Day 5: A Tale of Two Rivers

With a chilly, dew-hung start we woke this morning to see the familiar silhouette of the coastal mountains. We took a few minutes to shower, eat breakfast and pack our bags, opting to leave the tents up and get to doing geology right away. Our adventure began with a short drive to the base of the trail up to Franz Josef Glacier. Once there, we hiked for twenty minutes through the native coastal rainforest of the South Island. We got our first glimpse of what was to come in a large, metamorphic erratic overgrown with trees and moss.

Once we got out of the forest, we found ourselves standing on a vast glacial river plain. We were astonished at the wide span of the riverbed, although the current river only ran in a small carved channel with steep banks.

Between the two mountain walls, the river bed was a combination of extremely fine sediment, gray river water, and red-lichen-covered schists. 

As we walked toward the ice, we passed great metamorphic clasts of shiny-biotite- and green-chlorite-containing schist. Some of these were more angular in appearance than rounded, indicating they were probably scraped off by the glacier the last time it was at this level.

We also came across some very interesting fluvially-carved features, such as this tube-like scarp.

Note also the extreme dipping angle of the metamorphic rocks! This region has experienced massive tectonic forces

In the center of the valley was Franz Josef Glacier itself, a massive sheet of ice that last filled the valley 20, 000 years ago. We discussed the physics of the glacier: the fact that the ice is currently advancing by 1 m per year, even though the glacier's terminus is retreating; the way that Mie scattering causes the deep blue colouration of the ice; the way it moves due to a fluidized later at the base from hydrostatic pressure; and many other aspects. We also talked about how large-scale climate cycles (the Milankovitch cycles) are influenced by orbital motion of the Earth and have an impact on glacial maxima.

Though we were unable to get extremely close to the glacier, we were still able to easily see the impact it had on the geomorphology of the landscape.

The second stop of the day was the actual plate boundary between the Pacific and Indo-Australian plates. We hiked along yet another riverbed, but this time we were not in a tourist area (which was a nice change): Gaunt Creek.

Once we arrived at the fault, we discussed its formation and had several (educational) arguments about different possibilities for rocks and minerals. The green rock visible in the pictures below was supposed to mark a band of cataclastics, rocks that had been ground up by the strike-slip action of the fault and spit out as tiny, fine kaolin. Adjacent to those was a band of mylonite, rock that has been jumbled together and deformed, also under the immense pressure of the fault.

After hiking back for two hours to return to our cars, we drive another three hours to get to Westport, our next camp site. The beautiful drive consisted of the mountains on the right and the Tasman sea on the left. On the way we stopped for the best fish and chips of the trip so far, and eventually found ourselves at our campsite. 

Other highlights of the day included a possible Kiwi bird sighting this evening on the road, and visiting the limestone Pancake Rocks by moonlight!

 

These rocks consist of alternating layers of mud-poor and mud-rich limestone (known as "stylobedding"); the muddier layers erode away more easily, leaving the distinct "pancake" texture of the rocks.

Tomorrow we'll be doing more driving to get to Golden Bay. Hopefully it will be as warm as it was today (23 degrees)!  Thanks for reading! :)