Here's why:

* Plate Tectonics: The Earth's crust is made up of large plates that move and interact. At oceanic ridges, these plates diverge (move apart).

* Magma Upwelling: As plates separate, magma from the Earth's mantle rises to fill the gap. This magma cools and solidifies, forming new oceanic crust.

* Ridge Formation: This continuous process of magma upwelling and cooling creates the undersea mountain ranges known as mid-ocean ridges.

* Seafloor Spreading: The process of new crust formation at mid-ocean ridges is called seafloor spreading. As new crust is added, older crust is pushed away from the ridge, creating the movement of tectonic plates.

So, the oceanic ridge system is not only where new crust is formed, but it is also the driving force behind the movement of Earth's tectonic plates.