Volcanic activity in new zealand-a case study of ruapehu mountain
Volcanic activity in new zealand-a case study of ruapehu mountain
Ruapehu Mountain is a volcanic mountain located at the southernmost end of the series of volcanoes referred to as the North volcanoes spreading across New Zealand. The zone in which the mountain is located, which is highly volcanic is referred to as the Taupo Volcanic Zone (TVZ), which is a vast segment of the Earth's crust which has been significantly volcanic for the last over two million years. Geologists have suggested possible explanations as to the activity of the mountain, one common explanation being that the reduction in size of the middle axis of the TVZ have resulted to significant movements within the Earth's crust resulting to the eruptions. The mountain is approximately 9175ft (2797m) making it the tallest mountain in the North volcano region.. This case study is an attempt to evaluate the historical progress of major eruptions around the mountain and the consequential effects passed on to the surrounding human settlements and the environment. Every eruption in history is usually followed by a massive outflow of lava, debris of silt, sand and rock which flows to the surrounding areas under the influence of gravity. Such outflows, normally flowing at great speeds, are referred to as lahars and in the past have caused catastrophic damages and disruptions to the surrounding human environments.
Geological Composition of the Mountain
The large composition of the mountain is a rock, geologically referred to as andesite, regarded as a highly volcanic rock. Over a period of time, lava composed of the rock accumulates together with a composition of variety of other interblended debris and results to formation of high rising mini volcanoes referred to as strato volcanoes. According to Neal et al(2006, p1) these mini volcanoes become steep and usually collapse due to gravitational and other forces resulting to a great movement of rock debris and lahars onto the lowlands. The lahars normally form circular patches (rings) on the lowlands. The rings are comprised of variety of glacial debris, trapped water and massive composition of fragmented rocks and ash.
Pre Historic Volcanic Activity in the Mountain
Historical evidence is not precisely certain of the periods when eruptions first occurred at Ruapehu Mountain but the period has been intelligently assumed to be around two million years. The evidence available today is however certain of volcanic activities around 300 years ago (Neal, et al. 2006, p 1). The evidence is available in form of andesite debris located in the region of Wanganui district preserved on a marine bench. The debris, mostly pebbles are estimated to be 300years old. It is estimated that the debris was a result of eruptions in Ruapehu Mountain and were transported to their present position through continuous river flow along the adjacent Whanganui River. The debris consists of basic lava flow which accumulated and formed steep cones dated approximately 250,000 years ago. The period 200 to 30 years ago saw subsequent formations of cones, built up of a wide variety of lava composition suggesting an increase in the volcanic activities during this period. The voluminous lahars that resulted from these volcanic activities led to formation of a variety of physical features, notably two craters which appears to have been exploded out by lahar flow, which currently form the Ohakune lakes. The period 25,000 to 10,000 years ago also recorded significant volcanic eruptions followed by subsequent lahar flows. There is evidence of lava formations around 25,000 years ago that spread for approximately 4km2. The Rangataua lava which spread over 3km2 is dated around 18,000 years ago. Possibility of lava flow in the region during the Holocene period, less than 10,000 years, are also likely. Small scale eruptions which spread lahars especially to the North, both Eat and West appears to have occurred between 4,500 and 9,000 years ago (Neal, et al. 2006, p 1). It appears that a huge cone collapsed due to instability in the period 3,000 to 4,500 years ago resulting to a massive lahar flow that spread and destroyed approximately a forest covering 20km2. The lahar flow also spread to parts of the current Rangipo desert. A subsidence in extensive lahar flows seems to have occurred after this period, with the formation of the explosion crater lakes which appears to have reduced further movement of lahars.
