Taranaki/Egmont Volcano Geology

Please cite as:
Neall, V.E.; Alloway, B.V. 1993 Volcanic hazards at Egmont volcano. 2nd ed. [Palmerston North, NZ]: Ministry of Civil Defence. Volcanic hazards information series 1. 31 p.

The western 1500 km2 of Taranaki is a volcanic landscape that has been constructed from the products of volcanic eruptions principally derived from Mt Taranaki/Egmont (hereafter referred to as Egmont Volcano). Egmont Volcano last erupted about 200 years ago at the culmination of eight eruptions in the preceding 300 years. Deposits around the base of the Volcano record intermittent volcanic activity at this site for the last 130,000 years. Whilst the eruptions have not occurred at regular intervals there has been a moderate or major sized eruption on average every 340 years with numerous smaller events at more frequent intervals. There is therefore no evidence to suggest Egmont Volcano has finally ceased erupting and has become extinct. Rather it must be regarded as an active volcano in a state of quiescence and is one of a number of volcanoes in New Zealand where future eruptions are to be expected.
Egmont Volcano is of the "slumbering" type that could begin renewed activity in the next 100 years.

This booklet is designed to acquaint you with the types of volcanic hazards that have been recognised at this volcano, their expected distribution and what care you can take in the event of a volcanic emergency. Simplified hazard maps are presented based on the known prior behaviour of the Volcano and the distribution and frequency of each volcanic hazard. Whilst we currently have no scientific evidence for an imminent eruption, you can be fully informed about what to do in the event of a future eruption.

FUTURE HAZARDS AT TARANAKI/EGMONT VOLCANO

Potential danger to life and property due to a further eruption of Egmont Volcano can be grouped into two categories.

1. Hazards from "ground-hugging" flows that principally comprise lava flows and lava domes, pyroclastic flows and lateral blasts, landslides, lahars (or volcanic mud and debris flows) and associated floods.

2. Hazards associated with the spread of materials through the air. This comprises principally particles of ash, stones and pumice which are collectively called tephra, and the danger from volcanic gases which may be toxic.

Taranaki detailed 1

The image above shows the Mt Egmont Volcano from the east showing the upper cone composed principally of lava flows and the lower flanks composed principally of pyroclastic flow, landslide and lahar deposits. - DSIR photograph

LAVA FLOWS AND LAVA DOMES

Lava flows are the hot streams of molten rock that flow down the flanks of a volcano. The distance a lava flow travels is mainly dependent on the viscosity (or stickiness) of the lava. This is governed by the chemical composition and the gas content of the lava. Lava mobility is also governed by the volume of material erupted, the angle of slope and the configuration of the landscape. If a small volume of viscous lava oozes to a gentle gradient surface or a depression, the lava may congeal as a hemispherical lava dome over its source vent. Lava flows are not generated in all volcanic eruptions; in fact the most explosive eruptions may comprise huge volumes of explosively fragmented rocks, none of which gently oozed like lava from a volcanic vent.

Lava flows at Egmont Volcano were moderately viscous flows that now form most of the upper cone of the Volcano. However they form only a minor proportion of the total volume of erupted materials. Small lava domes have formed on the flanks of Egmont Volcano to form the Beehives (to the south), Skinner Hill and The Dome (to the northwest) and the summit dome in the present crater.

Lava flows offer no great hazard to life or movable property because lava tends to move slowly downslope and can therefore be easily avoided. Lava cools as it flows downhill until it begins to solidify, whereupon it comes to rest. The speed with which the lava is most likely to flow and to solidify depends upon its viscosity and slope angle of the ground surface. On the upper slopes of Egmont Volcano lava could travel up to 5 km/hour but would most probably move over large distances at less than 1 km/hour. High on the steep upper flanks of the cone it would be possible for flows to break off and fall, due to gravity, into deep gorges where broken pieces would accumulate. During a lava extrusion episode the heads of catchments would therefore be highly hazardous areas to frequent. If lava were to be extruded, the paths of flowage could be predicted very quickly after a flow is initiated, provided good visibility allowed immediate recognition of the hazard. Non-movable property is difficult to protect from advancing lava flows, and there is little one can do to control or prevent a lava flow’s progress, as it overwhelms most structures in its path. However lava flows from Egmont Volcano in the past 10,000 years have been of restricted distribution to within a 7 km radius of the summit. This is entirely within the confines of Egmont National Park, so there is only a minor risk to property.

