Stony Batter basalt karst boulders

The basalt boulders are all that remain of extensive lava flows fom the 8 Myr-old volcano. The solidified flows were cut by wide-spaced sets of cooling joints. Over hundreds of thousands of years weathering worked its way into the basalt along the joints and the resulting rotten rock was eroded away, leaving behind only the unweathered fresh core-stones between the joints - and these became the rounded boulders on the hillsides of today.
Until the arrival of humans the boulders would have been on the forest floor and most would have had vegetation growing all over them. Humus accumulated around the roots on the top of the boulders and over time this broke down and mixed with rain water to form weakly acidic humic acid. This weak acid sat in slight depressions on top of the rock and slowly dribbled down the sides of the boulders. Over tens to hundreds of thousands of years the humic acid very slowly dissolved the basalt surface creating basins on top and fluting down the sides of the boulders.
Limestones and marble (made of calcium carbonate) are well know to dissolve with acid water passing through and over it to form classic karst landforms and fluting. Until recently solution of basalt was not thought possible.

Can you see any boulders in the remaining patches of bush with vegetation growing on them?
Look for boulders that have well-developed fluting down their sides. Can you find any with shallow basins dissolved on top of them?
Do you think karst is still forming on the basalt boulders out in the open paddocks?
From the ridges around Stony Batter there are panoramic views over the Hauraki Gulf, Hunua Ranges and across to Coromandel Peninsula. How do you think the Hauraki Gulf and Firth of Thames was formed?