Prabighya - Geo Postcard

Braided Streams:
Braided stream was one of the most interesting features of a river system that I have learnt about during the trip. A braided stream is one where a channel divides into many small networks within the stream itself, in a pattern which to me appeared very much like its name suggests; braided. The characteristic feature of the braided river that we saw in Waimakariri river and Harunui river were that they were both in broad low land valleys adjacent to the mountain ranges. Although it is a river but the volume of water is very less and so the sediments are well exposed. This exposed sediment of the river bed is highly subjected to the effect of drying out due to the lack of enough water. This dry sediment provides an ample opportunity for the sediments to be easily affected by the wind action where the wind carries the dried out fine sediments and allows for loess deposition to occur in the wind blown direction. Loess is a blanket of un stratified , wind deposited fine grained sediment which is rich in clay minerals.
We also got the chance to see cloud of loess going above the braided river system of Waimakariri river during our drive.

Tower Karst Topography:
During our stop in Kura Tawhiti Conservation area which was about 920m from Castle Hill, we got to see some remarkable work done by nature on limestone which is a kind of sedimentary rock. Limestone is formed from layers of organic sediment deposited in deep oceans far from land. The resulting rocks can end up hundreds of metres above sea level during periods of mountain building. Limestone is composed of calcium and carbon which is soluble in the weak carbonic acids that are present in rain water which works on joints in soft rock, gradually enlarging them. Small differences is present in the rock structure and the solubility which leads to a wide range of pits and grooves and variation in structure that can be observed as each huge boulder is different from the other in terms of shape and structure. These sculptured landforms are the results.

Amanda Kern Postcards

Lady Knox Geyser The Lady Knox Geyser is named after Lady Constance Knox, the daughter of the 15th governor of New Zealand. It is located in the Wai-o-Taupo area of the Taupo Volcanic Field on New Zealand’s North Island. The geyser was discovered by convicts who were living and working in the region planting the largest man-made forest of pine trees, seen today on the hills beyond the geyser. The convicts were forced to live in rustic accommodations and took advantage of the hot springs for bathing and washing clothes. What the convicts discovered during their first bath, was that there was some sort of chemical reaction between the soap they used for washing and the water below ground which caused the geyser to explode. The convicts also realized that the smaller the hole through which the water could be released, the higher the geyser would spray. The cone formation seen today was originally a ring of rocks put in place by the convicts. Over time the cone has been coated with silicate rich water which solidifies in layers on a daily basis.

For the last 80 years, employees of Wai-o-Taupo Park have induced the geyser to explode every 24 hours at approximately 10:15am using the same reaction principle as the convicts, soap. Lady Knox Geyser is best thought of as a pipe beginning in a reservoir of water beneath the Earth’s surface and opening above ground as a geyser spout. The reservoir of water is heated from below by the Earth’s magma causing a build up in pressure. The heat is not able to convect and once the reservoir pressure exceeds the outside pressure, the geyser blows. It does not blow continually because the water in the spout is colder than the water in the reservoir. Adding soap breaks the surface tension and causes bubbles to rise up the spout. As the bubbles rise they bring with them water from the reservoir allowing the built up pressure to release in an explosion.

Lichenometry on Glacial MorrainesLichens are the ultimate pioneer species, meaning they are the first plant life to grow on recently exposed earth surfaces. Because of this, lichens are useful for determining the time of deposition for glacial morraines and therefore the time of glacial and inter-glacial periods. Glaciers deposit right and left lateral and end moraines as they retreat up valley. Moraines consist of a variety of material, including various sized boulders, which has been churned and piled up by the glacier during retreat. These boulders provide the perfect surface for lichen growth. By measuring and averaging the diameters of the original colonizing lichens across a morraine, an age can be extrapolated using a calibration curve. Difficulties arise when lichens begin to coalesce and determining the diameter of a single lichen becomes impossible. Also, lichen will often grow in a long, linear form and must be avoided in field or data can become skewed.

Sarah Holland- Postcard #2

There are many geological features present along the scenic Tongariro Crossing. The crossing passes through portions of both Tongariro and Ngauruhoe Mountain areas. The andesitic rocks that seem ubiquitous in this unearthly landscape show evidence of many lava flows over time. The volcanoes of the area are all stratovolcanoes. Eruptions are usually violent, and are followed by lava flow. The layering and jagged rocks in the area are both characteristic to lava flows. Cliff exposures include layers of very solid looking material sandwiched between layers of angular, unsorted substrate. The angular layer can be derived from two factors at play. Some of the material is colluvium, sediments (large and small) that gravity has pulled down the hill between eruptive events. Other parts of the angular layer is created during the cooling of lava flows. Due to exposure while traveling downhill, the outermost layer of lava congeals. As lava underneath continues to move downwards, it pulls the surface lava. This causes cracking, and piling of the recently hardened material. The valley that contains the first ascent of the Tongariro Crossing has a great deal of angular material which has been created by this process. The Tongariro Mountain area is still considered active.

Claire's other postcard

Shores of Lake Taupo

Lake Taupo, located in the center of the North Island, was created from the massive Taupo eruption, which released great quantities of ash and tephra into the air spreading over much of the northern North Island. Its shores consist of coarse volcanic sands which contain various minerals and volcanic rocks such as quartz, obsidian, and pumice. Pumice, a volcanic rock which traps gas as it cools, is very porous and light, enabling it to float on water. The shores of Lake Taupo have lines of pumice above the water where wave action has thrown them. Pumice deposits in banks up the shore from the lake show that the lake level has not always been at its current level, but has receeded from previous levels to its current state.

