Rock types are made up of phonolite and ignimbrite. The mountainous centre of the island is often covered with clouds, which feed numerous springs, as was already observed by the 19th century traveller Armand d'Avezac.
Phonolite and ignimbrite compose rock types. The elevated region of the island is commonly shrouded in clouds that sustain many springs, a phenomenon observed by 19th-century explorer Armand d'Avezac.