Here's a breakdown:

* Oceanic plates are the large slabs of Earth's crust that make up the ocean floor.

* Divergent plate boundaries are the areas where these plates move apart.

* Sea-floor spreading is the process where magma rises from the mantle (the layer beneath the crust) to fill the gap created by the moving plates. This magma cools and solidifies, creating new oceanic crust.

This process is responsible for the formation of mid-ocean ridges, underwater mountain ranges that are the most prominent feature of the ocean floor.