Here's why:

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

* Divergent Boundaries: Mid-ocean ridges are where two tectonic plates move apart (diverge).

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

* New Crust Formation: This magma cools and solidifies, forming new oceanic crust.

So, the process of seafloor spreading at mid-ocean ridges is where new crust is continuously being created.