Saddle near the top of Dun Mountain showing the clear vegetation break between the Dun Mountain Ultramafics (left) and Livingstone Volcanics (basalt; right).

By Alex

Reading some of the blog posts so far, you may have the impression that this trip was all jandals and beer, well that is not strictly the case. With this contribution I will attempt to impart a bit size morsel of geology.

To scale Dun Mountain on any normal day is no mean feat (26 km return; ≥900 m vertical ascent) but on this day it was always going to be a challenge… Thank god for a thing called twilight in New Zealand. Being marooned in the panacea that is Onekaka, we were up at 6 am and set out to climb Takaka Hill for the final time and make the two hour journey to Nelson. At least the lack of breakfast meant there was no car sickness, and everyone enjoyed breakfasting at a lovely French Patisserie on Motueka’s main road. Umm pies before 8am two days running got to be good for you.

Some quality directions for navigation purposes.

We found the track head with minimum drama (only lost the other van once) and everyone had set out by 9:20ish so not to bad really (still at least 11 hours of light left). Mike and I went to drop the vans at the Dam to shorten the hike (however, there were no public access signs anywhere), grab some zzz’s, and explore the camp ground before final setting off at about 10:40. Apart from the first 30 mins which is pretty tough (kinda straight up) the track gentle winds its way up the hill following an old railway line. Luckily we caught the other just before the Coppermine Saddle (there was never actually a copper mine; it’s a misnomer: they were mining chromite) just in time for lunch and the final ascent to the summit.

The famous fresh green olivine for which an Austrian made this the type locality (of dunite) in the world.

So what where we actually looking at: Dun Mountain is part of the Dun Mountain Ophiolite Belt, a major lineament in New Zealand’s geology and a major contributor to the initial recognition of the 600 km of dextral slip on the Alpine Fault. Dun Mountain itself is a large coherent block dunite with the highly variable Putaki Melange (a kinda mixture of all parts of the ophiolite, sheared and rotated about each other). The rocks in this region represent a piece of former oceanic crust that has subsequently been emplaced on the continent, the entire section (sediments, basalts, gabbros, mantle rocks) is now tilted almost vertical and exposed allowing us to walk right through the sequence.

Some guy in the mist at the top of a mountain talking about rocks again.

Dunite (a rock containing >95% olivine) was first described here in 1859 by Austrian geologist Ferdinand von Hochstetter. The really amazing thing is how fresh the olivine is given 1) its normal stable at >1000C and 2) these rocks are now exposed to surface weathering.

Chromite segregations (black layers) within a typical piece of Dun Mountain dunite. Note: the characteristic Dun colour of the rocks.

Everyone made it to the top of the mountain and thankfully safely back off the rock. The day was topped off with some vodka raros and a healthy serve of Burger King (family dinner anyone?).

A tantalising hint at the view of Tasman Bay that could be seen from the summit.
Looking south from near Coppermine Saddle the vegetation devoid ultramafic rocks extend into the distance.
The successful crew. At the top of Dun Mountain happy in the knowledge it’s all downhill from here.