By VE Neal, BF Houghton, SJ Cronin, SL Donoghue, KA Hodgson, DM Johnston, JA Lecointre, AR Mitchel
Ruapehu volcano is the southernmost of the large active volcanoes of the North Island. Rising to 2797m (9175ft), Mt Ruapehu is the highest mountain in the North Island and the most recent of the North Island volcanoes to have erupted. Ruapehu is located at the southern end of the Taupo Volcanic Zone (TVZ), a spreading segment of the Earth’s crust and the source of spectacularly explosive eruptions over the last 2 million years. Subsidence in the central axis of the TVZ has led to prominent active faults developing to the east and west of Ruapehu volcano, which are downthrown towards the mountain. These faults mark the boundary of the TVZ in this region, which terminates 20 km south of Ruapehu’s summit.
Ruapehu is largely comprised of the volcanic rock andesite. Accumulations of andesite lava flows interbedded with fragmental rubble radiate from the summit region forming a stratovolcano that rises 2000m from the surrounding lowlands. As stratovolcanoes build up they become steep and have a propensity to collapse generating debris avalanches and lahars that spread outwards onto the surrounding lowlands. These lowlands form a roughly circular apron of fragmented rocks (volcaniclastics) termed the ring plain, mainly derived from debris avalanches and lahars, but also including some river (fluvial) and glacial deposits. Mantling the lowlands are various thicknesses of volcanic ash forming the parent material of most of the soils in the region. Due to the dominantly westerly wind direction, ash thickness for a given distance from the summit is always considerably greater to the east than to the west.
Dominating the summit area is a crater lake which, when full to overflowing contains 8-10 million m3 of acid waters. During historical eruptions (in 1945 and 1995) the lake water has been ejected out of the crater and onto surrounding glaciers, and/or propelled across the outlet and into the Whangaehu catchment. Displacement of lake water during these eruptions was followed by dome extrusion and/or dry ash eruptions. When water again accumulates in the summit crater, phreatomagmatic (or wet) eruptions occur with accompanying water expulsion creating lateral surges and lahars. Lahars generated in such a way are one of the devastating and hazardous of volcanic events. Throughout history, lahars have been responsible for much loss of life in the countries of the Pacific margin, including New Zealand.
HISTORY OF RUAPEHU VOLCANO The pre-historic record
The beginnings of volcanic activity at Ruapehu are shrouded in the mists of time but early evidence comes from andesite pebbles preserved on an uplifted marine bench in the Wanganui district. These pebbles which are about 300,000 years old are the remnants of an andesite lava flow which was emplaced following eruptions from an early volcano in the vicinity of Ruapehu. The pebbles were then transported by a proto-Whanganui River to their present position. The oldest lava sequences preserved in situ are exposed on the northern flank of Ruapehu and seem to date from this time. This material comprises massive and brecciated lava flows of basic andesite which formed a steep cone up to about 250,000 years ago.
Subsequently, the range of lava compositions widened as a new phase of cone-building began about 130,000 years ago; these lavas appear to have emanated from one principal vent to the northwest of Mitre Peak. During this period, voluminous lahars were also generated. These inundated the nearby hill country of uplifted marine siltstones, sandstones and limestones, creating an extensive ring plain, especially to the north and to the west of the Whakapapa catchment. The resultant deposits total more than 12 km3 volume.
A second cone-building phase occurred between 60,000 – 15,000 years ago during which time further lavas were extruded from several vents established between Tahurangi and the northern Summit Plateau. Over time, lava composition became more variable, from olivine basalt to dacite. Lahars were also generated during this time, leading to further deposition on the western and southwestern ring plain, especially around Raetihi. Smaller lahars also inundated the eastern ring plain at this time, and considerable thicknesses of volcanic ash also accumulated downwind of the volcano.
At about 30,000 years ago, basaltic magma erupted scoria and surge deposits from a satellite vent (Rochfort crater) on the northern outskirts of Ohakune. South of Ohakune, the magma also encountered groundwater which flashed to steam resulting in two additional explosion craters that form Ohakune Lakes.
