Oceanic plates are primarily made of basalt. This is a dark-colored, fine-grained volcanic rock.

Here's why:

* Formation: Oceanic plates form at mid-ocean ridges where magma from the Earth's mantle erupts and cools, solidifying as basalt.

* Composition: Basalt is rich in magnesium, iron, and calcium, which makes it denser than the continental rocks (mostly granite).

* Thinness: Oceanic plates are much thinner than continental plates because of the basalt's denser nature.

While basalt is the primary rock type, other rocks like gabbro (a coarse-grained version of basalt) and peridotite (a rock found deeper in the mantle) also play a role in the composition of oceanic plates.