Potential hazards of explosive volcanic activity from Mount Ngauruhoe on surrounding infrastructure...

Potential hazards of explosive volcanic

activity from Mount Ngauruhoe on

surrounding infrastructure and property

A thesis submitted in partial fulfilment of the requirements for the degree of Bachelor of Science with Honours in Geology

at the University of Canterbury

by

Andrew Jonathon Cantwell 2011

Department of Geological Sciences

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Frontispiece

View of Mt. Ngauruhoe looking towards the southwest, with Red Crater in the foreground.

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Abstract

Explosive eruptions from Mt. Ngauruhoe cause a number of hazards including tephra fall

and the deposition of bombs and blocks. These pose a risk to infrastructure and property in

the central North Island of New Zealand. Mt. Ngauruhoe erupted on average every two to

three years between the years 1841 and 1975, but has not erupted since. Prehistoric

deposits from Mt. Ngauruhoe, including the Mangatawai Formation, have been mapped and

described by previous studies. The overall prehistoric record is incomplete, making it

difficult to understand the full eruptive history.

Historic and prehistoric eruptions from Mt. Ngauruhoe have been used to create a set of

parameters based around two eruption scenarios. The first scenario is the most likely

eruption, which is based around the 1975 eruption from Mt. Ngauruhoe. This is the largest

historic event, and was chosen because it is expected that following a longer than usual

period of low activity, a more substantial eruption will occur. The second scenario is the

maximum plausible eruption, which is based around the Mangatawai Formation, which was

deposited between 2,568 ± 508 cal. years BP and 1,717 ± 13 cal. years BP. Models of both

the most likely and the maximum plausible eruption from Mt. Ngauruhoe have been

created with these parameters using Tephra2, which gives resulting isopach maps of tephra

dispersal in the surrounding area.

Both eruption scenarios will be significantly affected by changing wind conditions, which

have the potential to disperse tephra across a large portion of the North Island of New

Zealand.

The model created using Tephra2 for a likely eruption scenario shows a number of impacts

on infrastructure and property. The most significant of these impacts is on electricity

production, particularly on the Tongariro Power Scheme. Electricity transmission lines to the

east of Mt. Ngauruhoe will be coated in up to 1 cm of tephra, so will be affected by insulator

flashover if the tephra is moist and of fine grainsize. A 14 km section of State Highway 1 will

be covered in more than 5 mm of tephra, causing hazardous driving conditions.

The model created using Tephra2 for the maximum plausible eruption scenario shows more

widespread damage to infrastructure and property. An area of more than 5,000 km2 will

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receive up to 50 cm of tephra, which will have a large impact on electricity production,

electricity transmission, transport and telecommunications. The area which the majority of

the tephra will fall is remote, with no major townships receiving significant levels of tephra.

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Acknowledgements

Firstly I wish to thank my supervisor, Professor Jim Cole for encouraging me to write this

thesis. Without his support and knowledge I would not have been able to achieve as much

as I have. I also thank Jim for the tour of the Taupo Volcanic Zone. It was the most

interesting, informative and valuable few hours in my entire time studying.

I would also like to thank my co-supervisor, Dr Tom Wilson for providing valuable advice on

the modelling of tephra, and for always being able to find relevant and appropriate sources.

To all the lecturers and technical staff in the Geology Department at the University of

Canterbury, you have been the most helpful, inspirational people I can imagine, and I thank

you for the time you have spent sharing your knowledge with me.

I thank Brian Mason and the Mason Trust for providing me with initial funding, which

allowed me to travel to GNS Wairakei. Without this I would not have met Dr Gill Jolly, who I

am grateful to for suggesting the topic of my thesis.

Finally I would like to thank my friends and family. I would not have got through this without

you. Thank you Mum, Dad, Nicole, Emma, Matthew, all the Geology students at the

University of Canterbury and everyone else that has helped me along the way.

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Table of Contents

Abstract...................................................................................................................................... ii

Acknowledgements .................................................................................................................. iv

Table of Contents ...................................................................................................................... v

List of Figures .......................................................................................................................... viii

List of Tables .............................................................................................................................. x

Chapter One – Introduction ...................................................................................................... 1

1.1 Geological Setting ............................................................................................................ 1

1.1.1 The Taupo Volcanic Zone .......................................................................................... 1

1.1.2 Tongariro Volcanic Centre ........................................................................................ 2

1.1.2.1 Tongariro Volcanic Complex ............................................................................. 2

1.1.2.2 Mount Ngauruhoe ............................................................................................ 3

1.2 Volcanic Risk Assessment ................................................................................................ 3

1.3 Objectives ........................................................................................................................ 5

1.4 Methods........................................................................................................................... 5

Chapter Two – Mt. Ngauruhoe Eruptive History ..................................................................... 7

2.1 Prehistoric Activity .......................................................................................................... 7

2.2 Historic Activity................................................................................................................ 7

2.2.1 Pre 1900’s Eruptive Activity ...................................................................................... 8

2.2.2 1948 – 1949 Eruptive Activity ................................................................................... 8

2.2.3 1954 – 1955 Eruptive Activity ................................................................................. 10

2.2.4 1973 – 1975 Eruptive Activity ................................................................................. 11

Chapter Three – Comparisons with Similar Volcanoes .......................................................... 13

3.1 Introduction ................................................................................................................... 13

3.2 Paricutin Volcano........................................................................................................... 13

3.2.1 Initial Observations at Paricutin ............................................................................. 13

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3.2.2 Continuation of Activity .......................................................................................... 14

3.2.3 End of Eruptive Activity .......................................................................................... 14

3.2.4 Characteristics of Paricutin Volcano ....................................................................... 15

3.2.5 Summary of Paricutin Volcano and Comparison with Mt. Ngauruhoe .................. 16

3.3 Mayon Volcano .............................................................................................................. 16

3.3.1 Eruptive History ...................................................................................................... 16

3.3.2 Summary of Mayon Volcano and Comparison with Mt. Ngauruhoe ..................... 17

3.4 Arenal Volcano .............................................................................................................. 17

3.4.1 Eruptive History ...................................................................................................... 17