The hazard created by formation of small lava domes at Egmont Volcano appears minimal, unless property or persons were located immediately adjacent.

PYROCLASTIC FLOWS AND LATERAL BLASTS

Originally termed nuée ardente or glowing ash cloud, a number of different types of hot gas-charged eruptive clouds are here collectively referred to as pyroclastic flows. The term pyroclastic flow is used to denote all flows where rapidly expanding hot gases transport particles in a fluidised mass down v alley and across surfaces of low gradient until the flow loses mobility by dissipation of the constituent gases. Such flows are dry and extremely mobile fluids. If the seed, volume and momentum are sufficient, pyroclastic flows may flow uphill or across areas of irregular relief, but usually then tend to flow along river courses and into depressions where they fan outwards. On Egmont Volcano pyroclastic flows have been directed along catchments to the northwest, northeast and east where they have extended up to 15 km from source.

Pyroclastic flows travel at speeds of up to 200 km/hour. They are usually extremely hot; temperatures up to 715oC have been measured in pumice flow deposits erupted from Mt St. Helens in October 1980. Some pyroclastic flows may reach 1000oC.

A lateral blast is a laterally directed explosive cloud of gases and entrained particles that travels at very high speeds from a volcano. Estimated velocities of more than 300 km/hour for the 1980 Mt St. Helens directed blast testify to the powerful acceleration a lateral blast can reach across a near horizontal trajectory and a dissected landscape. No lateral blast deposits have been identified at Egmont Volcano, but this does not mean that they have never happened. Nor does it mean they will not occur in the future. Lateral blasts are extremely unpredictable events. The hazard from any future lateral blasts at Egmont Volcano is grouped with the hazard from pyroclastic flows.

The principal hazards of pyroclastic flows are the dangers due to their extreme heat and rapid velocity. With little warning gas-charged clouds of particles may be erupted from vents and may be strongly laterally directed if a lava dome has cooled and inhibited the passage of upwelling magma. Once a cloud of expanding gases and entrained particles is released on a downward passage away from a vent, the mass becomes extremely mobile and rapidly expands outwards and upwards. Resulting damage is largely the result of searing heat that lasts momentarily, often for only two or three minutes. In that time, the heat is sufficient to kill most life forms, and few people survive; known exceptions being persons living underground or those who dived into water. The hazard to human life ranges from complete burial, to asphyxiation from poisonous gases or dust particles in the flow, to severe burning and irritation of the lungs, eyes and skin. Pressure waves also precede some pyroclastic flows killing persons in their path. The pyroclastic flows generated during eruptions or Mount Pelée in 1902 testify to their rapidity and destructive nature. The town of St. Pierre with a population of 30,000 inhabitants was razed over a period of a few minutes. Pyroclastic flows travel at speeds that make escape almost impossible. The hazard is reduced by evacuation of localities likely to be impacted before an event occurs.

The heat of a pyroclastic flow is often sufficient to instantaneously melt temperature sensitive materials such as plastic products, e.g. lampshades and car headlights. If the entrained particles are fine-grained, well constructed buildings 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 to bridges and concrete buildings, if they are not completely removed by the flow. Danger from impact of rock fragments moving at high speed is likely to be greatest in river channels where the dense basal load is likely to be most destructive. A cloud of gases and smaller ash particles often spread laterally onto the surrounding landscape and blanket surfaces irrespective of the relief.

One of the associated effects of pyroclastic flows is the melting of snow and ice to produce water that forms lahars and associated floods that may extend far beyond the limits of pyroclastic flows. At Egmont Volcano this would be most likely to occur in wintertime when much of the cone above 1500m is snow-covered. A further effect following pyroclastic flows would be the extreme erosion hazard created from rain storms activating denuded slopes to be rapidly eroded and leading to much flooding.