Sarah Holland- Postcard #1

Pictured here is a pumice terrace that marks the landscape next to Lake Taupo, on the North Island. Evidence of changes in water levels and other fluvial processes can be seen in the area’s many terraces which notch the topography. The pumice exhibited in the terraces was ejected over the landscape during an enormous explosion, possibly the Taupo explosion of 186 AD. The terraces have deposition layers of poorly-sorted and well-sorted material. The layers of well-sorted, finer sediment are volcanic ash. The ash can be deposited and redistributed for many years after an eruption and may also show evidence of stream or lake activity. For example, finer grained sediments such as volcanic ash are usually only deposited in low-intensity stream or wave environments unless they are of greater density than the larger substrate. In this case, since pumice is quite buoyant, the volcanic ash may have been deposited more easily. The poorly-sorted layers, where the pumice stones can be found, is evidence of not only an eruption, but also fluvial activity. The stones of pumice have been rounded, and in some areas, shingled. Both features are characteristic of the influence of streams on the landscape. For if they had been merely ejected and deposited, one would expect a more angular substrate. The youngest layer is eolian dust, or loess adding wind as another factor in the formation of this landscape. In summary, the pumice terraces have undergone processes of wind, water and time to appear as they do today.

Doug's Postcards (Otira Viaduct and Ngauruhoe)

Otira Viaduct
Upon first glance one may think this large bridge is spoiling the natural beauty of this spectacular, steep-sloped valley, but it undoubtedly demonstrates some of the difficulties faced while living in a geologically active area such as New Zealand. The Otira viaduct is a 440 metre stretch of road that connects two highpoints and spans both a river and a high frequency landslide path. The old road (located higher up the hill to the right) was constantly prone to rockfall, snow avalanches and landslides but was the only option until 1923 when the Otira train tunnel was completed as a way to accommodate heavy gold traffic. The road was also becoming increasingly unstable from the constant erosion of the base of the hill by the river at the bottom of the valley. This Otira Viaduct was the best solution that road engineers could come up with for this problem, but required a huge investment of time and money to build. The first foundations were poured in January 1998 and the first cars drove over the new viaduct in November 1999; costing New Zealanders a total of 25 million New Zealand dollars, the viaduct was deemed an engineering success! Many special features on this bridge were added due to the high geologic activity of this area. The combined high relief of this terrain with some of the highest uplift and erosion rates in the world caused engineers unimaginable grief while designing this viaduct. Besides the overall bend required of the bridge to traverse the topography, engineers also made this bridge forty percent more resistant to seismic activity than other comparable viaduct’s seismic maximums. They did so by incorporating 25 metre deep basal foundations for each pier, and adding triangular supports uphill of the two outer supports to help protect their integrity during landslide events (the middle pier didn’t receive one since it didn’t appear to be in a high frequency landslide zone). The difficulty of engineering design was also exasperated by the fact it is located within a national park, so the well-being of local wildlife such as the Kea (an endemic alpine parrot) was also placed at the forefront of importance when selecting bridge design methods. As part of the planning for this viaduct (which began in 1986) geologists obtained 14C dates of wood buried in landslide debris in the valley that suggests one large event occurred around 1900 years BP, that may correspond to the large eruption of Taupo during this time periof- though this has not yet been proven. One can only wonder how long this huge investment will last before being inevitably destroyed by the awesome powers of nature at work in this dynamic landscape.

Mount Ngauruhoe
(In Maori ‘Nga Uru Hoe’ means ‘throwing heated stones’)
Mount Ngauruhoe is a large (2,291 metres tall), young stratovolcano that first erupted about 2,500 years ago- making it the youngest vent in the Tongariro volcanic complex of the Central Plateau of New Zealand’s North Island. Although to many it appears to be a separate volcano, it is in fact a secondary cone of Mount Tongariro to the north. It erupted 45 times in the 20th century, making it one of the most active volcanoes worldwide in this period. It is New Zealand’s most active volcano having had over 70 eruptive events since 1839. Its most recent claim to fame comes not from its last eruption in February, 1975, but from its debut as Mount Doom in Peter Jackson’s The Lord of the Rings movie trilogy. The crater’s floor has steadily cooled since 1979, which may suggest that the main vent is becoming blocked (Snelling, 1998). Present in this photograph are many rocks of andesite (fine-grained equivalent of diorite) composition, which exhibit a ‘salt and pepper-like’ texture of dark mafic minerals mixed into a lighter felsic mineral matrix. Andesitic lavas are more viscous than basaltic due to slightly higher silica contents, but there is still evidence of lava flows here. Flows are evidenced by large angular boulders that have broken off after cooling on the surface of other mobile liquid lavas that carried these boulders down the slope. The best evidence of flows are further down the slopes out of the view of this photograph. Many of these rocks also appear to have small crystal sizes, and a moderate degree of vesicles which may mean the lava had little time to crystallize or allow volatiles to escape while underground. This moderate lava composition may result from the subduction of the (basaltic) Pacific plate under the Australian continental plate, reaching a critical depth, dewatering, and then melting these plates, sending the less dense lavas toward the surface where they eventually build up sufficient pressure and erupt. Also present in the foreground of this photo is some type of old inactive crater, which could represent a shift in active volcanism to the present crater around 2,500 years ago. The layering present on the far ridge of the old crater probably represent distinct volcanic events. Finally, in the old crater one can make out small alluvial fans beginning to form inside the old crater from the erosion caused by melting of large amounts of snow off Nguaruhoe each year.