The Mahaukura Bluffs are best observed from Ruapane, across the Mangakara Valley. They are spectacularly sheer and pale grey, about 70 m in height. The bluffs are cross cut by a series of imposing dikes at least 20 m thick which connect to domes of andesite on the ridgeline. You can stand on one of these domes at Wharauroa, a great lunch spot. I have come to believe, through my research, that Mahaukura is the eroded remnant of Pirongia’s earliest volcano. This is supported firstly by its intense degree of erosion, as well the oldest age dated rock in the Alexandra Volcanic Group at 2.49 million years old.
After spending a month in Mexico, I decided that when I returned I needed to find a way down to the bluffs and observe their stratigraphy close up. I was encouraged, because a local farmer told me there is an easy way to the base of the cliffs just south of the bluff.
Indeed, with careful GPS navigation and some bush bashing, I made it there after 4 hours. The area is far more overgrown than (evidently) it was in the 1970s when the farmer when there. That’s because the goat populations are significantly culled and the mountain is re-vegetating with native scrub.
When I reached a suitable viewing point, I quickly realised that the bluffs are composed of dozens of lava flows stacked upon one another. The lavas appear to be highly autobrecciated (rubbly) A’a’ flows with only thin layers of cohesive lava rock. Their compositions appear to be unanimously basaltic-andesitic with small phenocrysts of black clinopyroxene.
The lava sequence is cross cut by a spectacular array of andesitic dikes (vertical sheets of magma injected into the volcano). The dikes protrude from the bluff because they are far more erosion resistant than the lavas. This makes for incredible ‘wall’ like outcrops with well exposed joint patterns like the above photo shows.
I noted that the dikes project upwards to the small, randomly jointed domes of andesite on the ridgeline. It was a ‘smoking gun’ moment when I realised that all of the Mahaukura andesites found on previous trips were fed by the dikes I had long been aware of but never seen up close.
Where the dikes cross cut lava stratigraphy, there are small (<1 m) zones of breccia at the margins. These zones represent fragmentation caused when the dike cracked through the rocks. One can imagination that this was a violent process associated with the ‘last gasp’ of volcanism at this vent area. Both the andesite and older basaltic lava sequence were probably stored within a magma chamber at crustal levels below Pirongia.
The trip was symbolic because it is the last fieldwork I do before getting married in March (in eight days time). Perhaps accordingly, I also reached the very last page of my yellow field book on this day, and perhaps, closed a chapter of my young life.