Kohuora explosion crater

The Auckland Volcanic Field contains 53 identified volcanoes, all of which erupted within the last 200,000 years. The field is a monogenetic basalt field with the magma sourced from 60-90 km beneath the surface in the upper mantle. The volcanoes erupted in three styles - the first style is usually wet explosive (phreatomagmatic) and this produces base surges and ash eruptions that create a large explosion crater (maar) surrounded by a tuff ring. Kohuora is an example of a complex crater produced from many vents that erupted entirely in this style of eruption.

Kohuora is an L-shaped crater produced by wet explosive eruptions from at least four and possibly more vents as rising magma interacted with voluminous groundwater. As a result this is one of the few explosion crater volcanoes in Auckland that does not have a simple circular shape. The explosive eruptions threw out vast quantities of pulverised light-coloured pumiceous silt and Waitemata Sandstone that underlies most of the Manukau lowlands, and lesser amounts of darker volcanic ash. This built up an encircling rim of bedded tuff, 15–25 metres high, around the vents. Kohuora erupted about 34,000 years ago and its tuff ring and crater were mantled by dark scoriaceous ash blown from nearby Crater Hill when it erupted about 3000 years later. Probably the youngest of the explosion crater vents was in the southwest corner and occupies the low circular depression of Beaufort Reserve. Some of the ash thrown out from there forms the low land now underlying the houses in Beaufort Place.
Originally Kohuora’s complex crater was a lot deeper than today, but following its formation it became a lake which accumulated much sediment. By the time of human arrival the floor of Kohuora crater was a wetland swamp. A buried pipe beneath the natural overflow saddle drains the basin at the southwest corner from Beaufort Reserve.