LANDSLIDES, LAHARS AND ASSOCIATED FLOODS

One of the characteristic features in the history of Egmont Volcano has been the irregular occurrence of enormous landslides. On three occasions, twice within a very short period of geological time, former Egmont cones have collapsed to the northeast, southeast and west. In each instance extremely large volumes of material greater than 3.5 km3 flowed across the landscape to travel over 40 km distance, reaching the present Taranaki coastline. These rapid landslide events are technically referred to as volcanic debris avalanches. They have created the distinctive mounds or hummocks on the lowlands surrounding the Volcano. Near to source these landslides envelope everything in their path, even flowing uphill if there is a barrier in the relief. However beyond distances of about 20 km the landslide debris is guided by any pre-existing dissected relief and flows along channels or valleys.

The hazard from landslides is very similar to that presented by lahars. In this booklet the landslide hazard is grouped with potential hazards from lahars.

A lahar is a rapidly flowing mixture of rock debris and water (other than normal streamflow) originating from a volcano. Lahars behave in a fashion different to normal streamflow, resembling the behaviour of wet concrete as they flow. Lahars have low viscosities and are thus 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 incorporating loose material within the flow. The velocity of a lahar is dependent on the density of the flow, the gradient of the ground surface and the volume. If large enough volumes of material are incorporated on initiation of a lahar, and there is sufficient vertical drop, then it may travel great distances. Mudflows generated during the 1877 eruption of Cotopaxi in Ecuador travelled more than 320 km along one valley. Some mudflows are reported to have attained speeds of up to 180 km/hour on very steep slopes similar to the upper section of Egmont Volcano, and speeds of 20-40 km/hour can be reasonably expected on lower gradients. As lahars descent valleys, sufficient momentum may be attained for a flow to climb the walls at bends in valleys and even flow uphill over topographic barriers directly in their path.

Lahars are created by a variety of mechanisms. Generally water is ubiquitous with lahar formation and the sources of this water are many and varied. Large volumes of water are often stored in crater lakes and when portions of a crater-lake rim collapse or when eruptions occur through the lake, much water is released. If the crater lake is sited high on the flanks of a volcano or at the summit, a fast moving lahar may be generated due to the large vertical drop. Pyroclastic flows are also recorded as having generated lahars by becoming admixed with river or lake waters. Heavy rains on the flanks of a volcano may lead to saturation of unconsolidated materials which become unstable and flow as a lahar. These may be particularly common after eruptions when a thick blanket of ash coupled with vegetation destruction may lead to widespread remobilisation of the erupted 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 due to hydrothermal alteration of summit rocks has also been invoked as a mechanism for some large lahars. Steam explosions have also triggered collapses of sectors of volcanoes that have generated lahars.

Taranaki detailed 2

Lava flows comprising the upper cone of Egmont Volcano. View is from the north, showing Minarapa Stream lava flow (centre). The Turtle (upper right), Shark’s Tooth (upper left) forming the eastern margin of the crater, and the summit dome (upper centre), forming the highest point at 2518m - DSIR photograph.

If large volumes of water are released from volcanic terrain with little entrained detritus or if lahars deposit their load as it comes to rest, then a lahar may grade into a dominantly turbulent flow of water. Such floods are commonly associated with lahars at large distances from source. If volcanic vents are buried by ice and melting at the base of the ice is continual, large volumes of water may be stored that finally overcome the weight of ice above and flow from beneath the ice.

The hazards from lahars are due principally to their unusually mobile nature. By tending to be channelled along lower ground, lahars are of greatest hazard in valleys or in depressions. If sufficiently large, lahars may overtop embankments and spread outwards onto the surrounding landscape. Lahar flows are capable of burying or removing all persons and structures in their path. This is due to their high specific gravity (relative to floodwaters) and speed which enables them to carry objects of tremendous size and weight in a buoyant fashion. Buildings and vehicles may be smashed, buried or carried away and bridges are often lifted off their foundations to be carried along with the flow. 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 further than the outer limits of pyroclastic flow distribution around Egmont Volcano. Lahars may be initiated over very short periods of time giving little warning of their impending presence. As a lahar grades downstream into a fine mudflow or flood, large quantities of silt may be deposited, or remobilised in subsequent rains, creating substantial floods. Deposition of volcanic sediment in the river channels leads to reduction in channel capacity which leads to a further likelihood of more flooding. On the lower flanks of a volcano like Egmont 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 destruction of life and property.

