Rapa Nui is entirely volcanic. It marks the highest point of a 2700 km long underwater mountain range called Sala y Gómez Ridge.
Basically the whole island is basaltic, which means among other things that the rocks appear to the naked eye to be of only one color. A large portion of these basaltic rocks are classified as scoria, which means that they are full of air bubbles. These bubbles were formed by gases in the magma as the volcanoes erupted.
Three main volcanoes make out a large part of the landmass and give the island its triangular shape; Rano Kau, Terevaka and Poike.