Around 25,000 years ago basaltic andesite erupted from the southwest flank of Ruapehu, spreading lavas across 4km2. Later similar flows were generated from the southern flank to form the Rangataua lava flows around 18,000 years ago. During this period the climate was colder than at present and most river beds were choked with sediment (aggraded) as further lahars spread laterally both to the west and to the east. Their deposits total in excess of 3 km3 volume. During this time more explosive magmas were erupted showering the eastern ring plain with successive thick pumice layers. The last of these events occurred about 10,000 years ago when hot pumice flows descended the eastern and western flanks to 13km from source. These pumiceous deposits were actively resorted by rivers to the lower slopes of the ring plain and are included in a suite of lahar deposits. The climax of these events was a major structural collapse of the northwest flank that produced a debris avalanche that swept down the Whakapapa catchment to beyond State Highway 47, covering 23km2. The resultant deposit has distinctive mounds or hillocks on its upper surface through which the road passes to the Chateau Tongariro.
All later lava flows on Ruapehu are of Holocene (<10,000 years) age. They are principally distributed to the north-west and to the east, but also include flows from four satellite vents. Small-scale ash eruptions accompanied by lahars occurred between 9,500 and 4,500 years ago.
Sometime between 4,500 and 3,500 years ago, instability of the upper cone led to 34 million m3 debris avalanche of largely hydrothermally altered rock and clay that swept to the east across the Rangipo Desert. It covered 20 km2 and destroyed a widespread beech forest on the southeastern ring plain at the time. Lavas were then directed for a time to the east, until the amphitheatre formed by the collapse event became buttressed by the construction of the present eastern rim of the Crater Lake. A series of 19 widespread but thin tephra deposits have been locally preserved, since deposition of the Taupo Pumice around the volcano’s flanks about 1,800 years ago. It appears the modern crater lake came into existence since that time, resulting in a wide range of lahar sizes, some reaching estimated maximum discharge rates of up to 6,000 m3 per second, this over three times the maximum discharge of any Ruapehu lahar in historical time (since 1860 A.D.).
The historical record
In historical time the earliest report of eruptive activity at Ruapehu was in 1861 when a lahar was sighted in the Whangaehu River. Mr Henry Sergeant described the lahar in the vicinity of Wyley’s Bridge: "In the mid-summer of 1860-61…I was standing on the bank (of the Whangaehu River) …when I suddenly saw coming around a corner in the distance a huge wave of water and tumbling logs. They filled the whole trough of the stream…As it passed us it appeared to be covered with what we first thought to be pumice but the intense cold which soon made us shiver and turn blue caused us to discover that …[this] was no less than frozen snow. Mixed with this was a mass of logs and debris. Very soon a bridge passed us stuck in the roots of a giant tree and a few minutes later about a dozen canoes came down". Campion et al., (1988).
A major eruption was recorded in 1895 when further lahars were generated in the rivers draining the eastern flank. Smaller eruptions occurred intermittently until March 1945 when a volcanic dome was slowly emplaced within the crater. Crater Lake waters were gradually displaced into the Whangaehu River (no lahars were recorded) until explosive eruptions began tearing the new dome apart. Volcanic ash was spread from Wellington to the Bay of Plenty, with ash eruptions peaking in August to September. By December 1945 the activity subsided leading to a deep, vertically walled crater which slowly refilled with water. By 1953 the lake had filled to a level 8m above the pre-1945 level. Suddenly, on the evening of 24 December 1953 an ice and volcanic debris barrier built across the outlet collapsed into an ice cave above the Whangaehu River. About 1,650,000 m3 of water were rapidly released entraining boulders and sand across the Whangaehu fan to form a substantial lahar. About 42km downstream the lahar was reaching its peak discharge at Tangiwai (810m3 per second) when the Wellington-Auckland passenger train was crossing the rail bridge. The bridge’s piers were unable to sustain the force of the debris-laden water and the weight of the train, causing the engine to plunge into the far bank and taking 6 of the passenger carriages into the raging torrent. In what was to become New Zealand’s worst rail disaster, 151 lives were lost (Fig 1.). As a result a lahar detection gauge was built 15km upstream to warn of any future lahar events that had the potential to damage the bridge.