Recent Volcanic Activity in Ruapehu Mountain
The historic period earliest record of a lahar flow was in 1861 along Whangaehu River. Then, it occurred an hour before dusk and according to observers who were at the river banks at the time; the lahar consisted of massive and extremely cold mixture of debris and logs. The lahar washed down bridges and canoes as it sped downwards at great speeds under the influence of gravity. There is a record of a major eruption in 1895 when significant lahars formed and flowed into the adjacent rivers. Up to 1945, occasional eruptions occurred and during this period, most of the crater lakes formed by earlier activities, emptied into the adjacent rivers without significant formation of lahars. The disappearing crater lakes eventually developed into a huge volcanic dome, which began filling with water around 1945 (Neal, et al. 2006, p 1). Subsequent smaller eruptions reshaped the dome and transformed it into a massive crater which filled with water up to eight times the initial level by the year1953. The water collected burst the crater walls in 1953 and released a torrential water swirl approximated to 1.65 million cubic meters that developed into a massively huge and high speed lahar that collected down way along Whangaehu River. The consequential massive down flow, entrenching rocks, debris and silt swept the river banks, wiping away everything on its way. In what was to become the worst rail accident in New Zealand, the lahar discharging over 800m3 per second hit, engulfed and swept a rail line connecting Auckland and Wellington, at Tangiwai, at the same time that the passenger train was passing. Unable to withstand the huge force, the bridge collapsed tearing with it six carriages carrying passengers, eventually killing 151 passengers. A significant ash eruption, under the influence of southerly winds, occurred in 17th June of 1996 where the ash was continually swirled for over seventy two hours and spread in a vast region within Rotorua and Taumarunui. There have been slight eruptions recorded in 2007, which formed as a result of the collapse of the crater lakes formed in 1996. The lahors caused were however insignificant. Scientists continually warn of the instability of the mountain and the GNS has put up state of the art facilities to monitor minute by minute activity of the mountain
Volcanic Hazards Posed by Ruapehu Mountain Volcanic activities
The extent of hazards is proportional to the type and magnitude of the eruption. When lava domes are extruded to the outside, there is normally a possibility of the lava eventually flowing as per the topography. The consequence of this flow is normally non-disruptive since it normally occurs at slow speed. In the event where the lava dome is trapped within a vent, there is normally a danger of extensive explosion due to build up of pressure. This can lead to pyroclastic flows where molten lava flows out of the vent massively in great speeds due to internally built up pressure (Miller & Savage, 2001, p 2231). The significant hazard would be as a result of a pyrocrastic flow where the hot lava, of up to 2000o melts everything and spews gases and can travel uphill due to its speed. Such flow is precipitated by entrapped magma which finds its way out through lava domes. Their formations are characteristically short timed making them dangerous to human settlement that would be caught unawares (Neal, et al. 2006, p 1). The temperatures of these flows are normally enough to cause rapid melting of materials such as glass. The spread of hot gases during flow is dangerous since these gases may cause instantaneous fires, burning of bodies, eyes damage and irritation of the lungs. Though not recorded in Ruapehu, occurrence of such a flow would have extensively devastating destruction especially around the region of Mangaturuturu and Whangaehu areas. Another hazard is the weaknesses of lava cones, normally caused by weaknesses in the interlayer connecting rocks (Neal, et al. 2006, p 1). Such weaknesses may cause a sudden avalanche of volcanic debris. An avalanche from a large cone would produce significant explosion, gas emission and flow of rocks especially along the Earth's topographical channels. A significant hazard is associated with lahars. A lahar may collect massive tones of debris and flow with significant speed (over 1800 m/s) in 1945. Lahars pick speed depending on the topography and amount of debris (Siebert & Simkin, 2002, p 45). They are potential to cause flooding since hey solidify on their way and eventually stops. The water flowing along will then accumulate resulting to flooding and consequential flow alongside the river banks and possibly encroaching into human settlement.
Recomendations to Avert Human and Environmental Crisis around Ruapehu
The hazards created by these eruptions can be attributed to their physical presence and the emissions in form of poisonous and hot fumes. The principal premeditative action to avert the physical destruction would be to avoid human settlement along topographic channels that can support lahars. The accumulation of gases and fumes may lead to extensive lung damage especially to personnel involved in evacuations (Neal, et al. 2006, p 1). The fumes also destroy vegetation and poisons water putting at risk animal and vegetation life supported by the water sources. This can be averted through well controlled scientific surveys to predict such occurrences in order to assist with evacuations. Efforts should be made to demarcate areas potentially dangerous, especially along channels and river valleys. The efforts by GNS to monitor the activity in the mountain are as positive step in preventing major catastrophes (Miller & Savage, 2001, p 2233).
Conclusion
Information from GNS indicates that Ruapehu Mountain is still a highly potential eruption site and this call for an improvement in link between all agencies that are crucial in case of an eruption. These agencies include GNS science, the emergency department and department of conservation in New Zealand.
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