WHAT'S HERE?
A unique occurrence of a partly intrusive finger of lava flow beneath tuff (ash) and into soft sediment that it baked into small natural brick columns.
LEARN ABOUT IT
Earliest eruptions were wet and explosive, and built up a circular tuff ring of hardened wet ash. The south side of the tuff ring is still present although partly eroded and some of its layers can be seen in the high tide beach on the south west side of the island. In this tuff there are a number of blocks of Waitemata Sandstone that were ripped from the wall of the volcano's throat when it was erupting, and thrown out in explosive blasts. Some of these blocks have evidence of baking around their exterior in the heat of the erupting column.
The eruption style later switched to 'dry fountaining' producing a complex of scoria cones (some of which have been quarried away) and fluid basalt lava poured out in all directions from around the base of the cones forming an apron of cooled basalt lava. A finger of this lava on the south side pushed its way under the tuff ring and into the soft rhyolitic sediments beneath. It formed a 300 m long finger that can now be seen as an erosion resistant reef extending south across the Manukau Harbour mud flats. The southern part of this basalt finger may have broken out onto the surface as there is no evidence of baked sediment above. The northern part of the finger near the island has a carapace of up to 40 cm thick of baked sediment in baked columns. The intrusive lava finger pushed up the overlying sediment and tuff into a narrow anticline and basalt lava intruded along fractures along the crest of the anticline as a narrow basalt dike. An intrusive lava flow like this is unique in the Auckland volcanic field.
HAVE A LOOK!
See if you can find the upfolded tuff layers near the coast of the island and the intrusive dike of basalt that came up along the axis of the flow.
On the foreshore 200-300 m away to the east, walk on the hard eroded layers of tuff and see the impacts of the sandstone blocks where they landed in the soft wet ash.
Turn off Oruarangi Rd down Island Rd towards the sewage treatment station and then turn left across the causeway to Puketutu Island. Park in carpark on left at end of road.
The muddy sand flat is firmest just out beyond the mangroves and softest adjacent to the water course near the reef. Best footwear for the flats is water shoes, wet suit booties, old sandshoes or maybe gumboots. Do not venture out into the soft mud beyond the rocky reef - you may get stuck.
From the carpark take the gravel roadway to the southwest around the south side of the island. Do not take the walkway back across the causeway. Walk all the way along the gravel access road to its end (about 1 km), then take the trodden track through the coastal vegetation, just past the fence line, down to the muddy sand flats. Only accessible 2 hours either side of low tide. Walk west across the muddy sand flats on the seaward side of the mangroves all the way to the rocky reef that sticks out across the flats (about 1 km). To see the volcanic block impacts in the tuff venture up to the high tide beach in the middle of the bay where rock is exposed. Total time return 2 hours plus.