Three volcanic fields erupted between 2.7 and 0.5 million years ago, migrating northwards from Mount Pirongia to the Bombay Hills. These formed the Alexandra Volcanics, Ngatatura Volcanics and the South Auckland basalts respectively. The hot spot that created the Auckland volcanic field is considered to have caused these outpourings as well. Unlike typical hot spots such as the one underlying Hawaii, it does not seem to have stayed still, but instead is migrating northward at a faster pace than the surrounding Indo-Australian Plate. Its motion has been explained as the tip of a propagating crack produced by the twisting of the North Island's crust.