Here's why:

* Plate Tectonics: The Earth's outer layer (the lithosphere) is broken into large pieces called tectonic plates. These plates are constantly moving, interacting with each other.

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

* Magma Upwelling: As plates pull away, magma (molten rock) from the Earth's mantle rises to the surface.

* New Crust Formation: The magma cools and solidifies, creating new oceanic crust. This process is called seafloor spreading.

In summary: New crust is formed at mid-ocean ridges as a result of the movement of tectonic plates and the upwelling of magma.