The archipelago is purely volcanic in origin, and remains one of the most volatile such regions on the planet; the most recent eruption was La Cumbre on Fernandina in 2009. Unlike most of the world’s volcanic areas, the islands don’t lie on the borders of two tectonic plates, a fact that has puzzled scientists. The hot spot theory, in which a fixed area of extraordinary heat in the magma occasionally bubbles up to form a volcano, offers the most plausible explanation for this. The archipelago sits on the Nazca plate, which is moving eastwards and downwards to South America at a rate of 3.4cm a year: as the plate shifts, the volcano comes off the hot spot, becomes extinct and is eventually eroded by the elements and submerged beneath the sea. Meanwhile, new volcanoes appear over the hot spot. This would explain why the most easterly islands are the oldest and most weathered; San Cristóbal is thought to be between 2.3 and 6.3 million years old. In the west, Isabela and Fernandina are thought to have been created less than 700,000 years ago and are the most active and clearly volcanic islands. Two chains of extinct and eroded underwater mountains and volcanoes – the Cocos and Carnegie ridges, which extend for hundreds of kilometres northeast and east respectively of the Galápagos – are evidence the hot spot has been working for millions of years.