3.4.2 Summary of Arenal Volcano and Comparison with Mt. Ngauruhoe ...................... 18

Chapter Four – Modelling Using Tephra2............................................................................... 19

4.1 Tephra2.......................................................................................................................... 19

4.2 Tephra2 Calculations ..................................................................................................... 19

4.3 Simplifying Assumptions ............................................................................................... 20

4.3.1 Particle Size Distribution ......................................................................................... 20

4.3.2 Near-Vent Processes ............................................................................................... 21

4.3.3 Well-Mixed Plume .................................................................................................. 21

4.3.4 Layered Atmosphere .............................................................................................. 21

4.4 Input Parameters for modelling using Tephra2 ............................................................ 21

4.4.1 Eruption Parameters ............................................................................................... 22

4.4.2 Particle Parameters ................................................................................................ 23

4.5 Weather Patterns Above the Central North Island ....................................................... 25

4.5.1 Average Wind Velocity and Direction ..................................................................... 27

4.6 Results from Tephra2 Modelling ................................................................................... 32

Chapter Five – Potential Impacts from Volcanic Activity at Mt. Ngauruhoe ........................ 36

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5.1 Proximal Impacts ........................................................................................................... 36

5.2 Distal Impacts ................................................................................................................ 36

5.2.1 Energy Infrastructure .............................................................................................. 36

5.2.1.1 Electricity Distribution Systems ...................................................................... 36

5.2.1.2 Electricity Production Systems ........................................................................ 40

5.2.2 Transport Infrastructure ......................................................................................... 41

5.2.3 Water Supply Infrastructure ................................................................................... 45

5.2.4 Telecommunication Infrastructure ......................................................................... 46

5.2.5 Tourism ................................................................................................................... 47

5.2.6 Residential, Commercial and Industrial Property ................................................... 48

5.2.7 Other Infrastructure ............................................................................................... 49

Chapter Six – Discussion and Conclusions .............................................................................. 52

References ............................................................................................................................... 54

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List of Figures

Figure 1: (A) Geological setting of the Taupo Volcanic Zone, showing location within the

North Island of New Zealand and relation to the Kermadec Trench. The extent of the

modern Taupo Volcanic Zone is shown by dashed lines. (B) Map of the Taupo Volcanic Zone

showing location of calderas and compositional changes between andesite and rhyolite

across the area. Calderas: (1) Rotorua; (2) Okataina; (3) Kapenga; (4) Reporoa; (5)

Mangakino; (6) Maroa; (7) Whakamaru; (8) Taupo (Spinks et al. 2005). .................................. 1

Figure 2: Overview of the Tongariro Volcanic Centre showing major topographical features

in relation to Mount Ngauruhoe (GeoMapApp 2011). .............................................................. 4

Figure 3: Recorded historic eruptions and their volcanic explosivity index. Black lines

represent ash columns, red lines represent lava flows and blue lines represent pyroclastic

flows (Moebis 2010). ................................................................................................................. 8

Figure 4: Evolution of Mt. Ngauruhoe summit crater viewed from the north- west, from

oldest (A) to present day (F) (Hobden et al. 2002). ................................................................... 9

Figure 5: Pyroclastic avalanche flowing down the northern slope of Mt. Ngauruhoe on

February 19, 1975. Ejecta in eruption column and at base of flow were incandescent at the

time of photograph (Nairn and Self 1977). .............................................................................. 12

Figure 6: Total energy of the Paricutin volcano, showing a decline in energy over time (y-axis

measured in ‘ergs’ – one erg is equivalent to 10-7 Joules) (Scandone 1979). ......................... 15

Figure 7: Locations of North Island volcanoes, as well as selected NIWA climate data

observation sites. ..................................................................................................................... 26

Figure 8: Frequency distribution of annual surface winds in azimuth bins, from selected

Windrun and CLIFLO datasets. ................................................................................................. 27

Figure 9: Frequency distribution of annual winds at approximately 2,000 m in height in

azimuth bins, from selected Windrun and CLIFLO datasets. ................................................... 28







Figure 13: Frequency distributions of annual winds at above 9,000 m in height in azimuth

bins, from selected Windrun and CLIFLO datasets. ................................................................. 29

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Figure 14: Annual resultant vector values versus elevation for the three observation sites

most relevant to Mt. Ngauruhoe (Ruapehu, Taupo and Hamilton CLIFLO), as well as the

average of these three sites..................................................................................................... 30

Figure 15: Annual average wind speed versus elevation for the three observation sites most

relevant to Mt. Ngauruhoe (Ruapehu, Taupo and Hamilton CLIFLO) as well as the average of

these three sites....................................................................................................................... 30

Figure 16: Map of the North Island of New Zealand, showing isopachs measured in kgm-2,

modelled using Tephra2 for most likely eruption scenario. 1 kgm-2 is equal to 0.66 mm, and

10 kgm-2 is equal to 6.6 mm..................................................................................................... 34

Figure 17: Map of the North Island of New Zealand, showing isopachs measured in kgm -2,

modelled using Tephra2 for maximum plausible eruption. 1 kgm-2 is equal to 0.66 mm, 10

kgm-2 is equal to 6.6 mm, 100 kgm-2 is equal to 6.6 cm and 1,000 kgm-2 is equal to 66.6 cm.

.................................................................................................................................................. 35

Figure 18: 220kV transmission lines to the east of Mt. Ngauruhoe, with Mt. Tongariro to the

right of photo. .......................................................................................................................... 37

Figure 19: Map of central North Island of New Zealand showing isopach map for most likely

eruption scenario, as well as key infrastructure in the affected area. .................................... 38

Figure 20: Map of central North Island of New Zealand showing isopach map for maximum

plausible eruption scenario, as well as key infrastructure in the surrounding area. .............. 39

Figure 21: (A and B) Penstock priming butterfly valve taken from the Rangipo Power Station

after seven months exposure to suspended volcanic tephra within the Tongariro River

(Kieran Devine pers. comm. 2011). (C) New penstock priming butterfly valve (Casting Quality

2011). ....................................................................................................................................... 42

Figure 22: Damaged cheek plate from the Rangipo Power Station after seven months

exposure to suspended volcanic tephra within the Tongariro River (Meredith 2002). .......... 42