Displacement of Crater Lake waters on 26 April 1968, 22 June 1969 and 24 April 1975 created further hazardous lahars on the Whakapapa skifield and down the Whangaehu River, the latter event involving nearly 2 million m3 of the expelled lake water, travelling at almost 5m per second at Tangiwai. The 1969 and 1975 events caused extensive damage to skifield facilities and to alpine huts on Ruapehu.
Beginning in early 1995 small steam eruptions were forerunners of 2 moderate phreatomagmatic explosions on 18 and 21 September. A spectacular though small phreatomagmatic eruption spread ash, bombs and water across the skifields in the late afternoon of 23 September. Besides producing the third lahar in the Whangaehu River that week, it also created a lahar in the Whakapapa Skifield which split into two flow paths close to the western T-bar. About 30 hours later, regular eruptions climaxed in the early afternoon, with the first widespread lobe of thin tephra, waters from Crater Lake were being propelled across the outlet in waves to feed a major surging Whangaehu lahar. This lahar, lasted for at least 15 hours and peaked at Tangiwai with a discharge of 230 m3 per second.
By early October nearly all the Crater Lake water had been expelled; at the same time the intensity of eruptive activity had increased. On 11 October, the lake was finally removed at the beginning of a major ‘dry’ ash eruption which commenced during the night, spreading ash northwestwards to Gisborne. In conjunction with overnight rain, the ash turned to a slurry on nearby roads, closing the Desert Road until the ash was cleared. Three days later (14 October) a second major dry ash eruption was spread by strong northwesterly winds across Tikokino and Waipukurau. Activity then subsided and in early November 1995 a new lake began to form in the crater. Ruapehu’s third major ash eruption of this episode began in the early morning of 17 June 1996 when a strong southerly wind carried the ash in a narrow plume northwards across Rotorua. Over the next 3 days the wind velocity decreased and gradually swung to the southeast distributing ash across Taumarunui. Minor ash emissions occurred in July and August before lake refilling recommenced.
TYPES OF VOLCANIC HAZARDS AT RUAPEHU VOLCANO
LAVA DOMES AND FLOWS
Lava domes have been extruded at Ruapehu once and possibly twice in historical times (in 1945 and possibly 1861) but little remains of these today. Lava domes are bulbous masses of viscous lava, which are extruded slowly from a vent (Fig 2.). The radius of a lava dome is typically between a few hundred metres and 1-3 kilometres. Extrusion of a lava dome in the currently active summit vent poses few direct problems, all of which can be managed with use of an "exclusion zone" around the summit. The eruption is likely, however, to lead to more widespread hazards during explosive disruption of the dome e.g. ash fall and potentially pyroclastic flows. In contrast, dome extrusion from a flank vent onto the steep slopes of the volcano would be a more serious hazard, creating an eruption scenario similar to the 1990 eruption of Mt Unzen in Japan when collapse at the front of the lava sent rock, hot gases and ash plummeting down the volcano. The impact would be particularly severe if a northern flank vent were to be active as this could lead to an extended evacuation of Iwikau and possibly Whakapapa villages. Such a dome-forming eruption is likely to be prolonged (months to possibly years) and create an extended management problem for officials.
The extrusion of lava flows at Ruapehu has occurred from summit vents during the Holocene. Lavas are channelled down existing valleys; i.e. along paths defined by the pre-existing topography, at rates that seldom threaten human life. Hazard zones in which all structures would be destroyed are elongate and would extend typically several kilometres from the active vent. As with lava domes the position of the vent is critical in determining the hazard zonation for lava flows. Lava flows can often become unstable on steep slopes or on ice, generating other hazards such as lahars.
The distribution of lava flows at Ruapehu both prior to and since 10,000 years ago is shown in Fig. 3.
