According to the theory of seafloor spreading, new rock is added to plates at mid-ocean ridges.

Here's why:

* Mid-ocean ridges are underwater mountain ranges that are the sites of volcanic activity.

* Magma from the Earth's mantle rises to the surface at these ridges, cools, and solidifies, creating new oceanic crust.

* This process pushes the existing crust away from the ridge, causing the seafloor to spread.

This constant creation and spreading of new crust is a key driver of plate tectonics and the movement of continents over millions of years.