Most of the seafloor is covered with basalt, a dark-colored, fine-grained volcanic rock. This basalt is formed from non-explosive volcanic eruptions along mid-ocean ridges, where tectonic plates pull apart and magma rises from the mantle.

Here's a breakdown:

* Mid-ocean ridges: These underwater mountain ranges are where new oceanic crust is created.

* Magma: Molten rock from the Earth's mantle.

* Non-explosive eruptions: Magma flows out slowly and spreads over the seafloor, forming sheets of basalt.

This process of seafloor spreading and basalt formation is a major component of plate tectonics.