Onepoto

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. Onepoto is one of the best examples in Auckland of a volcano produced entirely by this style of eruption.
Onepoto is a 600-m-diameter explosion crater and high surrounding tuff ring at Northcote that erupted about 185,000 years ago. When it erupted, sea level was lower than at present and mature kauri forest grew on this site. Hollow moulds of some of these trees that were killed and buried by volcanic ash were uncovered when the southwestern portion of the tuff ring (Tarahanga St area) was removed in the late 1950s to provide fill for the northern approaches to the harbour bridge.
At the time of eruption, Onepoto Stream was another arm of the Shoal Bay tributary of the Waitemata River. Initially the stream would have been dammed by the Onepoto tuff ring but soon it eroded a new course around the southern side of the blockage. After the eruption the crater filled with freshwater to become a lake that overflowed over the lowest part of the tuff ring in the south into the recently diverted Onepoto Stream.
Geologists from the University of Auckland had two cores drilled near the centre of the crater floor in 2000–2001. They passed through 36 metres of marine mud then 25 metres of laminated silt that accumulated on the floor of a freshwater lake, before bottoming in the base of the crater at 61 metres down. This sequence of crater-fill sediment showed that the lake was not breached by the high sea level (6 m higher than today) during the Last Interglacial about 120,000 years ago. With more erosion of the overflow sill, the sea was able to flow up the Onepoto Stream valley and enter the lake as sea level slowly rose after the end of the Last Ice Age. After the breaching 8,100 years ago, the freshwater lake became a saltwater lagoon for several thousand years as it rapidly filled with marine mud picked up from the Shoal Bay tidal mudflats and carried in suspension into the lake with every incoming tide. Once the mud filled up to mid tide level it was colonized by mangroves and fringing salt marsh.
Onepoto is the southernmost of a line of three explosion crater (including Tank Farm/Tuff Crater and Lake Pupuke) where the magma from depth appears to have risen up along a North-South fault line. These are the three oldest volcanoes in the Auckland Volcanic Field, all erupting between 180,000 and 190,000 years ago.

From the parking area inside the crater you can admire the size and height of the surrounding tuff ring or go for an explore on flat walkways around various parts of the crater floor. There are sports fields on part of the reclaimed land in the floor of the crater; two small lakes are popular for sailing model boats or watching eels. There is a children’s adventure playground and an excellent sealed bike track for young children to learn to cycle on. Come for a BBQ or picnic. There is also a short boardwalk and bush walk loop around the north-eastern part of the crater floor.