The Birth Of The Boulders
Around 2.8 million years ago a volcano at the site of Lake Omapere erupted. This eruption was so huge that a basalt lava flow was spread across a massive area, that extended to Horeke. In fact there may have been multiple eruptions that extended this far resulting in a deep basalt layer around 30m thick.
This basalt lava flow started to crack as the ground beneath it was eroded over time by the action of rain. The Wairere stream formed, widening the gap below the basalt crust, so more and more chunks of basalt broke off. The boulders started to move towards the valley floor. The underlying soil was clay and as this washed away from under the boulders they moved further down the valley, finally ending up at the valley floor in the river. It has taken the boulders 2.8 million years to get from the top of the hill to the positions they lie in today.
This is the name given to the type of erosions we see on our boulders. Fluting is sometimes also known as lapiez or solution pits and is caused by water running across the rock surface. It is most commonly seen on limestone, and it is very rare for basalt to erode this way. Basalt requires acidic water to cause fluting, and only here at Wairere Boulders, due to special set of circumstances, did the water become acidic enough for the fluting to occur.
2.8 million years ago the whole area would have been covered in thick native bush, with Kauri a prevalent species. The forest would also have been rich in mosses and epiphytes. Kauri trees promote podzolisation, which is the turning of silicate rock into soil rich in silica. It has been speculated that kauri is probably the best agent for podzolisation of any tree, and creates an “eggcup” of this type of soil near the tree. This soil is exceptionally acid and this promotes the erosion of basalt. It takes 5000 years on average to produce viable soil from rock, and then the process of fluting can begin. The presence of the epiphytes and mosses, which have high organic acid levels around their exposed roots, would have been another contributing factor in the development of the flutes. Rainwater washing through the canopy would have turned mildly acidic and then dripped down onto the boulders.
The large size of the boulders is another unusual factor which contributed to our unique rock formations . For the fluting to be so marked the boulders need to be of a very large size. These massive lumps of rock would have been slower to roll down the hill, and it is thought they would need to sit in one position for at least 1000 years for a flute to develop. The fluting in different directions on different surfaces of the rocks, shows how they rolled down the valley, exposing different surfaces to the canopy over time.
Below are a collection of papers written on the Wairere Boulders:
1.LAPIEZ AND SOLUTION PITS AT HOKIANGA, NEW ZEALAND
By J.A. Bartrum and A.P. Mason, Auckland University College
(Received for publication, 28th May, 1948)
2. Origin of the Wairere Boulders from the Head of Geology Department of Auckland University (Philippa Balck).
3. How Kauris erode basalt. Dr. Neil E.Whitehead (ex GNS) August, 2013.
4. Fluting, Lapiez and Solution pits at Wairere Boulders. News Letter Article, Institute of Geology and Nuclear Science