PYROCLASTIC FLOWS AND LATERAL BLASTS
Whilst pyroclastic flows appear to be uncommon at Ruapehu, occasionally they have been projected into the Mangaturuturu and Whangaehu catchments, reaching as far as the Desert Road (Fig 4). Pyroclastic flows are dense rapidly expanding clouds of hot gases which transport particles in fluidised masses down valleys and across surfaces of low gradient until the flows lose mobility by dissipation of the gases. If the speed, volume and momentum are sufficient, pyroclastic flows may travel uphill or across areas of irregular relief, but usually they tend to be channelled into river courses and into depressions. Pyroclastic flows travel at speeds of up to 200km/hour. Temperatures of up to 828oC have been measured in pumice flow deposits erupted from Mt St. Helens in October 1980.
The principal hazards of pyroclastic flows are the dangers due to their extreme heat and high speed. They can be highly destructive and unpredictable. Pyroclastic flows may form with little warning catching people unawares. They may burst out sideways if for example a lava dome has cooled and inhibited the passage of upwelling magma.
The hazard to human life includes a lethal pressure wave preceding the flow, complete burial, asphyxiation from poisonous gases or dust particles in the flow, and severe burning and irritation of the lungs, eyes and skin.
The heat of pyroclastic flow is often sufficient to instantaneously melt temperature-sensitive materials such as glass and plastic products e.g. car headlights and lampshades. If the entrained particles are fine-grained, well constructed structures may survive with only partial damage. However, if much coarse detritus is carried at the base of the flow considerable damage or burial may result, if the structures are not completely removed by the flow.
Danger from impact of rock fragments moving at high speed is likely to be greatest in valleys where the dense basal load is likely to be most destructive. The turbulent cloud of gases and smaller ash particles in the upper portions of pyroclastic flow can often spread laterally onto the surrounding landscape as a thin ash blanket which has been referred to as an "ash cloud". Pyroclastic flows travel at speeds that make escape almost impossible. The hazard is reduced only by evacuation of localities likely to be impacted before an event occurs. One of the associated effects of pyroclastic flows is the melting of snow and ice to produce lahars and associated floods that may extend far beyond the limits of pyroclastic flows. At Ruapehu this would be most likely to occur in winter-time when much of the cone above 1500 m is snow-covered. A further effect following pyroclastic flows would be the extreme erosion and flooding hazard created when heavy rain falls on hillslopes denuded of vegetation (often by prior eruptions), allowing rapid runoff.
The distribution of primary and reworked pyroclastic flow deposits at Ruapehu is shown in Fig. 5.
A lateral blast is a laterally directed eruption plume of gases and entrained particles that travels at very high speeds from a volcano. Estimated velocities of more than 300 km/hr for the 1980 Mt St. Helens blast testify to the powerful acceleration a lateral blast can reach across even a hilly terrain. As yet no lateral blast deposits have been identified at Ruapehu, but this does not mean that they will not occur. Lateral blasts are extremely unpredictable events. The hazard from any future lateral blasts at Ruapehu is grouped with the hazard from pyroclastic flows. The blasts generated during eruptions of Mount Pelee in 1902 testify to their rapidity and destructive nature. The town of St. Pierre with a population of 30,000 inhabitants was razed by blasts over a period of a few minutes.
VOLCANIC DEBRIS AVALANCHES
On some of the large andesite cones of the world, the interlayered strong and weak rock units lead to planes of weakness. Infrequently, failure may occur along these weaknesses triggering extremely large, high velocity gravitational landslides called volcanic debris avalanches. The classic modern example is the failure of the north flank of Mt St. Helens on 18 May 1980 when approximately 2.5km3 of the cone avalanched 27 km downstream.
