The submarine basement of Bequia, is a hornblende andesite, and has been dated with a K/Ar age of 6.25 Ma. While lavas found on land have been dated at 3.6 to 5 Ma. The island has three main formations, that include basaltic lavas, tuffs, hyaloclastites and vertical dykes. The central portion of the island is composed of andesite and basaltic andesite breccias. The basalts found on Bequia can be divided into three main types. The oldest are highly porphyritic with large clinopyroxene and olivine phenocrysts. Next there are high alkali basalts. Lastly there are quartz-normative lavas. The last two are most likely related through fractional crystallization as they show a gradual change in chemistry. However, the most primitive does not seem related.