TEPHRA

Tephra is a term that encompasses all the products of a volcanic eruption that are aerially ejected from a vent. 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 2 mm diameter), lapilli (2-64 mm diameter) and blocks (greater than 64 mm diameter). Unlike the other forms of volcanic hazard that travel close to the ground surface, tephra can be distributed widely through the atmosphere to all altitudes. Principally governed by the height of an eruption (which may be up to 30 km high) and the predominant wind directions and wind velocities, tephra may be widely dispersed to different directions at varying altitudes.

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 feedstuffs. The effects are very much dependent on particle size, tephra thickness and distribution. If both molten and solid particles are eruptive, it is the molten particles that will tend to be restricted to near the vent. Here tephra particles may be very coarse 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 tephra deposition which may not be restricted to just close to source. However, by far the greatest problem is the physical occurrence of ash in the home, at work, on highways, in water supplies, in the air, and 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 every crevice, crack and cranny. Hence problems experienced from volcanic ash eruptions are as varied as the people and places where ash has fallen.

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 0.01 g/m3 may endanger health. This can lead to a breathing problem despite the natural filtering mechanisms in the nasal passages, because particles finer than 0.01 mm manage to penetrate to air exchange portions of the lungs. Those people involved in clearing the ash and scientists investigating the eruptions may be exposed to high dosage rates. If free silica particles are abundant within the ash, the danger of silicosis is also increased in those exposed to high TSP loads. 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 1 cm of ash falls, and more especially when greater than 5 cm 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 personal respiratory protective devices 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 heavily ash prone areas, similar hazards will be experienced by domestic and agricultural livestock that rely on foodstuffs, such as grass, hay and lucerne exposed to ash deposition. Besides the direct effects of ash inhalation, there is also the secondary problem of deep ash affecting the palatability of feed or even burying it completely. Thus livestock on farmland and natural fauna would face an impairment of their bodily functions in a future tephra eruption and could experience respiratory diseases, grinding down of teeth and starvation.

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 and lettuce are likely to be most sensitive.

Ash may accumulate on the soil surface and compact to form a water-resistant crust that inhabits soil water infiltration and increases surface runoff with possible resultant erosion.

Ash in farm machinery can be a major problem. In Taranaki where the large bulk of the rural community own dairy farms with attendant dairy shed equipment, a difficult problem in any future eruption would be reducing the intake of ash particles into milking equipment as well as clogging moving 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.

Heavy ash fall disrupts transport services and communications. Impaired visibility for drivers of motor vehicles can be a dangerous problem during eruptions. At times of maximum tephra fall, complete darkness may prevail, visibility may be reduced to almost zero and driving may not be possible. This is due to the opaque nature of ash. Even vehicles being driven with their headlights on find that the lights penetrate only very short distances in front of a moving vehicle during heavy tephra deposition. 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 most air and oil filters on all exposed equipment in ash blanketed areas.

Ash deposition may interrupt telephone communications, and disrupt radio and electrical services as ash particles penetrate contact breakers and induce shorts. Intense electrical activity in and near the ash cloud may produce frequent lightning strikes to buildings and services.

The weight of ash upon house roofs can lead to collapse if ash is not removed. The danger of roof collapse is related to the angel of pitch of a roof, with flat roofed houses being at greatest risk. As soon as dry ash has been brushed or hosed from problem areas it immediately creates a secondary problem of ultimate disposal. If the ash becomes wet it may form a slurry causing a traction problem and on redrying may harden like cement. This may lead to blockage of storm water systems. Ultimately the ash will enter waterways and create a sedimentological problem. Increased sediment loadings will effect town water supplies downstream and by sheer volume block water intakes, reduce the storage volume of channels and increase the likelihood of flooding. The impact of even a very small eruption on Taranaki’s major water supply catchments is likely to be the first and most inconvenient effect residents many kilometres from the mountain might experience.