Volcanic debris avalanches appear to be triggered most frequently by magma intrusion, but some are triggered by large magnitude tectonic earthquakes, or by exceptionally heavy rains saturating the volcanic pile. Also lava flows, pyroclastic flow deposits and tephras forming the upper slopes of stratovolcanoes like Ruapehu may often be deeply altered and thus rendered unstable by acid fluids percolating from vapour-rich underlying geothermal systems. In particular, brecciated pyroclastic material is quickly transformed by acid fluids into an unstable, saturated, clay-rich deposit, easily prone to mechanical failure that may initiate a volcanic debris avalanche. Such avalanches may spread unimpeded on the relatively flat terrain of ring plains but where they flow into deeply dissected landscapes they are strongly topographically controlled and become confined to river valleys. All physical structures in their paths will be destroyed or buried. Sometimes volcanic debris avalanches may block drainage systems, and thus form permanent or temporary lakes which may break out to form lahars. The principal mitigative measure is to plan to avoid building valuable structures in known or likely flow paths.
At Ruapehu, two volcanic debris avalanches have occurred in the last 10,000 years. The first travelled northwards down the Whakapapa Catchment about 9,900 years ago, the second down the Whangaehu Catchment about 4,500 years ago. In the Whakapapa, the avalanche deposited the distinctive mounds (Fig. 6) which extend from the Chateau Tongariro (near 1100 m altitude) northwards to State Highway 47 (at the 800 m contour).
LAHARS AND ASSOCIATED VOLCANIC FLOODS
A lahar is a rapidly flowing mixture of rock debris, sand, silt and water (other than normal streamflow) originating from a volcano. Lahars resemble wet concrete as they flow. They are readily influenced by relief, being guided along stream channels and into deep gorges or even along shallowly incised stream channels at low gradients. Close to source a lahar may be erosive and scour underlying soft materials on steep slopes, often adding loose material to the flow leading to a larger flow volume (bulking). The velocity of a lahar is dependent on the density of the flow, the gradient of the ground surface and the volume and depth of flow. If large enough volumes of material are incorporated on initiation of a lahar, and there is sufficient fall in elevation then it may travel hundreds of kilometres. Lahars generated during the 1877 eruption of Cotopaxi in Ecuador travelled more than 320 km along one valley. Some lahars are reported to have attained speeds of up to 180 km/hour on very steep slopes, similar to the upper section of Ruapehu. Speeds of between 10 and 90 km/hour were recorded for lahars during the 1995 eruptions at Ruapehu (Fig.7). As lahars descend valleys, sufficient momentum may be attained for a flow to climb up the river banks at bends in valleys and even flow up and over topographic barriers located directly in their path.
Lahars are created by a variety of mechanisms. Water is essential to lahar formation. Large volumes of water are often stored in crater lakes (e.g. Crater Lake, Ruapehu) and when the rim of a crater lake collapses or when eruptions occur through a lake, large volumes of water are released. If the crater lake is sited high on a volcano or at the summit, a fast moving lahar may be generated due to the fall in height. Pyroclastic flows have generated lahars as they mix with river or lake waters. Heavy rains on the flanks of a volcano may saturate loose materials which become unstable, fail and then flow as a lahar. These may be particularly common for months to years after large eruptions when a thick blanket of ash coupled with vegetation destruction may lead to widespread failure of the saturated products. Melting of snow and ice by a variety of processes including high heat flow from the ground or lava flows may create large volumes of water which may be temporarily stored in depressions or directly create lahars. Collapse of summit rocks that have been altered by hot acidic geothermal fluids and/or steam may also generate some large lahars. Steam explosions have also triggered collapses of sectors of volcanoes that have generated lahars.
The hazards from lahars are due principally to their unusual mobility. By tending to be channelled along lower ground, lahars are of greatest hazard in valleys or in depressions. If sufficiently large, lahars may overtop river banks and spread outwards onto the surrounding landscape. Lahars are dense and fast which enables them to carry objects of tremendous size and weight. Buildings and vehicles may be demolished, buried or carried along with the flow. People caught by lahars are unlikely to survive as they become sucked into the flow and drown. Some lateral erosion may occur leading to removal of embankments and channel side collapse. Although lahars do not generally travel as fast as pyroclastic flows they are often more fluid and may travel enormous distances, many times farther than the outer limits of pyroclastic flow distribution around Ruapehu. Lahars may be initiated in minutes giving little warning of their impending arrival. Deposition of volcanic sediment form lahars in river channels leads to a reduced channel capacity and in consequence a greater likelihood of more flooding. On the lower flanks of a volcano like Ruapehu it is also possible for lahars and floods to overflow into adjacent catchments and thus change from an expected flow path.