Figure 23: Map of Tongariro Volcanic Complex showing proximity to State Highway 1 (east)

and State Highway 47 (northwest). ......................................................................................... 43

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List of Tables

Table 1: Input parameters for most likely eruption and maximum plausible eruption for

modelling using Tephra2.......................................................................................................... 25

Table 2: Average wind speed and direction at different elevations around the central North

Island, using data taken from sites in Ruapehu, Taupo and Hamilton. ................................... 31

Table 3: Average wind speed and direction at different elevations around the central North

Island, using data taken from sites in Ruapehu, Taupo and Hamilton, including estimations

for elevations above 9,114 m. ................................................................................................. 32

Table 4: Conversion between mass per unit area of tephra and tephra thickness, using an

average tephra density of 1,500 kgm-3. ................................................................................... 33

Table 5: Overview of risk to each type of infrastructure from tephra fall. ............................. 51

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Chapter One – Introduction

1.1 Geological Setting

New Zealand is situated on the boundary between the Pacific and Australian Plates, which

are moving between 42 mm and 50 mm/year (Bibby et al. 1995). Stress between these

plates in the South Island is released mostly as dextral movement through earthquakes

along the Alpine Fault. In the North Island, the westward subduction of the Pacific Plate

beneath the Australian Plate forms a volcanic arc and back arc basin, which is represented

by the Taupo Volcanic Zone (Cole 1990). The subduction of the Pacific Plate in this manner

has formed the Taupo-Hikurangi trench system (Cole and Lewis 1981).

1.1.1 The Taupo Volcanic Zone

The Taupo Volcanic Zone is located in the central North Island, and is the main area of active

volcanism in New Zealand at the present time (Figure 1). Initial volcanic activity in this area

Figure 1: (A) Geological setting of the Taupo Volcanic Zone, showing location within the North Island of

New Zealand and relation to the Kermadec Trench. The extent of the modern Taupo Volcanic Zone is

shown by dashed lines. (B) Map of the Taupo Volcanic Zone showing location of calderas and

compositional changes between andesite and rhyolite across the area. Calderas: (1) Rotorua; (2) Okataina;

(3) Kapenga; (4) Reporoa; (5) Mangakino; (6) Maroa; (7) Whakamaru; (8) Taupo (Spinks et al. 2005).

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is thought to have occurred around 2 million years ago with andesite volcanism, while

rhyolitic activity commenced around 1.6 million years ago (Wilson et al. 1995). The present

day Taupo Volcanic Zone is around 300 km long and up to 60 km wide, and is defined by the

positions of structural boundaries of calderas as well as vent locations (Wilson et al. 1995).

The central part of the Taupo Volcanic zone consists predominantly of calderas associated

with rhyolite volcanism, while the northern (Whakatane – White Island) and southern

(Ruapehu – Tongariro) ends consist of andesite to dacite stratovolcanoes (Spinks et al 2005).

The Taupo Volcanic Zone has an average magma production rate of 0.3 m3s-1 over the last 2

million years, with a total volume of over 15,000 km3 rhyolitic material erupted in this time.

The most recently active volcanoes of the Taupo Volcanic Zone are White Island, which last

erupted in 2000 (GNS Science 2011), Mt. Ruapehu, which last erupted in 2007 (Jolly et al.

2007), and Mt. Ngauruhoe, which last erupted in 1975 and is part of the Tongariro Volcanic

Centre (Hobden et al. 2002).

1.1.2 Tongariro Volcanic Centre

The Tongariro Volcanic Centre is located at the southern end of the Taupo Volcanic Zone. It

consists of two major andesite stratovolcanoes; Tongariro (the Tongariro Volcanic Complex

including Mt. Ngauruhoe) and Ruapehu, as well as Kakaramea, Pihanga, Maungakatote,

Pukeonake, Hauhungatahi and Ohakune, which are smaller centres (Cole 1978). The

Tongariro Volcanic Centre is underlain by Tertiary mudstone and sandstone (Grant 2006),

above Mesozoic greywacke basement (Graham et al. 1995).

1.1.2.1 Tongariro Volcanic Complex

The Tongariro Volcanic Complex has been active for approximately 275 ka, showing rapid

growth from 210 ka – 200 ka, 130 ka – 70 ka and 25 ka to the present day (Hobden et al.

1996). During this time, the predominant product has been basaltic andesite to andesite

lavas and pyroclastic deposits.

The Tongariro Volcanic Complex has been divided into an older (>20,000 years) and younger

group (<20,000 years). The older group consists of lavas which have been strongly eroded

and show little topographic expression. These lavas were erupted in a NW-SE trending line,

and include eruptions from the Tama Lakes. The younger group exhibits clearly visible

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topographic features such as levees and troughs, and are sourced from vents aligned in a

NNE-SSW orientation. Vents include North Crater, Red Crater and Mt. Ngauruhoe (Cole

1977).

The postglacial history of the Tongariro Volcanic Complex is well understood through

tephrostratigraphy, as well as by 14C dating. The Tongariro Volcanic Complex covers an area

of c. 200 km2, and a volume of c. 60 km3 with an additional 15 km3 in the surrounding ring

plain (Hobden 1997).

1.1.2.2 Mount Ngauruhoe

Mount Ngauruhoe is an active andesite stratovolcano located within the Tongariro Volcanic

Complex (Figure 2) (GeoMapApp 2011). Volcanic activity commenced approximately 7,000

years B.P. and has alternated between effusive strombolian, vulcanian and subplinian

eruption styles since then (Moebis 2010). Historical volcanic activity in general lasts days to

months, with years to decades between events (Hobden et al. 2002). Major historical

eruptions occurred in 1870, 1949, 1954 and 1975. No major volcanic activity has occurred

since the 1975 event, meaning Mt. Ngauruhoe is currently undergoing its longest quiescent

period in historical times (Hobden et al. 2002).

1.2 Volcanic Risk Assessment

Volcanic risk can be assessed by combining potential hazards, the exposure to these hazards

and the vulnerability of infrastructure and property. To allow volcanic risk to be calculated, a

number of areas must be investigated. These include the type of volcano, its eruptive

history, hazards from previous eruptions and similarities to other volcanoes.