Taranaki detailed 3

Egmont Volcano from the southeast, showing the summit dome within the main crater and the parasitic cone of Fanthams Peak constructed principally of lava flows erupted over the last 3300 years - DSIR photograph.

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 concentrations. These solutes may be strongly acid or alkaline. Under natural conditions these concentrations will quickly become diluted downstream.

VOLCANIC GASES

The eruption of large quantities of acidic and highly poisonous gases have been known to accompany some eruptions as well as in areas of hydrothermal activity. These gases comprise principally steam, carbon dioxide and sulphur dioxide, which may be emitted in large quantities, followed in abundance by hydrochloric acid, hydrofluoric acid and ammonia. All these gases are harmful if inhaled because they affect the respiratory system and act as an irritant to eyes and skin, some causing acid burning. On the basis that the past volcanic vents on Egmont Volcano are sited in the centre of the largely unpopulated Egmont National Park, the hazard of toxic gases to the populace is regarded as minimal, and the wind is likely to disperse the gases to low concentrations quickly.

EXPLANATION OF HAZARD MAPS

Five maps on the following pages delineate zones likely to be affected by each volcanic hazard. Since all the major eruptions at Egmont Volcano have been sited within the summit crater or the nearby parasitic crater of Fanthams Peak, future eruptions can be expected to occur from either or both of these craters. The hazard maps have been based on this premise.

Map 1 - Lava Flow and Lava Dome Hazard Zone Map

Taranaki detailed 4 map

LAVA FLOW AND LAVA DOME HAZARD ZONE

Lava flows from Egmont Volcano have seldom extended beyond 4 km distance for their source crater, and the most distant is the Dawson Falls lava flow, 5 km from its presumed source. Thus all lava flows extruded in the entire history of volcanic activity at Egmont Volcano have never extended beyond the area now delineated as Egmont National Park, an area largely unpopulated (Map 1).

In the past, nearly all of the lava flows have been erupted from the central vents of Egmont crater and Fanthams Peak. These vents are regarded as the likely source-areas for future lava flows in future eruptions of Egmont Volcano.

The hazard zone for occurrence of future lava flows and lava domes, based on their past distribution, behaviour and known locations for former vents, is contained entirely within a single circular zone (Map 1), bordered to the north by the steep slopes of the Pouakai Range. Lava flows would be expected to travel slowly, perhaps up to 5 km/hour on upper slopes, but would probably move over large distances at less than 1 km/hour.

Of greatest potential hazard when lava flow extrusion recurs is the melting of large volumes of snow and ice on the upper slopes of Egmont Volcano. Large volumes of ice and snow are stored on the upper slopes of the mountain in winter. If lava was extruded in winter much melting could occur generating explosions and small lahars that would rush down any of the catchments draining the mountain. However if ice was melted in the crater to form a lake, collapse of any part of the crater rim could produce lahars.

Map 2 Pyroclastic Flow and Lateral Blast Hazard Zones Map

PYROCLASTIC FLOW AND LATERAL BLAST HAZARD ZONES

Taranaki detailed 5 map

The distribution of pyroclastic flow deposits on Egmont Volcano shoes strong directional patters to the west, north and east. No lateral blast deposits have yet been recognised at Egmont Volcano. In the northeast, pyroclastic flows have been distributed throughout most of the Stony River Catchment to as far west as Wiremu Road. Based on this distribution of pyroclastic flows in the last 15,000 years, the outer limit of the hazard zone was constructed (Map 2). This hazard zone A delineates the areas likely to be affected most severely and most frequently by pyroclastic flows or lateral blasts resulting from future eruptions at Egmont Volcano. Some parts of this zone have not been covered by pyroclastic flows in the past, but this is due to past configurations of the summit topography. In a future eruption, changes in the form of the crater rim could be rapid and where there is no impediment to their flow could lead to pyroclastic flows in directions not previously taken. All the pyroclastic flow deposits recognised at present, have originated from Egmont crater. Thus future pyroclastic flows will likely affect the area in the hazard zone A more than any other area; over 50% of hazard zone 1 has been covered by pyroclastic flows in the last 15,000 years.

