Crater Hill is one of the volcanoes of the Auckland Volcanic Field

Auckland Volcanic Field

The Auckland volcanic field is a monogenetic volcanic field in the North Island of New Zealand. Basaltic in nature, it underlies much of the metropolitan area of Auckland....

. It consists of an explosion crater about 600m wide, partly filled with water.

Late in the eruption sequence lava welled up inside the explosion crater creating a lava lake. This lake began cooling on the surface and around the edge, creating a solid basalt crust. When the molten lava withdrew back down the volcano's throat, the crust surface collapsed, creating the island in the middle of the present lake, and some of the solid basalt was left around the inside walls of the crater marking the former level of the lava lake. Two lava caves – Selfs and Underground Press Lava Caves – exist beneath the remnant crust on the south side of the crater. Quarrying has removed the small scoria cone in the crater and some of the tuff ring on the northeast side. The Southwestern Motorway was cut through the northeast crater rim and a small explosion crater.