Hochstetter Pond collapsed lava cave

Lava flows from One Tree Hill Volcano flowed south to the Mangere Arm of the Manukau Valley system (when sea level was lower) about 67,000 years ago. Molten lava flowed along inside a partly cooled and solidified lava flow. As it flowed along, some of the solidified roof above the moving molten lava collapsed and was rafted away through the flow. The collapsed roof grew bigger and bigger into a 60 m diameter depression. Eventually most of the molten lava drained away leaving a hole. There is a smaller but deeper collapsed lava cave (Puka St Grotto) 100 m upstream in private land behind 5 Puka St.
Lava flows are full of cooling joints (cracks) and water would not normally pond in a collapsed lava cave. The lava flow here is mantled by volcanic ash, presumably erupted from nearby Mt Smart 20,000 years ago. This ash would also have accumulated in the collapsed lava caves and obviously sealed the 'Grotto St' one so that it became an open freshwater lake from that time on. Freshwater phytoplankton (tiny organisms) were abundant in the pond and their silica skeletons accumulated on the floor, forming a bright white sedimentary deposit called diatomite.
This diatomite filled up the pond to the level of the present swamp. In the 1940s and 1950s the pond was drained (see the concrete walls) and the shallower diatomite deposits were quarried and sold locally as an abrasive cleaner called Grotto Maid. The property was recently purchased for a reserve by Auckland Council and named Hochstetter Pond in recognition of the work on Auckland's volcanoes in 1860 by the father of New Zealand geology Ferdinand von Hochstetter.

Look both ways along Grotto St - can you make out one or both the sides of this lava flow?
Can you see some of the jointed basalt of the original lava flow in the sides of the collapsed cave?
Freshwater springs flow out of the basalt lava flows near the Onehunga foreshore downhill from here. How do you think that water gets there?
Can you see some of the concrete wall remains in the collapsed lava cave? What do you think they were used for?
There are a number of lava caves in One Tree Hill's lava flows - how were they formed?