In the past, the study of the mantle has relied primarily on experimental and geophysical data. It has not been until very recently that interest has turned towards studying the actual rocks in the exhumed portions of the mantle. The study of the mantle is important because we can gain a better understanding of the rheology and deformation of the largest portion of the Earth, improve interpretation of seismic data, better grasp interactions between the Earth's layers and identify the processes and changes that occur as a mass of mantle is exhumed through the crust.
Needless to say these rocks are very unique. Their high iron content ensures that nothing grows on them except tussock grass and very hardy alpine plants. For this reason, bushline begins at 600m elevation (sorry I'm on metric now!) in the Red Hills and the contact between the ultramafic rocks and neighboring rock terrains is clearly marked by a meandering line where the trees abruptly stop.
The stark landscape is spectacular because of the lack of trees and recent glaciation. When the weather is clear, the ridges offer some of the most amazing panoramic views I've seen in this country- Tasman Bay to the north, several ranges in Kahurangi National Park far to the west, the Southern Alps to the south and the Kaikoura Ranges and Pacific Ocean to the east- but that's when the weather is clear. We had our fair share of clouds, fog, rain, wind and occasional hail as well.
To start things off we drove up a four-wheel drive track to the Red Hills hut on the edge of the massif. From here we were helicoptered into our first camp next to a beautiful alpine tarn (lake) in the shadow of a prominent 1600m peak. We did four days of fieldwork staged from this previously studied area before another helicopter picked us up transported us to our second camp, at a higher elevation closer to the headwaters. From Camp II we more or less covered the whole northern half of the Red Hills. When the forecast was particularly nasty, we retreated down to a hut at bushline to do some work based from there. For most of one day we were completely confined to the hut. That night fierce winds bombarded the hut and surrounding trees and I was thankful to be in a warm, dry hut rather than a drafty backpacking tent exposed to the elements 1000m higher where snow was forecasted. Note that the photos are biased towards nice weather...
Every night we set up a mountain radio which involved stringing out two 15m wire aerials. At 7:30PM the weather forecast would be transmitted across the radio from the base in Christchurch and then afterwards we would have a chance to radio in our current location and place a message or telephone call if we needed. It became something to look forward to as it was our only communication with the outside world and it was interesting to hear what the other mountain radio transmitters were doing. It never lost its novelty.
Fieldwork was, for the most part, very enjoyable. The sharp rocks tore my new boots up pretty efficiently. The rocks were comparable to alpine karst or a fractured obsidian dome, so face plants were to be avoided. When the weather was nice in the evenings (it did not get dark until after 9:30PM), I would bathe in an alpine tarn, which was surprisingly refreshing. We also found ourselves in a couple steep places- traversing knife-edge ridges and ambling down angle-of-repose scree slopes. Outcrop was exceptional for New Zealand and the rocks very interesting. Vast expanses of harzburgite and dunite with monomineralic dikes of clinopyroxene, orthopyroxene, spinel, and plagioclase; some in some very intriguing relationships I had never considered before. We also saw some neat serpentine shear zones and faults. I've seen enough olivine to last a lifetime.
In short: The mantle is very cool, but thank God for plate tectonics and crustal rocks.
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| Camp I near the tarn |
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| Mantle rocks |
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| Camp II |
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| Geologist working |
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| Geologist posing |
1 comment:
very impressive, nic :) great pictures!
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