Formed mostly as the result of volcanic eruptions, Java is the 13th largest island in the world and the fifth largest in Indonesia by landmass. A chain of volcanic mountains forms an east–west spine along the island. Three main languages are spoken on the island: Javanese, Sundanese, and Madurese. Of these, Javanese is the dominant; it is the native language of about 60 million people in Indonesia, most of whom live on Java. Furthermore, most residents are bilingual, speaking Indonesian (the official language of Indonesia) as their first or second language. While the majority of the people of Java are Muslim, Java has a diverse mixture of religious beliefs, ethnicities, and cultures.