Lahars are not easily controlled or channelled, although many engineering schemes have attempted, with partial success, to reduce their destructive power.
The distribution of volcanic debris avalanches, lahars and associated floods at Ruapehu in historical times, and over the last 10,000 years and 20,000 years is shown in Fig. 8.
Tephra is a term that encompasses all the products of a volcanic eruption that are aerially ejected from a vent. Tephra is subdivided into three dominant particle sizes, ash (less than 2mm diameter), lapilli (2-64mm diameter) and blocks or bombs (greater than 64mm in diameter). Unlike the other forms of volcanic processes that travel close to the ground surface, tephra can be dispersed widely through the atmosphere. Tephra dispersal is principally governed by the height of an eruption column (which may be up to 40 km high) and the predominant wind directions and wind velocities. The finest fraction, ash, is widely distributed for up to hundreds of kilometres and at high altitudes from the volcano. In contrast blocks and bombs are usually found within a few kilometres from source. Lapilli fall from eruption columns up to tens of kilometres from the volcano.
The hazards presented by tephra may be considered as (1) the problem created by the physical presence of tephra and (2) the presence of potentially harmful substances adhering to tephra particles that create a poison or pollutant to water supplies and animal foodstuffs. The effects are very much dependent on particle size, tephra thickness and distribution. Near to the vent "ballistic" projectiles may be metre-sized and may travel at great velocity causing injury to persons and property by direct impact or burial. Asphyxiation may also result in areas of high ash deposition which may not be restricted close to the source. However, by far the greatest problem is ash in the home, at work, on highways, in water supplies, in the air, and where it mantles the ground from grazing pastures to airport runways that creates a major nuisance effect. Downwind from source, tephra particles become finer and, as they fall and become redistributed by the wind, are able to penetrate cracks and crevices.
Of immediate concern in a tephra eruption is the danger to public health and safety. Although inhalation of ash particles on infrequent occasions may be more of a discomfort than a health hazard, ash concentrations in the atmosphere exceeding a total suspended particle (TSP) concentration of 0.01g/m3 may endanger health. This can lead to breathing problems despite the natural filtering mechanisms operating in the nasal passages, because particles finer than 0.01 mm manage to penetrate to air exchange regions of the lungs. People involved in clearing the ash and scientists investigating the eruptions may be exposed to high dosage rates. If free silica particles are abundant in the ash, the danger of silicosis is also increased in those exposed to high TSP concentrations. Incidence of diseases such as "industrial" bronchitis, acute and chronic obstructive pulmonary disease (COPD) or emphysema and asthma are likely to increase in areas where more than 10mm of ash falls, and more especially when greater than 50mm of ash deposition is experienced. Susceptibility to these diseases depends on the condition of people being exposed to the ash, but clearly is greatest for heavy smokers and those with pre-existing respiratory diseases. The use of facemasks is the most effective way to reduce inhalation of particles less than 0.01 mm (by 98-99%) that would otherwise penetrate to the air exchange portion of the lungs. If protection devices are not available, dampened handkerchiefs are probably the next best screening material.
In areas of thick ash fall similar hazards will be experienced by domestic animals and agricultural livestock. Besides the direct effects of ash inhalation, there is also the secondary problem of ash covering vegetation and affecting the palatability of feed or even burying it completely. Thus livestock on farmland and natural fauna would face impairment of bodily functions and could experience respiratory diseases, grinding down of teeth and starvation. In the eruptions of 1995 some fluorine toxicity was also experienced with pregnant sheep grazing ash-coated pastures.
The consequences of ash deposition on a variety of high value horticultural crops is very much dependent on the susceptibility of the plant. Larger leafed plants such as potatoes, tomatoes, lettuce and cauliflowers are likely to be most sensitive.
