Here's why:

* Plate Tectonics: The Earth's outer layer is made up of tectonic plates that constantly move and interact.

* Divergent Boundaries: Mid-ocean ridges are divergent plate boundaries, where two plates move apart.

* Magma Upwelling: As plates separate, magma from the Earth's mantle rises to the surface.

* Solidification: The magma cools and solidifies, creating new oceanic crust.

This process is known as seafloor spreading, and it continuously adds new crust to the ocean floor.