The Kermadec–Tonga Trench runs between the North Island of New Zealand and the island of Tonga. It is a subduction zone, where the Pacific Plate is converging with the Australian Plate. At the northern end of the Kermadec-Tonga Trench, closure rates of 9 in (24 cm) per year have been measured—the fastest plate motion yet recorded. The older oceanic crust of the Pacific Plate is sinking below the more buoyant, young oceanic crust of the Australian Plate. The Tonga Ridge and the older Lau Ridge to the west formed as arcs of volcanoes above the subduction zone. The rapid speed of this subduction has caused extension of the overriding Australian Plate and the opening of a back-arc basin (an isolated basin behind a subduction zone) between the two ridges, in the Lau Basin. Together with Fiji and Samoa, the 36 inhabited islands of Tonga are the cradle of the Polynesian seafaring culture, which had stretched across the South Pacific by the 12th century.