Ash may accumulate on the soil surface and compact to form a water resistant crust that inhibits soil water infiltration and increases surface runoff with possible resultant erosion. Remobilised tephra may lead to river aggradation and blockage of water intakes for both domestic consumption and hydroelectricity generation.
Ash penetration of farm machinery can be a major problem. On farms with dairy shed equipment, a problem in any future eruption would be preventing ash particles entering milking equipment and clogging mechanical parts. Farm pumps would experience rapid wear on seals and bearings. The use of settling basins to reduce grit flowing through pumps would alleviate this problem. Covering valuable machinery is another alternative.
Heavy ash fall disrupts transport services and communications. Impaired visibility for drivers of motor vehicles can be a danger during eruptions. Volcanic ash is opaque in nature so visibility may be reduced to zero even with car headlights on. In addition to the attrition effects on aircraft, particularly aeroengines, decreased visibility in the atmosphere and traction difficulties may prohibit aircraft from landing on or taking off from runways.
Ash is likely to clog moist air and oil filters on all exposed equipment in ash covered areas.
Ash deposition may interrupt telephone communications, and disrupt radio and electrical services as ash particles penetrate contact breakers and induce short circuiting. Intense electrical activity in and near the ash cloud may produce frequent lightning strikes on buildings and equipment
The sheer weight of thick tephra deposits on house roofs can lead to collapse if the tephra is not removed. The danger of roof collapse is related to the angle of pitch of a roof, with flat-roofed houses being at greatest risk. As soon as dry tephra has been brushed or hosed from problem areas it immediately creates a secondary disposal problem around the building perimeter. If the tephra becomes wet it may form a slippery slurry and on redrying may harden like cement. This may lead to blockage of storm water systems. Ultimately the tephra will enter waterways. Increased sediment loadings will affect town water supplies downstream and by sheer volume block water intakes, reduce the storage volume of channels and increase the likelihood of flooding.
Contamination of water supplies can take a number of forms resulting from physical and chemical changes in water quality. The most common contamination problem results from the suspension of ash in water (turbidity).
The influence volcanic ash has on water quality is highly variable. Water-soluble materials clinging to glassy and crystalline ash particles may be potentially harmful for drinking purposes, if in sufficient concentration. These solutes may be strongly acid or alkaline. Under natural conditions these concentrations will quickly become diluted downstream.
Based on the previous eruptive history of Ruapehu, a tephra hazard-zonation map has bee compiled with expected tephra thicknesses for (1) a small magnitude eruption e.g. 1995 AD and (2) a large magnitude eruption, typical or larger eruptions that occurred at Ruapehu between 10,000 to 20,000 years ago (Fig.9).
VOLCANIC GASES AND ACID RAIN
Volcanic gases consist predominantly of steam (H2O), followed in abundance by carbon dioxide (CO2) and compounds of chlorine and sulphur. Minor amounts of carbon monoxide, fluorine and other compounds are also released. Concentrations of gases will dilute rapidly away from a volcano and pose little threat to people more than a few kilometres from the active vent.
Eruptions at Ruapehu volcano are accompanied by very high fluxes of acidic gas especially sulphur dioxide (SO2). This is because large quantities of these gases are extracted in Crater Lake from gas vents and fumaroles during non-eruption times and "stored" in the lake water until an eruption occurs. During the 1995-96 eruption the flux of SO2 reached values in excess of 15,000 tonnes/day, exceptionally high by world standards. Other significant gases emitted by the volcano include carbon dioxide, hydrogen chloride and hydrogen fluoride. The impacts of these gases on human health in distant communities are poorly studied. In contrast there is a clear problem on the volcano during eruptions and scientists working on Ruapehu during recent eruptions encountered dangerous concentrations of SO2 in 1995-96. A significant gas hazard is present in Crater Lake basin at most times.
