Castigo sin crimen

3.3.1 Lahars

Lahars are the main volcanic hazard on Ruapehu in the early stages of an eruption (Johnston et al., 2000), and are the greatest volcanic threat to life and property at Whakapapa ski area (Leonard et al., 2004). Lahar is an Indonesian word meaning a slurry of water and volcanic debris that flows down the side of a volcano. At Ruapehu, lahars are specifically created by the mixing of ejected

rocks and lake water with snow (Nairn et al., 1996). Lahars may occur during an eruption or be a secondary occurrence as volcanic debris may be re-mobilised during heavy rainfall. Lahars may also occur by overflow of a crater lake, which may or may not be triggered by a volcanic event. Death is generally caused by severe crush injuries, drowning, or asphyxiation (Johnston and Houghton, 1995). Nearly all major eruptions in the historical record at Ruapehu have produced lahars, draining into valleys around the summit (Nairn et al., 1996). Ruapehu is unique

internationally as lahars on the mountain are produced by depositing water from a Crater Lake onto surrounding snow/ice (Otway et al., 1995).

Figure 3.3 1995 eruption lahars down Whakapapa ski area, viewed from above the Crater Lake (Photograph from GNS)

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Figure 3.4 Lahar at Mt. St. Helens, produced by melted snow from the March 21, 1982 eruption (Photograph by Tom Casadevall, March 21, 1982, USGS)

Figure 3.5 Damage done by a lahar at Mt. St. Helens (Photograph by Don Swanson, May 19, 1980, USGS)

Lahars at Ruapehu most commonly run down the Whangaehu river valley, but can also run down the Whakapapaiti and Whakapapanui valleys to affect the ski areas. Recent lahars on Ruapehu originated by two processes. The first is by collapse of the tephra barrier on the edge of Crater Lake to form a lahar which typically travels down the Whangaehu valley (see Section 2.3 for a more detailed explanation). The second process involves an explosive eruption which ejects contents of the Crater Lake onto surrounding snow/ice/rock, and the mixture will begin to flow downslope (Otway et al., 1995). In order to get lahars in the Whakapapaiti and Whakapapanui valleys, which will affect the ski area, a northerly headwind must prevail to direct the surge into these headwaters (Otway et al., 1995) or the eruption must be laterally-directed towards these drainage basins, as occurred in the 1995 eruption (Department of Conservation, 1996).

Background Information on Skiing at Ruapehu 25

3.3.2 Ballistic bombs

Another threat on Ruapehu is the ejection of rocks during an eruption from the vent. These rocks are known as ballistic bombs or blocks, depending on their angularity. They follow ballistic trajectories and can be highly damaging. Generally, they land within 2 kilometres of the vent from which they were ejected (Johnston and Houghton, 1995). Therefore, they can be hazardous to anyone or anything within 2 kilometres, but are of low risk to anyone outside this zone. This area of high hazard at Ruapehu is known as the Summit Hazard Zone, and can be easily managed during periods of volcanic activity by simply restricting access to the area (Sherburn and Bryan, 1999). The two upper chairlifts of Turoa ski area fall within this 2 kilometre Summit Hazard Zone. The zone also passes close to the top of the Far West T-bar on Whakapapa. However, in the event of a 100-year frequency eruption, this Summit Hazard Zone would increase to 3 kilometres (Houghton et al., 1987). There would be little chance of survival in this area. This 3 kilometre Summit Hazard Zone would extend further down into Whakapapa and Turoa ski areas, increasing the hazard for people on the ski area. This Summit Hazard zone continues to increase in distance from the crater as the frequency of event decreases (Houghton et al., 1987).

3.3.3 Ash fall

Ash fall is a common hazard occurring with eruptions from Ruapehu and can affect a large area, particularly downwind of the eruption. Ash from the 1995 eruption fell on Hastings (Johnston et al., 2000), which is approximately 120 kilometres from the vent. The impacts of ash falls on structures, people, and equipment will vary depending on the thickness of the ash layer. Ash fall can severely disrupt infrastructure in the effected areas, for instance by polluting water supplies, causing vehicle damage, reducing visibility, covering roads which effects vehicle traction, and disrupting electrical supplies (Johnston and Houghton, 1995). In a large eruption (frequency 100- year) it is likely that the zone of ash fall will extend for 100 kilometres downwind, particularly causing problems for areas close to the mountain (Houghton et al., 1987). Ash fall could adversely affect the surrounding area including: closing the ski areas, closing the Desert Road, severely disrupting the electricity supply, pose problems for power scheme intakes, tunnels and equipments, disrupt the NZ Army’s activities, and ruin the trout fishing in surrounding rivers (Neall et al., 1995). Ash fall on the snow at the ski area would disrupt skiing until subsequent snowfall occurs. Ash is not usually a direct cause of loss of life, but it can act as an irritant affecting the eyes and throat (Johnston and Houghton, 1995).

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3.3.4 Volcanic Gases

Volcanic gases may also be hazardous during an eruption episode. Volcanic gases are continuous products of volcanic activity, and eruptions may produce lethal quantities. Besides being dangerous to humans, volcanic gases also affect the regional and global environment. Commonly emitted volcanic gases include carbon dioxide (CO2), sulphur dioxide (SO2), hydrogen sulphide (H2S),

radon (Rn), hydrochloric acid (HCl), hydrofluoric acid (HF), and sulphuric acid (H2SO4)

(Williams-Jones and Rymer, 2000). Concentration of the gases in the air decreases as distance for the source increases, so threat is only posed to areas within a few kilometres of the eruption vent. Volcanic gases can affect respiration and eyes, and can also corrode metals (Johnston and Houghton, 1995). Volcanic gases can cause acid rain, which may damage equipment on the ski areas over time.

3.3.5 Avalanches

Avalanches can also be a hazard during an eruption, particularly to the ski areas. It is possible for a large explosive eruption to trigger wet slab avalanches from steep areas of the mountain, or

possibly elsewhere depending on the stability of the snow pack (Otway et al., 1995).

3.3.6 Pyroclastic flows

Another hazard from a volcanic eruption is the generation of pyroclastic flows. A pyroclastic flow is a “laterally transported, fluidized mass of hot dry rock fragments mixed with hot gases” which moves away from the volcano at very high speeds (Lipman, 2000). Anyone caught in a pyroclastic flow will likely die or be severely injured. Dome collapses during a magmatic eruption at Ruapehu are likely to cause pyroclastic avalanches and/or hot, dry pyroclastic surges (Houghton et al., 1987).

Historically, pyroclastic flows have not been seen on Ruapehu. However, new geological evidence suggests that pyroclastic flows have occurred on Ruapehu in the past, particularly during the Taurewa eruptive episode which consisted of closely space Plinian eruptions (10,500 years B.P.). Therefore there is potential for another pyroclastic flow to occur on Ruapehu, although it is likely that these events would be very infrequent, possibly on time scales of thousands of years

(Donoghue et al., 1999).

Background Information on Skiing at Ruapehu 27