The Wairere boulders have moved down the hillsides from an eroding
Pliocene basalt lava flow of the Kerikeri volcanic group formerly known as
Horeke basalts. Erosion of the clay under the basalt plateau (cap) started to create a valley; the edges of the cap broke off and blocks moved towards the bottom of the valley, where they accumulated. They now fill a portion of the valley which is about 1.4 km long and up to 350 m wide. Many of the boulders have deep
solution basins and
fluting formed on their surfaces as they very slowly slid down the valley sides - a particularly good example of the relatively rare phenomenon of
karst formation on basalt (sometimes known as proto-karst). This phenomenon was documented by geologists as early as the 1920s-1940s in
Hawaii and New Zealand. Usually karst landforms are formed by solution of
calcareous rocks (e.g. limestone and marble) by mildly acidic percolating water. At Wairere, and elsewhere, basalt has been dissolved, probably over a much longer interval of time, by the production of weakly acidic
humic acid in the leaf litter that collects around the roots of plants that grow on the top of the boulders, usually beneath a forest canopy. On the top of the boulders this humic acid has etched out solution basins 20–50 cm across and of similar depth. Humic acid seeping down the sides of the boulders has, over thousands of years, dissolved deep, near-vertical flutes out of the hard basalt. In some places the fluting is no longer vertical as the boulders have rolled over or tilted since it was formed. Basalt karst occurs in a number of places in northern New Zealand with some of the best examples at Wairere Boulders, but also at Stoney Batter,
Waiheke Island; Ti Point,
Leigh;
Lake Manuwai,
Kerikeri; and Stoney Knowe, Helena Bay. Excellent examples of karst features developed on basalt boulders can be seen on
Norfolk Island, in the Tasman Sea. ==See also==