The modelling of potential future eruptions using the program Tephra2 requires information

gathered from the eruptive history, as well as wind data to create two eruption scenarios;

the maximum plausible event and the most likely event. Resulting isopachs can then be

overlain on maps to determine areas which may be exposed to tephra fall. The vulnerability

of these areas can then be determined.

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N

Hauhangatahi

Tongariro Volcanic Centre

Pihanga

Kakaramea

Ohakune

Mount Ruapehu

Central North Island, New Zealand

Mount Ngauruhoe

Mount Tongariro

Figure 2: Overview of the Tongariro Volcanic Centre showing major topographical features in

relation to Mount Ngauruhoe (GeoMapApp 2011).

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Two approaches can be undertaken for volcanic risk assessment; deterministic and

probabilistic. Deterministic hazard assessment focuses on a single hazard event, which

ensures that the event is realistic. A most likely scenario as well as a maximum plausible

event are often used in deterministic hazard assessments. The major limitation of this

approach is that the chosen scenarios may not be realistic, meaning results may not be

representative of events which may occur in the future. Probabilistic hazard assessments

model all possible scenarios to determine the most likely event. This takes into account the

uncertainty involved with creating likely scenarios. Limitations of this approach are that it

excludes scenarios which are unexpected, and does not allow for complexities in systems.

This study takes a deterministic approach. It uses information from historic events as a basis

for developing two scenarios (most likely eruption and maximum plausible eruption) for

modelling, as no eruptive activity has occurred from Mt. Ngauruhoe since 1975. Modern

wind data is required to produce average wind speed and directions for the affected area,

however this is highly variable.

1.3 Objectives

There are three main objectives that this study aims to achieve: (1) To identify areas

vulnerable to volcanic hazards from Mt. Ngauruhoe using information from past events; (2)

To model the potential dispersal of tephra for the most likely eruption, and the maximum

credible eruption, from Mt. Ngauruhoe across the North Island of New Zealand using the

program ‘Tephra2’; and (3) To analyse the impact of tephra on infrastructure using results

from modelling with Tephra2.

1.4 Methods

A literature review of Mt. Ngauruhoe and the surrounding area has been carried out in

chapters one and two. Paricutin Volcano, Mayon Volcano and Arenal Volcano have been

reviewed in chapter three to give a comparison of similar volcanoes to Mt. Ngauruhoe.

Tephra2 is a computer program which uses the advection-diffusion method to calculate

dispersal of tephra across an area. This has been used in chapter 4 to model two eruption

scenarios from Mt. Ngauruhoe; the most likely eruption and the maximum plausible event.

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Tephra2 requires a number of input parameters, including variables relating to the eruption,

particles and wind.

The variability of wind is one of the most important aspects of carrying out accurate

modelling of volcanic tephra dispersal. This study uses the calculated average wind direction

and speed at different elevations from three measuring stations in the central North Island.

Any number of weather patterns could cause tephra to be deposited in different locations,

however only the most common weather pattern has been modelled due to the limited

time available.

Volcanic hazards which pose a threat to life and property from Mt. Ngauruhoe have been

recognised in chapter 5. The vulnerability of different types of infrastructure and property

have been also been assessed using results from modelling. Volcanic hazards have been

combined with vulnerability of each type of infrastructure to allow the risk of damage to be

identified.

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Chapter Two – Mt. Ngauruhoe Eruptive History

2.1 Prehistoric Activity

Little is known about prehistoric activity from Mt. Ngauruhoe because of its recent

formation and lack of deeply incised drainage channels. Eruptive products prior to European

arrival are poorly exposed at the surface, meaning most of the current information available

on Mt. Ngauruhoe has been gathered from eruptive styles and rates from the more recent

deposits, as well as by direct observation of historic eruptions (Hobden et al. 2002). Deposits

from large prehistoric eruptions at Mt. Ngauruhoe have been exposed in road cuttings

(Topping 1973), however these often contain deposits from other volcanoes including Mt.

Ruapehu, Blue Lake, Red Crater, Te Maari Craters and larger eruptions from the Taupo

caldera. These prehistoric deposits from the Tongariro Volcanic Centre include the

Ngauruhoe Formation, Tufa Trig Formation, Mangatawai Formation, Papakai Formation and

the Mangamate Formation (Moebis 2010).

The Ngauruhoe Formation includes tephras from Mt. Ngauruhoe, Red Crater and Te Maari

Craters, and represents activity since 1,717 cal. years BP. The Tufa Trig formation includes

19 tephras sourced from Mt. Ruapehu, and was deposited below the Ngauruhoe Formation

(Moebis 2010). The Mangatawai Formation is considered to have been formed mostly from

Mt. Ngauruhoe (Topping 1973; Donoghue 1991). The Mangatawai Formation is thought to

be a result of the most vigorous period of growth in the history of Mt. Ngauruhoe, therefore

will provide a good example of the maximum plausible eruption from Mt. Ngauruhoe. The

Mangatawai Formation was deposited between 2,568 ± 508 cal. years BP (Fergusson and

Rafter 1959) and 1,717 ± 13 cal. years BP (Lowe et al. 2008). The Papakai Formation was

formed by both Mt. Ruapehu and the Taupo Caldera, while the Mangamate is older than the

first deposits from Mt. Ngauruhoe so is not relevant.

2.2 Historic Activity

There have been at least 73 eruptive episodes from Mt. Ngauruhoe greater than 100,000 m3

in volume since records began in 1839 (Bebbington and Lai 1996). The majority of these

events consist of explosive ash eruptions, with a smaller number of a’a and pahoehoe lava

flows, fire fountaining and small pyroclastic flows (Figure 3). Historic activity has significantly

8

changed the summit crater since records began in 1839 (Figure 4). Records prior to the

1900’s are poor because it was uncommon for Europeans to visit the area. This may explain

the significant increase in eruptive activity between the first observed activity and more

recent activity (Figure 3).

2.2.1 Pre 1900’s Eruptive Activity

The first European to view and record eruptions from Mt. Ngauruhoe is thought to be John

Bidwill, who observed an eruption on 3 March, 1839. The activity observed by Bidwill is

described as a loud eruption, followed by a “thick black mushroom cloud” which lasted for

half an hour. Large, continuous steam emissions were present throughout the event (Bidwill

1841). Following the 1839 event, small ash eruptions occurred approximately every four

years until 1870, when a more significant event which extruded two lava flows occurred.