Zone A Areas likely to be affected most severely and most frequently by future pyroclastic flows or lateral blasts.

Zone B Areas likely to be affected less severely and less frequently by future pyroclastic f

Hazard zone B (Map 2) delineates areas likely to be affected less severely and less frequently by future pyroclastic flows or lateral blasts. It is based on the outermost limits of pyroclastic flow deposits known from the last 130,000 years of volcanic activity at Egmont Volcano. The zone is defined in the south by extension of information from the northwest and northeast, where no known pyroclastic deposits occur but where a minor change in the configuration of the summit crater or renewed activity at Fanthams Peak could direct future pyroclastic flows.

Taranaki detailed 6

Pyroclastic flow deposits exposed in Maero Stream on the northwest flanks of Egmont Volcano. These deposits overlie the Newall eruption pyroclastic flow deposits dated between c. 1450 and 1500 A.D. The youngest deposit (at top) is younger than 1750 A.D. and contains charred tussock, evidence of its very hot mode of emplacement – V.E. Neall photograph.

The principal danger to life and property from future pyroclastic flows is likely to be residents and buildings within Egmont National Park. In addition residents living within the zone outside of Egmont National Park, particularly to the west of the mountain would be placed at risk.

LANDSLIDE, LAHAR AND ASSOCIATED FLOOD HAZARD ZONES

The hazard zones delineated in Map 3 have been constructed on the basis of the distribution of former historic and pre-historic landslide, lahar and associated flood deposits. The zones take into account the most likely event of renewed activity in the existing summit crater, which is presently breached to the west, determining a principal westward component to the majority of flows likely to occur. Included in this appraisal is the scenario for removal of the entire eastern crater rim which would lead to unconfined distribution of landslides and lahars in all directions from the summit, although throughout the last 25,000 years these have totalled a considerably smaller volume than westward directed events. Recognition is given to the physiographic effect Fanthams Peak would have on diverting southward-directed landslides and lahars. Based on the known records for the last 130,000 years, an average incidence of landslide and lahar occurrence in Taranaki has been interpreted for each of the zones. These figures should not be directly compared to recurrence intervals as used by engineers and hydrologists, because within these time periods lahar recurrences were often grouped into specific time intervals.

Three landslide and lahar hazard zones (Map 3) are recognised and these are, in decreasing order of risk, hazard zones A, B and C (Map 3). The chances of land being affected by a lahar, decrease with increasing distance from the volcano and with increasing distance from drainage channels. The boundaries between these hazard zones does not reflect a sudden change in the level of risk over a sharp boundary zone but rather reflect gradational changes that occur throughout all zones.

Hazard zone A delineates the area that would in part be the first to be affected by landslides and lahars in future eruptions as it represents the area where the greatest frequency of landslides and lahars has occurred throughout the last 25,000 years. The average incidence interval of landslides and lahars in this zone ranges from 1 per 500 years to 1 per 3000 years. Hazard zone A is judged to be the region of initial and immediate danger from small landslides and lahar flows in any future eruption at Egmont Volcano. The outer limits of this zone have been set by taking into account the potential of landslides and lahars to ride tens of metres upwards onto higher areas that may be in the path of flows. This is particularly the case along the northern boundary of the zone where future landslides and lahars would abut against the southern flanks of the Pouakai Range. Within this zone safety increases with height about channel floors, with distance from river courses and towards the outer boundaries of the zone. Of highest risk are the gorges and river channels within the confines of Egmont National Park where many lahars have travelled in the last 500 years.


Taranaki detailed 7 map

Zone A Areas likely to be affected most severely and most frequently by future landslides, lahars and associated floods.

Zone B Areas likely to be affected less severely and less frequently by future landslides, lahars and associated floods.

