They cover an area of 30 sqkm , and are the remnants of a caldera along with islets on the surrounding fringing reef, They are generally considered a separate island group from Tuamotu both because their culture and language (Mangarevan) are much more closely related to those of the Marquesas Islands, and because, while the Tuamotus comprise several chains of coral atolls, the Gambiers are of volcanic origin with central high islands.