Te Hopua tuff ring

BY BRUCE HAYWARD (GEOLOGIST)
Accessibility: WHEELCHAIR ACCESS
Tuff exposure beside two storey sea scout club house.
One of the explosion craters and tuff rings of the Auckland Volcanic Field and Onehunga's only local volcano. See what the tuff ring is made of on the foreshore beside the Sea Scout Hall.
Blocks of One Tree Hill lava flow basalt in Te Hopua tuff, Sea Scout Hall shoreline.
Te Hopua volcano erupted through underlying Waitemata Sandstone capped by basalt lava flows from One Tree Hill. Blocks of both of these rocks can be seen in the foreshore where they landed in the tuff (layered ash) having been thrown out by the explosive eruptions that built the low tuff ring. After these wet eruptions stopped, the deep crater filled with freshwater to become a lake. Sediment accumulated slowly on the floor of the lake until the tuff ring was breached by rising sea level at the end of the Last Ice Age, about 8000 years ago. After that the crater became a tidal lagoon that quickly filled up to mid tide level with marine mud.
The breach widened and most of the tuff ring on the SW side was eroded away.
Geddes Basin, the European name, was reclaimed and transformed into land in the 1930s. Since then it has had various uses and now is bisected by the southwest motorway and the NE floor is playing fields in Gloucester Park.
Onehunga Wharf was constructed on the south side of the circular tuff ring.
Gloucester Park fields and the SW Motorway on the floor of Te Hopua crater. Low tuff ring beyond.
Can you trace the original circular shape of the volcano, the extent of its flat-floored crater and the crest of the low tuff ring?
Can you identify the blocks of black basalt from One Tree Hill lava flows that were thrown out by the explosive eruptions?
How far did the lava flows from One Tree Hill flow to get to here?
What is the large volcano just across the Mangere Arm of the Manukau Harbour?
Can you imagine what this area looked like 18,000 yrs ago at the height of the Last Ice Age when sea level was up to 130 m lower than today?
Directions/Advisory

Drive into Gloucester Park carpark at 68 Onehunga Mall for easy access into the NE part of Te Hopua crater floor and surrounding tuff ring slopes
Walk along the footpath of lower Onehunga Mall and Onehunga Harbour Rd along the approximate crest of the Te Hopua tuff ring.
View the tuff ring exposure when the tide is out from the footpath just south of Aotea Sea Scout Hall on Onehunga Harbour Rd (historic two storey building) or clamber around on the rocks below when the tide is out..

Beware of cars in Gloucester Park carpark. If you visit the foreshore tuff exposure by the Sea Scout Hall be careful getting down the sloping sea wall or boat ramp and walking around the muddy foreshore and slippery tuff exposures.

Google Directions

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Accessibility: WHEELCHAIR

Walk around Gloucester Park to appreciate the crater's flat floor and low surrounding tuff ring.
At mid tide and lower view from the footpath or clamber around on the foreshore to see the internal structure of the ash and blocks that were erupted from Te Hopua.

Features
Volcanic Landform
Geological Age
Age of eruption unknown - younger than One Tree Hill (67,000 yrs old) and older than 30,000 yrs ago.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
Links
Hayward, B.W. 2019. Volcanoes of Auckland: A field guide. Auckland University Press: p.162-165. https://aucklanduniversitypress.co.nz/volcanoes-of-auckland-a-field-guide/ See Hayward;B.W.;Murdoch;G.;Maitland;G.;2011. Volcanoes of Auckland: The Essential Guide. Auckland University Press. p.152-153.;