Basalt is a dark-colored, fine-grained igneous rock with a composition rich in magnesium and iron. Its color can range from dark gray to black.

Here's why basalt forms at mid-ocean ridges:

* Mid-ocean ridges are sites of divergent plate boundaries. This means that tectonic plates are moving apart, creating a gap.

* Magma rises from the mantle to fill this gap. The magma is less dense than the surrounding rock and therefore rises to the surface.

* As the magma cools and solidifies, it forms basalt. Basalt is a common type of volcanic rock that forms from the rapid cooling of magma.

So, in summary, the oceanic crust at mid-ocean ridges is primarily composed of dark-colored, fine-grained basalt that forms from the cooling of magma.