During an eruption a portion of the ejected SO2 and hydrogen chloride and hydrogen fluoride dissolves in water droplets in the eruption plume to form aerosols (droplets and tiny particles) which rain out over the landscape with the ash. This mixture often creates acid rain and an atmosphere haze known as "VOG" (volcanic smog). These very acidic water droplets can irritate humans’ and animals’ eyes and impair respiration, cause much damage to crops, and corrode metals similarly to effects described for tephra fall in the previous section.
Explosive eruptions at Ruapehu include a range of hazards that define an approximately circular zone of extreme risk to facilities and human life that extends about 2km from the centre of Crater Lake. In historical eruptions these hazards have included:
- fall out of dense blocks up to 5 m in diameter
- inclined tephra-laden steam jets
- metre-thick fall out of scoria and ash
- laterally directed turbulent and strongly erosive pyroclastic flows
The associated eruptions often occur with few or no precursors. The sole management option to mitigate risk is implementing "red" closure zones of a radius to be decided in consultation with the scientific monitoring team. This option was successfully applied at Ruapehu during the 1995-96 episode.
STABILITY OF THE CRATER RIM
The 1995-96 eruptions produced significant changes to the crater area. Portions of the southeastern rim of the crater were eroded and deposition of new tephra has raised the elevation of the rim by 6-7m in the former outlet area. Studies suggest that the potential for collapse of the main crater wall is low but that the weak, poorly compacted and permeable new tephra barrier is strongly prone to erosion and/or collapse once the lake refills. The worst possible scenario in these circumstances is sudden collapse of the new tephra dam causing a lahar as large or bigger than the 1953 Tangiwai lahar.
VOLCANO MONITORING AND HAZARD MITIGATION
Ruapehu is the most intensively monitored volcano in New Zealand, reflecting its recent past of damaging eruptions. The principal surveillance tool is volcano seismicity. Ruapehu and neighbouring Tongariro and Ngauruhoe are monitored by a single network of 6 seismometers telemetered to Whakapapa Village and linked by fibre optic cable to the Wairakei Research Centre. Levels of seismicity are audited on a 24-hour basis using a pager system to prompt scientists to log into the seismic network. The system is designed to be triggered by either elevated levels of volcanic tremor or by discrete volcanic earthquakes.
The second monitoring tool used at Ruapehu is surveillance of Crater Lake. Crater Lake is fed by direct precipitation, runoff of melt water, and discharge of steam and gases from fumaroles on the floor and walls of the lake. The chemistry, temperature and volume of discharge of the lake reflects the balance between these three sources and also the extent to which the lake water is able to react chemically with hot andesite rock. Significant rises in temperature (30 to 60oC) and changes in its chemistry precede major eruptions at Ruapehu. While the lake is in a process of refilling after the 1996 eruption, Crater Lake monitoring is not a real-time or continuous process but relies on fine weather and safe access to the lake shore for spot sampling at a frequency which is monthly at best. Once the lake has stabilised at a new level it may be possible to resume remote monitoring of lake temperature and other parameters, via satellite telemetry.
Information on the alert status of Ruapehu and monitoring data is available at the web site http://www.geonet.org.nz.
Serious risks occur at Ruapehu during even moderate-sized eruptions, and a series of measures are in place to mitigate the impacts of Ruapehu volcanism. A warning system on the northwestern skifield is designed to detect the onset of lahars threatening the Whakapapa skifield. This eruption detection system combines seismic data from high on the cone with detection of acoustic waves associated with an explosive eruption, and triggers an automatic warning on the skifield. Airwaves are used to differentiate local seismic signals at Ruapehu from other regional earthquakes. A second scheme on the northwestern slopes is designed to permit closure of intakes to the Tongariro power scheme, before passage of a lahar down the Whakapapa catchment. To the east and south of Ruapehu, Tranzrail operates a warning scheme which will detect lahars in the Whangaehu River at a distance of 28 km from source and permit closure of the Tangiwai rail bridge a further 14 km downstream. The Minister of Conservation is currently considering options for hazard mitigation east of Crater Lake in the event of a significant wall collapse similar to the 1953 Tangiwai disaster.