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Cruise Report for NBP 95-07

New Zealand Geology

After the cruise, I spent some time traveling, hiking, and camping in New Zealand. The country of New Zealand is composed of two large islands that lie near the southwestern edge of the Pacific Ocean, some distance east of Australia. The North Island and a portion of the South Island are part of the Indian-Australian Plate, while the remaining portion of the South Island is part of the Pacific plate. The two plates are sliding past each other along the Alpine Fault. These tectonic origins for the islands of New Zealand have given it a complicated geologic history and interesting morphology.

The South Island is mountainous with the Southern Alps running along the western coast of the island. There are many sheer cliffs of highly metamorphosed schists, tall peaks with permanent snow caps, deep fjords, glaciated valleys, numerous waterfalls and fast moving braided mountain streams. The tallest peak, Mount Cook, reaches 3,764 m. The North Island is less vertical with smooth sandy beaches, rolling hills, large lakes, pock marks of volcanic activity, hot springs, boiling mud, and steam vents. The highest point on the North Island is Ruapehu a member of a centrally located group of volcanoes that reaches 2,797 m.

During my travels in New Zealand I was able to observe dramatic geologic processes in action. On the South Island, the motion associated with active glaciation may be imperceptible but the periodic popping and cracking echoing in the valley and the melt-water rushing from underneath, give a strong sense of the dynamics involved. I was also witness to the role weather plays in contributing to geologic effects. A stalled low pressure system over the country produced heavy rains that extended over several days. The thinly sedimented steeply sloping mountainous regions quickly channeled the water into fast moving rivers and streams producing episodic flooding and substantial erosion. There were numerous landslides, some of which nearly blocked the narrow winding roads, while other roads were not only washed out but washed away completely. While driving around, I was able to observe a bridge ramp disappearing below my feet and river flood plains overflowing their banks. Lake Wakatipu had already put parts of Queenstown under water and was working its way through town.

On another evening, I set up my bedding about ten feet from the calm settling waves of Lake Te Anu. It was a relaxing and scenic spot as the sun went down over the Southern Alps and the southern hemisphere stars came into view. Through the night, the westerly winds off the Tasman Sea whipped their way through the fjords and churned up the lake. I was suddenly awakened to the sound of waves crashing within inches of me and covering me with spray. Needless to say, I picked up and moved to a protected knoll back from shore but I now understood why the lake had a beach of well weathered, rounded pebbles and almost no sand or silt.

Tongariro National Park on the North Island contains several of New Zealand's large volcanoes. The ancient and more recent volcanic colderas appear as gentle, sleeping giants. Although this area is not obviously active, attention to subtle signs can give one a hint that this is not the case. In some areas the air is thick with a sulfurous odor, some of the peaks periodically release puffs of steam and smoke, ash from the September '95 eruption of Ruapehu is clearly visible, and boiling mud, hot springs, and steam vents support this view.

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