Question: Ever imagined standing inside a volcano? That is what it feels like here at Onepoto Basin. It is a fascinating maar or volcanic explosion crater.
Onepoto Basin is located on Auckland’s North Shore in New Zealand. It is part of the Auckland Volcanic Field. Do not confuse it with Onepoto Hill in South Auckland.
Onepoto Basin formed about 185,000 years ago. A series of eruptions created it. These eruptions occurred when the sea level was lower. Onepoto and the nearby Tank Farm were freshwater lakes back then. During the Last Glacial Maximum, the water levels rose. The Waitematā Harbour breached the tuff rings of the craters, forming tidal lagoons.
In the 1950s, people quarried parts of Onepoto’s tuff ring. They used the material to fill in the Northern Motorway approaches. Later, in 1975, the crater was reclaimed. This reclamation created sports fields. Today, Onepoto Domain offers sports fields, ponds, playgrounds, and walking paths.
Onepoto is one of the best examples of a specific volcanic eruption style. It is known as a wet explosive or phreatomagmatic eruption. This type of eruption produces base surges and ash eruptions. These create a large explosion crater or maar. A tuff ring surrounds the crater. Onepoto has a 600-meter-diameter explosion crater and a high tuff ring.
When Onepoto erupted, mature kauri forests covered this area. The volcanic ash buried and killed these trees. When the southwestern part of the tuff ring was removed, hollow molds of these trees were found. This happened in the late 1950s.
At the time of the eruption, Onepoto Stream was part of the Waitemata River’s Shoal Bay tributary. The Onepoto tuff ring initially dammed the stream. But the stream soon carved a new path around the southern side. After the eruption, the crater filled with freshwater. It became a lake, overflowing into Onepoto Stream.
In 2000 and 2001, geologists drilled cores near the crater’s center. They found 36 meters of marine mud. Below that, they found 25 meters of silt. This silt had accumulated on the floor of a freshwater lake. The base of the crater was 61 meters down. This sediment showed that the lake was not breached during the Last Interglacial.
About 8,100 years ago, the sea breached the overflow sill. The freshwater lake became a saltwater lagoon. Marine mud from Shoal Bay filled the lake with each tide. Eventually, mangroves and salt marsh colonized the mud.
Onepoto is the southernmost of three explosion craters. The other two are Tank Farm or Tuff Crater and Lake Pupuke. Magma rose along a North-South fault line to create these. They are the three oldest volcanoes in the Auckland Volcanic Field.
Researchers have found 80 tephra beds in Onepoto Basin. These beds date from about 9,500 to over 50,000 years ago. The tephra came from the Taupo Volcanic Centre, Okataina Volcanic Centre, Taranaki volcano, and Tongariro Volcanic Centre. Eight tephra layers are from the Auckland Volcanic Field.
The Onepoto tephra record provides some of the best age constraints for the AVF. It gives reliable data for hazard recurrence calculations. Auckland is frequently impacted by ash fall from many volcanoes. The Onepoto Basin gives us important insight into volcanic activity here. It also provides a continuous record of the last 200,000 years of climate. Today, Onepoto Domain is a reminder of the area’s fiery past. It is a place for recreation and reflection.