The lava flows travelled over a bluff on the north side of the cone, breaking up into blocks.

Incandescent scoria and ash plumes from explosive eruptions heard 190 km away also

occurred during the 1870 eruption (Hobden et al. 2002).

2.2.2 1948 – 1949 Eruptive Activity

After the 1870 event, small ash eruptions occurred approximately every three years until

April 1948, when the next major event began. The 1948 – 1949 event is classed as vulcanian,

and commenced with major explosive activity releasing an ash plume to more than 600 m

Figure 3: Recorded historic eruptions and their volcanic explosivity index. Black lines represent ash

columns, red lines represent lava flows and blue lines represent pyroclastic flows (Moebis 2010).

9

above the summit. In the following days, violent activity erupted both ash and large,

incandescent blocks up to 3,000 m above the summit (Hobden et al. 2002). In February 1949

this period of increased activity continued with a series of violent explosions similar to those

in 1948. These were accompanied by several block and ash flows, as well as a single a’a lava

flow on the north-west flank of the cone. During the extrusion of the lava flow very little ash

was emitted from the vent, however large ballistic blocks were ejected. The 1948 – 1949

Figure 4: Evolution of Mt. Ngauruhoe summit crater viewed from the north-

west, from oldest (A) to present day (F) (Hobden et al. 2002).

10

eruption series ended in February 1949, culminating with explosive ash, volcano-tectonic

tremors, loud explosions and the formation of ash plumes up to 6,000 m above the summit

(Hobden 2002).


The next major event in the history of Mt. Ngauruhoe occurred between 1954 and 1955.

This event was comprehensively reviewed by Gregg (1956) and Krippner (2009). The

eruption began in May 1954 with an ash plume rising to around 4,000 m above the summit.

Minor ash eruptions continued until the end of May, and during this time red-hot lava was

seen in the crater. On 2 June 1954 lava fountaining to c. 300 m above the summit was

observed. This could also be seen in Taupo, located c. 60 km to the north-east of Mt.

Ngauruhoe. The lava fountaining continued until 4 June, and formed two lava flows down

the north-west flank of the cone. The most westerly of these flows covered a hot avalanche

which was deposited earlier in the day. On 6 June 1954 a number of sharp explosions were

heard from Mt. Ngauruhoe, and ash clouds rising to several hundred metres above the

summit were observed. Explosions and ash clouds continued intermittently until the end of

June, along with occasional lava fountaining which reached a maximum for the 1954 – 1955

eruption period on 30 June 1954. Near-continuous fountaining to a height of approximately

300 m above the summit, as well as the formation of another lava flow down the north-west

flank of the cone occurred on this day (Gregg 1956; Krippner 2009).

Lava fountaining and the emission of ash continued throughout July, and on 8 and 9 July two

further lava flows were extruded down the western flanks. These continued to flow until 13

July 1954. On 29 July 1954 a new lava flow was extruded, accompanied by explosions that

produced both shock waves (seen as bright arcs of light moving away from the vent) and

vortex rings (circular puffs of ash). Three lava flows were extruded in August, the first

reaching Pukekaikiore on 15 August and the second on 18 August. The second flow was 183

m wide and 15 m high, and moved at around 15 cm per minute. The third flow travelled

around the southern end of Pukekaikiore, also at around 15 cm per minute (Gregg 1956;

Krippner 2009).

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In September 1954 loud rumbling explosions and the emission of ash intensified. Lava

fountaining on 15 and 16 September caused lava to flow over the western rim of the crater,

and a hot avalanche fell to the saddle of Pukekaikiore. Further visible shock waves were

seen later in September, and ash fell at the Tongariro Chateau, as well as in Taupo. On 26

September the final lava flow of this eruption took place (Gregg 1956; Krippner 2009).

Intermittent ash eruptions occurred throughout October, and only one ash eruption was

observed during November. Activity during December was increased from the previous

months, with lava fountaining and ash emissions, as well as loud explosions occurring

(Gregg 1956; Krippner 2009).

January 1955 saw activity increase again, with loud explosions and the emission of ash

reaching a peak on 28 January. Lava fountaining occurred several times during January.

Activity decreased in February with a small number of ash explosions, however it was

reported that small fragments around the vent were being carried up by a blue gas

discharge. The final ash explosion in this eruptive sequence occurred on 10 March, although

incandescent material remained in the vent until at least 25 June 1955 (Gregg 1956;

Krippner 2009).


Following the 1954 – 1955 eruptive activity, seventeen years of semi-continuous steam

emission and small ash eruptions occurred. The next period of increased activity from Mt.

Ngauruhoe began on 29 December 1972, with the emission of fresh ash and incandescent

material. Activity then diminished until September 1973, when effusive activity took place.

In November it was observed that the crater of Mt. Ngauruhoe had near-vertical walls at

least 180 m in height, with a secondary crater at the bottom. Minor eruptive activity

occurred in December 1973.

Major eruptive activity for the 1973 – 1975 event commenced on 22 January 1974, with the

emission of a black cloud of ash accompanied by the ejection of incandescent bombs.

Sporadic ash emission continued until 26 January, when an eruption produced a large,

dense cloud of ash which partially collapsed to form a pyroclastic flow (Nairn et al. 1976).

The next significant event occurred on 27 March, when a large ash column rose to 1,700 m

12

above the vent. This was followed on 28 March by rapid degassing of Mt. Ngauruhoe,

producing ash columns and associated pyroclastic flows. Bombs and blocks up to 5 m in

diameter were ejected onto the slopes of Mt. Ngauruhoe, with shock waves from these

events reaching as far as Chateau Tongariro. The most explosive eruptions occurred from 28

to 29 March, throwing bombs and blocks 40 m above the crater rim. An ash cloud with

internal lightning extended to 2,500 m above the vent (Nairn et al. 1976).

