The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, about west of Ecuador. They are driven by the Galápagos hotspot, and are between approximately 4.2 million and 700,000 years of age. The largest island, Isabela, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española, the oldest island, and Fernandina, the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform. This platform creates a shallow water depth of at the base of the islands, which stretch over a diameter. Since Charles Darwin's visit to the islands in 1835 during the second voyage of HMS "Beagle", there have been over 60 recorded eruptions in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active.