Seafloor spreading does not make Earth steadily larger. At deep-ocean trenches, older oceanic crust is commonly destroyed by subduction, balancing much of the crust created at ridges.