Volcanic islands in the deep ocean are primarily composed of basalt.

Here's why:

* Oceanic Crust Formation: These islands form from volcanic eruptions at mid-ocean ridges, where new oceanic crust is created.

* Magma Composition: The magma that erupts at these locations is typically mafic (rich in magnesium and iron). This composition results in basalt, a dark-colored, fine-grained igneous rock.

* Cooling and Solidification: As the basalt lava flows out and cools, it solidifies into the characteristic igneous rock that forms the foundation of these islands.

While other igneous rock types might be present in smaller quantities, basalt is the dominant rock type found in deep ocean volcanic islands.