Mid-ocean ridges are areas where new rock forms, increasing the size of the ridges. When rock material is buried deep within the Earth, pressure from the rock above causes the rock to be heated above the melting point. As the rock melts, it expands and forces the melted magma to rise. This magma escapes through cracks and vents. Often, the escaping magma cools quickly in the deep waters, adding to the size of mountain ridges.

There are two forces at work that account for the spreading of the mid-ocean ridges. However, it is not known which force is dominant. Ridge-push happens when the growing volcanic mass on the ridge pushes the plate away from the ridge and towards the subduction zone, usually at the opposite side of the plate. Slab-pull forces come into effect near the subduction zone, which is caused by the weight of one plate being pulled below another plate.