An 11 month lull in activity was broken on 12 February 1975, with the emission of ash

followed by intermittent ash eruptions until 17 February. The largest explosion of the 1973 –

1975 eruption sequence occurred on 19 February, creating a 6,000 m high ash cloud which

collapsed to form a pyroclastic flow. This was accompanied by the continuous emission of

gas, and volcanic tremor. A block and ash flow was caused by the collapse of a build-up of

bombs and blocks between the inner and outer craters. This produced a co-ignimbrite

plume that rose to 500 m above the block and ash flow. By the afternoon of 19 February

1975, the eruption column had risen to 10,000 m above the crater rim, depositing ash 170

km from the vent. Pyroclastic flows caused by column collapse continued until the

afternoon, when the ash column declined to 4,000 m (Figure 5). The eruption ended during

the night of 19 February, 1975.

Figure 5: Pyroclastic avalanche flowing down the northern slope of Mt. Ngauruhoe

on February 19, 1975. Ejecta in eruption column and at base of flow were

incandescent at the time of photograph (Nairn and Self 1977).

13

Chapter Three – Comparisons with Similar Volcanoes

3.1 Introduction

Mt. Ngauruhoe has been compared with three other volcanoes using a literature review, in

order to identify similarities and differences in the eruptive history and potential future

activity from each volcano. The volcanoes used to compare to Mt. Ngauruhoe are Paricutin

Volcano in Mexico, Mayon Volcano in the Philippines and Arenal Volcano in Costa Rica.

These volcanoes are similar in composition and size, and are all younger than c. 25,000

years.

3.2 Paricutin Volcano

Paricutin is an andesite stratovolcano located in the state of Michoacán, Mexico. Paricutin

was born in 1943 with intensely explosive emission of magma during the first year of

activity. Paricutin is situated in the monogenetic Michoacán-Guanajuato volcanic field,

which contains more than 1,000 volcanoes as well as flood basalts (Pioli et al. 2008). The

normal eruptive style of Paricutin was strombolian, however effusive activity from the base

of the volcano also occurred frequently. Eruptions declined in explosivity and volume from

birth until 1952, when activity ceased (Bullard 1962, Pioli et al. 2008).

3.2.1 Initial Observations at Paricutin

Paricutin Volcano was the first volcanic eruption to be witnessed by humans in the

Michoacán area of Mexico. Events were recorded by locals as well as at least two scientists,

who collated descriptions from the local farmers. The first signs of eruption occurred on 5

February, 1943, with a series of earthquakes which increased in number and intensity until

the start of eruption on the morning of 20 February. The initial activity was witnessed by

four farmers in the area, who noticed a small fissure extending across a paddock. The fissure

began emitting sparks and smoke, and the surrounding ground swelled upwards by two

metres. Blocks were initially thrown to a height of around five metres, until the early

evening when the volcano started erupting larger bombs and blocks which could be seen

several kilometres away in Parangaricutiro. During the night of 20 February, large

incandescent bombs were ejected from the volcano and a large ash column was produced.

By the morning of 21 February, the volcano had grown to around 10 m high and continued

14

emitting rocks and ash. By midday the ejection of bombs and ash was significant enough to

have grown the cone to 50 m high. The first lava flow was emitted on 21 February and was

sourced from the base of the cone, flowing at approximately 5 m per hour (Bullard 1962).

3.2.2 Continuation of Activity

By the end of the first week of the eruption, Paricutin had grown to over 140 m high. The

noise of explosions could be heard 350 km away, and material was frequently ejected to

over 1,000 m above the vent. Eruptions were normally strombolian in style, however

hawaiian-style eruptions also occurred. Alternating lava flows, pyroclastic activity and ash

columns built the cone to 325 m above ground level by the end of the first year of activity. It

was during the first year that cone building was at its greatest. Following the first year, the

effusion of lava was the dominant process. Paricutin grew only 73 m in the following 6 years

(Bullard 1962).

During the second year of eruption the main activity was from bocas, which are lava flows

sourced from the base of the crater. By the end of 1944 the general outline of the Paricutin

lava field had been defined. A large parasitic cone developed in 1943, quickly growing to

over 100 m in height. Subsequent flows were deposited on top of existing material, but the

lateral extent of deposits did not increase (Bullard 1962). Total energy of the Paricutin

eruption declined each year (Figure 6).

3.2.3 End of Eruptive Activity

The final activity from Paricutin came suddenly, rather than a more gradual decline as may

have been expected. During the evening of 22 February 1952 a lava flow on the north-

eastern side of the volcano slowed, and by 25 February 1952 it had stopped completely. The

last day of continuous eruptive activity from the vent was 24 February 1952, when large

quantities of ash were deposited from the volcano. During the night of 24 February 1952,

the deposition of ash stopped. Several intense explosions the next day occurred, and

smaller eruptions until March occurred until 4 March 1952 when activity ceased entirely,

nine years and twelve days after the first activity was observed (Bullard 1962).

15

3.2.4 Characteristics of Paricutin Volcano

Lavas from Paricutin Volcano are similar to those from Mt. Ngauruhoe, consisting mainly of

andesites. The 1943 lavas are classed as olivine-bearing andesites and have an SiO2 content

of approximately 55 wt%. They contain phenocrysts of olivine and plagioclase feldspar

within a fine groundmass. As the eruption progressed, the SiO2 content increased to

approximately 60 wt% for the 1952 lavas. The later lavas contain fewer olivine phenocrysts

and no plagioclase feldspar phenocrysts, and are classed as orthopyroxene andesites

(Bullard 1962). This change is believed to be due to a combination of crystal fractionation

and assimilation of the country rock by the original melt (Scandone 1979).

The age of Paricutin is accurately known, as it relies on direct human observation rather

than the dating of material. Paricutin was born on 20 February 1943 and its eruptive period

ended on 24 February 1952. The total volume of Paricutin is around 1.3 km3.This means the

long term growth rate of Paricutin is approximately 0.14 km3 y-1 (Yokoyama and de la Cruz-

Reyna 1990).

Figure 6: Total energy of the Paricutin volcano, showing a decline in energy over time (y-axis

measured in ‘ergs’ – one erg is equivalent to 10-7 Joules) (Scandone 1979).

16

3.2.5 Summary of Paricutin Volcano and Comparison with Mt. Ngauruhoe

Paricutin Volcano appears to show similar characteristics to Mt. Ngauruhoe, however

following the first year of activity, the effusion of lava was the dominant process. Mt.