Zone C Areas likely to be affected infrequently by very large landslides or lahars.

If in future eruptions, lahars are distributed on the lowlands within hazard zone A, then many of the areas would be inundated from a few centimetres of silt and sand near the outer limits of the zone to thick gravels and mud in proximity to the river channels. This is because shallowly incised river beds will tend to be raised by debris deposited by lahars and flood waters. Flood waters would then overflow onto surrounding land and even into other catchments. Hazard zone A also represents areas of potential flooding where channel widening could lead to undermining of roads and buildings situated close to channel sides, as well as road and railway bridges.

Within hazard zone A there are five instances of rivers within the zone passing very close to urban development. These specifically include the Waiwhakaiho River where it flows through New Plymouth, Stony River and its tributaries adjacent to Okato, the Kaupokonui Stream and Kaponga, and the Waitara River at Waitara. In the event of an eruption of Egmont Volcano, it would be desirable to warn residents to stay away from river courses in hazard zone A.

Hazard zone B represents those areas likely to be affected less severely and less frequently by future landslides, lahars and associated floods. It comprises those areas outside of hazard zone A that have been inundated by landslide and lahar deposits dated between 5000 and 20,000 years old. In this period of time at least 6 lahars travelled 25 km to beyond the present coastline. The average incidence interval of landslides, lahars and floods in hazard zone B ranges from 1 per 3000 years in proximal areas and river channels to 1 per 20,000 years in distal areas. Four principal districts are included. The first is to the northwest of Egmont Volcano in the Warea-Okato district. The principal guiding channels for landslides, lahars and floods here are between Stony River in the north and Warea River in the south. The second is the largest area in zone B, to the southwest of Egmont Volcano in the Opunake district. Here between the Okahu Stream in the north and near Punehu Stream in the southeast, numerous channels radiate from Egmont Volcano. The third area is to the south of Egmont Volcano between Mangawhero and Kaupokonui streams extending almost to the coastline. The fourth district, to the northeast, extends from Tariki north to Inglewood. An estimated population of about 7000 persons live within the confines of hazard zone B.

Hazard zone C represents those areas likely to be affected infrequently by very large landslides or lahars. The hazard zone is based on the distribution of the four largest (greater than 3.5 km2) volume debris avalanches produced during the entire volcanic history of Egmont Volcano. Deposits in this zone extend up to 48 km from source. This zone represents the lower risk landslide, lahar and associated flood zone.

Three principal areas are recognised. The first is to the west of Egmont Volcano in the Pungarehu and Rahotu districts. The second area is to the south between Midhirst in the north to Ohawe Beach in the south and from Auroa in the west to Stratford in the east except where hazard zone C continues southeastwards along the Patea River. The third area is to the northeast along the Waiongana, Manganui and Waitara Valleys and onto the north Taranaki coastal plain. The incidence of landslide and lahar deposits in this zone ranges from 1 per 20,000 years to 1 per 120,000 years. Hazard zone C represents those areas likely to be affected only infrequently by very large landslides or lahars.

In any future eruption of Egmont Volcano landslides and/or lahars can be expected to be a major hazard, affecting all catchments on the mountain. Of considerable potential economic loss will be loss of stock, principally dairy cows, as well as lost production from surviving animals and damage to pastures. However, of the short term effects, one factor likely to create the largest problem is damage to water quality and to intake structures of water supplies used for domestic and rural reticulation. Eight town water supplies and at least one rural water supply depend upon good quality water derived from catchments on Egmont Volcano. In future eruptions this supply would be highly variable, unpredictable and of lower quality. Lack of quantities of clear, fresh water in Taranaki would be of major inconvenience to a large sector of the population with no alternative water supplies.

TEPHRA HAZARD ZONES

In any future tephra eruptions at Egmont Volcano, a tephra cloud will be primarily influenced by the prevailing wind pattern. From high altitude wind direction data available from Ohakea, 150 km to the southeast, wind directions and thus likely tephra distribution occur to the sector between northeast and southeast for 91% of the time between 12,000-16,300m. For the 3000-10,400m altitude range, tephra is likely to be carried into the same sector (northeast to southeast) for 73% of the time and below 3000m for 55% of the time.

