Of the millions of pilgrims that visit the holy city of Medina, in Saudi Arabia, every year to pray in the Prophet’s Mosque, few people are aware that the city is build upon the basalt flows of a past volcano, with the now dormant volcano lying very close to the city. This volcanic field, known as Harrat Khaybar, contains some of the rarest examples of white volcanoes, so called because of their light colored rocks caused by the presence of a kind of alkali and silica-rich light blue-gray igneous rock called comendite.

An aerial view of the Jebel Bayda, a white volcano made by silica-rich lava (comendite) in Harrat Khaybar. Photo credit: Luigi Vigliotti