Ngauruhoe has emitted both effusive material and explosive material since historic records

began, and it is likely that both eruptive styles also occurred in prehistoric times. The total

energy released from Paricutin Volcano decreased over time, rather than remaining similar.

Paricutin Volcano is unusual in that it had an extremely short life of nine years, after which

no further volcanic activity has occurred. Mt. Ngauruhoe has been active over a much

longer period of time, making it significantly different to Paricutin Volcano.

3.3 Mayon Volcano

Mayon Volcano is an active andesite volcano, located in the Bicol volcanic chain which is

formed through subduction associated with the Philippine Trench (Ramos-Villarta et al.

1985). Mayon Volcano has a summit elevation of 2,462 m above sea level, and covers an

area of 314.1 km2. Lavas erupted from Mayon Volcano are typically basalt to olivine-bearing

pyroxene andesite, ranging from strombolian to plinian in style (Philippine Institute of

Volcanology and Seismology 2011).

3.3.1 Eruptive History

The first recorded eruption from Mayon Volcano occurred in February 1616, with the

production of pyroclastic flows, lava flows and ash fall, as well as associated lahars. Around

150 years passed until the next eruption was recorded. In 1766 a series of vulcanian

eruptions, lava flows, pyroclastic flows and ash falls were emitted from Mayon Volcano.

Heavy rain followed these eruptions, causing lahars which damaged six small townships as

well as causing 39 fatalities. In 1800 and 1811 further vulcanian eruptions, lava flows,

pyroclastic flows and ash falls occurred. The largest historical eruption from Mayon Volcano

occurred in 1814, causing up to 1,200 deaths. This was plinian in style, and had associated

pyroclastic flows, volcanic lightning, bombs and blocks (Ramos-Villarta et al. 1985).

Between the years 1827 and 2006, vulcanian eruptions, pyroclastic flows and ash fall were

produced on average every three to four years, with a maximum gap between events of 26

years. No activity was recorded from 1902 until 1928, when another vulcanian eruption with

17

associated pyroclastic flows, lava flows and ash fall occurred. This was followed by a ten

year gap until 1938, after which regular eruptions every three to four years recommenced

(Philippine Institute of Volcanology and Seismology 2011).

The most recent eruption from Mayon ceased on the first of January 2010, and little activity

has been observed since (Philippine Institute of Volcanology and Seismology 2011).

3.3.2 Summary of Mayon Volcano and Comparison with Mt. Ngauruhoe

Mayon Volcano is c. 25,000 years old, so is significantly older than Mt. Ngauruhoe. Despite

this, eruption styles and sequences are similar to Mt. Ngauruhoe, with vulcanian eruptions

occurring on average every three to four years. This gives an indication that Mt. Ngauruhoe

may follow a similar trend to Mayon Volcano over the next several thousand years.

3.4 Arenal Volcano

Arenal Volcano is the youngest volcano in the Tilarán Ranges, which are located in the

volcanic arc of Costa Rica. Arenal Volcano is an active andesite stratovolcano which has an

elevation of 1,680 m above sea level. The slopes of Arenal Volcano consist of blocky lava

fields of basaltic andesites (Soto et al. 2006). Arenal Volcano has been continuously

erupting lava for more than 37 years, making it the longest running lava eruption on Earth at

present (Gill J et al. 2006). During its c. 7,000 year history, there have been four plinian,

eight subplinian, seven violent strombolian and two vulcanian eruptions recognised in the

tephra sequence, with many others too small to be apparent. Tephra volumes of up to 0.44

km3 per event have been recognised from these deposits (Soto et al. 2006).

3.4.1 Eruptive History

Arenal Volcano has produced four plinian eruptions which have been recognised in the

tephra sequence. These were c. 3,200 BP, 2,120 BP, 1,300 BP and 550 BP. Between each of

these events, eruptions large enough to be preserved in the tephra sequence occurred on

average every 300 years. Minor events almost certainly occurred between the larger

eruptions (Soto et al. 2006).

18

3.4.2 Summary of Arenal Volcano and Comparison with Mt. Ngauruhoe

Arenal Volcano has been active for c. 7,000 years, which is similar to the age of Mt.

Ngauruhoe. Arenal Volcano has had several plinian eruptions which have been recorded in

the tephra sequence. This may have also occurred at Mt. Ngauruhoe, but due to poorly

exposed deposits these cannot be seen.

19

Chapter Four – Modelling Using Tephra2

4.1 Tephra2

Modelling of two eruption scenarios has been carried out using data collected both from

past eruptions at Mt. Ngauruhoe, and from a general literature review. Modelling was

carried out using Tephra2, which is a computer program that uses an advection-diffusion

model to calculate the dispersal of tephra across an area, under a particular set of

conditions. This uses a deterministic approach to investigate how a given scenario will

impact the surrounding area. Tephra2 has been used to model two eruption scenarios; the

most likely eruption and the maximum plausible eruption.

4.2 Tephra2 Calculations

Tephra2 was designed to allow modelling of tephra accumulation around a volcano based

on a set of eruption parameters. Tephra2 uses an advection-diffusion model, which

describes the solution of the equations of particle diffusion, transport and sedimentation,

and can estimate tephra accumulation relative to the source vent. The numerical simulation

of tephra accumulation is based on an advection – diffusion equation (Suzuki 1983, Connor

et al. 2001), which is expressed by a simplified mass-conservation equation:

Where x, y and z are spatial coordinates expressed in metres (downwind, crosswind and

vertical respectively); Cj is the mass concentration of particles (kgm-3) of a given particle size

class, j; wx and wy are the x and y components of the wind velocity (ms-1) and vertical wind

velocity is assumed to be negligible; K is a horizontal diffusion coefficient for tephra in the

atmosphere (m2s-1); vt,j is the terminal settling velocity (ms-1) for particles of size class j, as

these particles fall through a level in the atmosphere; is the change in particle

concentration at the source with time, t (kg m-3 s-1). The algorithm implemented in Tephra2

assumes negligible vertical wind velocity and diffusion, and assumes a constant and

isotropic horizontal diffusion coefficient (K = Kx = Ky). The terminal settling velocity, v, is

calculated for each particle size, j, at each atmospheric level, l, as a function of the particle’s

Reynolds number, which varies with atmospheric density. Wind velocity is allowed to vary

20

as a function of height in the atmosphere, but it is assumed to remain constant within each

specific atmospheric level.