In constructing the tephra hazard zones map for Taranaki, information on the wind directions as well as the past distribution, thickness and frequency of moderate to large magnitude tephra eruptions has been used. Past eruptives give a likely indication of the magnitude of eruptions one can expect in the future. Based on the thinning patter of Inglewood Tephra (which is representative of the larger eruptions from Egmont Volcano), together with known data for other tephra eruptions, four zones of tephra hazard are recognised (Maps 4 and 5).

Taranaki detailed 8

View to southeast from the west Taranaki coast, near Pungarehu (to right centre). The numerous mounds were formed by a massive debris avalanche from a former cone at the site of the present Egmont Volcano, about 22,500 years ago. This avalanche spread to the coastline between Okato in the north to Opunake in the south. The southern flanks of Egmont Volcano form the skyline to the left - V E Neall photograph.

Hazard zone A is bounded where greater than 0.25m thickness of tephra could be expected to be deposited during an eruption of similar size to the Inglewood Tephra. Within this area the danger from tephra impact would be greatest. This zone covers as area from NNE to SSE within 21 km of the present Egmont Volcano summit and includes the communities of Kaponga, Stratford, Tariki, Inglewood and Egmont Village. The hazards produced by tephra fall in this zone result principally from the accumulation of the deposit; danger also exists where the impact temperature of lapilli may be high enough to ignite a variety of materials. Extreme darkness during tephra falls may restrict persons in their homes or places of work. Vehicular transport would be impossible on roads covered to greater than 30 cm depth of tephra.

Hazard zone B is the area where between 0.25m and 0.10m of tephra could be expected during a moderate to large magnitude eruption like that which produced the Inglewood Tephra. It occupies an area between 21 and 28 km from the present Egmont summit in a NNE to SSE direction and includes the communities of Kapuni, Okaiawa, Eltham and Kaimata.

Taranaki detailed 10 map

Hazard zones A, B, C and D in North Island from future eruptions of Egmont Volcano.

Hazard zone C is the area where between 0.10m and 0.01m of tephra could be expected. This zone covers an area in the NNE to SSE between 28 and 60 km from the present summit. This zone includes the urban areas of Hawera, Waitara and New Plymouth and extends to as far as Uruti in the northeast, Whangamomona in the east and Patea in the southeast.

Hazard zone D is the area where between 0.01m and 0.001m of tephra could be expected during a moderate to large magnitude eruption like that which produced the Inglewood Tephra. This zone covers an area between 60 and 200 km in a NNE to SSE direction from the present summit. Dustings of tephra could reasonably be expected in the larger urban centres such as Wanganui, Palmerston North, Taupo and Hamilton.

Tephra deposition in zones C and D will not seriously endanger human life or health but may have significant economic consequences.

These three zones express outward degrees of risk expressed as relative magnitudes of effects one might expect to encounter in future tephra eruptions. Risk does not change abruptly at the boundaries between zones but should be regarded as gradational. Severity of risk would be expected to parallel the boundaries between tephra hazard zones. In any future eruption only part of each of the tephra hazard zones may be influenced by tephra deposition. The areal extent of tephra deposition in each zone will be determined by the size of the eruption, the height of the eruption column and the wind velocities and directions at a range of altitudes. It is most likely that areas due east of the volcano will be affected most.


This text is taken from one of a series booklets which cover volcanic hazards at each active or potentially active volcanic centre in New Zealand.

The series was produced by the Volcanic Hazards Working Group of the Civil Defence Scientific Advisory Committee, which includes scientists from the Institute of Geological and Nuclear Sciences and the Universities.

Booklets published in the series so far are:

Number One ‘Egmont Volcano’.
Number Two ‘Okataina Volcanic Centre’.
Number Three ‘White Island’.
Number Four ‘Kermadec Islands’
Number Five ‘Auckland Volcanic Field’
Number Six ‘Mayor Island’
Number Seven "Taupo Volcanic Centre’