Tephra2 allows mass accumulation, M (kg m-2), at each location (x, y) surrounding the

source vent to be calculated;

( ) ∑ ∑

( )

Where ml,j (x, y) is the mass fraction of the particle size, j, released from atmospheric level, l,

accumulated at location, (x, y). Hmax is the maximum height of the erupting column, and dmin

and dmax are, respectively, the minimum and maximum particle diameters. This means the

distribution of tephra mass following an eruption depends on both the distribution of mass

in the eruption column and the distribution of mass by grain size. The algorithm used in

Tephra2 assumes that mass is uniformly distributed in the eruption column, or can be

specified to be uniformly distributed in some fraction of the uppermost column, to be

consistent with observations of past volcanic plumes. Grain size distribution is assumed to

be log-normal, and is deduced from comparison with studies of well-preserved past deposits

(Connor et al. 2001).

4.3 Simplifying Assumptions

Several assumptions are made during modelling with Tephra2. These include particle size

distribution, near-vent processes, mixing of the plume and layering of the atmosphere.

4.3.1 Particle Size Distribution

Many volcanic deposits show a bimodal rather than Gaussian distribution of particle sizes,

however these distributions are often characterised by a dominant peak, with a much

smaller secondary maximum. Although distributions are often bimodal, the dominant peak

of grain size fits a Gaussian distribution, making the use of Gaussian distribution in the

Tephra2 model adequate (Connor et al. 2001). Particle sizes in deposits from Mt. Ngauruhoe

also show bimodal distribution, with a dominant peak which fits a Gaussian distribution

(Moebis 2010).

21

4.3.2 Near-Vent Processes

Tephra2 does not account for near-vent processes such as pyroclastic flows, meaning it

cannot accurately predict tephra deposition in areas close to the vent. Results from the

current version of Tephra2 have been compared with deposits from Cerro Negro Volcano, in

Nicaragua, showing that Tephra2 significantly underestimates tephra accumulation on the

flanks of the volcano (Connor et al. 2001).

4.3.3 Well-Mixed Plume

The Tephra2 model assumes all particle sizes are evenly spread throughout the entire

plume. It is more likely that larger particles fall out of the plume at lower elevations, and

smaller particles at higher elevations. This assumption is made in order to simplify the

mathematics of the model. Additionally, it is not known exactly how tephra is dispersed

within the plume, so the use of a different model may prove to be inaccurate. The current

version of Tephra2 has been shown to accurately display the distribution of tephra on the

ground, suggesting that the majority of each deposit is not sensitive to the mass distribution

of tephra in the plume. This assumption is allowed because the model compares well with

actual deposits (Connor et al. 2001).

4.3.4 Layered Atmosphere

Tephra2 divides the atmosphere into horizontal layers, which each have uniform wind speed

and direction. The real atmosphere consists of updrafts, downdrafts and eddies which

change over time. The approximation of the atmosphere as static and of constant value

within each horizontal layer simplifies the model and works well for smaller eruptions that

do not spread out over larger areas or last for prolonged periods of time. Larger eruptions,

or those which occur during highly variable weather conditions are likely to be poorly

modelled, with an inaccurate prediction of tephra dispersal on the ground (Connor et al.

2001).

4.4 Input Parameters for modelling using Tephra2

The input parameters for Tephra2 fall into two main categories; eruption parameters and

particle parameters. Atmospheric conditions at the time of eruption are also required.

22

These include the eddy constant of the Earth’s atmosphere, which is 0.04 (Courtland 2011),

as well as wind speed and direction.

4.4.1 Eruption Parameters

Eruption parameters include the location and height of the volcano, erupted mass, particle

sizes, plume height and the number of integration steps.

The vent easting and northing was found using Google Earth, and is measured using the

Universal Transverse Mercator (UTM) coordinate system. The vent coordinates of Mt.

Ngauruhoe are 60 H 5664923 m E 381803 m S. This is equal to coordinates in decimal

degrees of -39.156921° 175.631963° (Google Earth 2011). The elevation of the vent above

sea level is 2,200 m (Google Earth 2011).

The erupted mass of the volcano can be calculated by multiplying the total volume of the

fall deposit by the average density of the fall deposit (including lithics and pumice).

There has been no activity from Mt. Ngauruhoe since 1975, so it is assumed that the next

eruption will be larger than average, as the magma has had time to build up beneath the

vent. The eruption that will be used in modelling as the most likely eruption is the 1974

vulcanian eruption, which is the largest historic eruption from Mt. Ngauruhoe. The total

volume of this eruption was c. 0.74x106 m3 (Self 1975).

The maximum plausible eruption has been based around the largest prehistoric eruption

from Mt. Ngauruhoe. This is the Mangatawai Deposit, which has been mapped in the field

to produce an isopach map (Gregg 1960). The volume of this deposit can be estimated using

the formula:

Where a represents the thickness of the tephra at the source and b represents the distance

over which that thickness is halved (Cole and Stephenson 1972). Using 180 cm as the

thickness of tephra at source, and 9.7 km as the distance over which that thickness is

halved, the total volume can be calculated as 3.987x109 m3 (Gregg 1960).

23

The average density of a fresh fall deposit from Mt. Ngauruhoe was calculated for the 1974

vulcanian eruption (Self 1975). One other density was found from literature review,

however this was for the pyroclastic flow which occurred during the 1975 eruption, which is

not included in the modelling carried out using Tephra2 (Nairn and Self 1978). The average

density of the 1974 tephra deposit was calculated to be 1.5 g/cm3, or 1,500 kg/m3. This

takes into account the different densities of each component of the deposit, including

lithics, vitrics and crystals (Self 1975).

Using the densities and volumes above, the erupted mass can be calculated. For the smaller

Mt. Ngauruhoe event, this is calculated as (0.74x106 m3) x (1,500 kg/m3), giving an erupted

mass of 1.11x109 kg. For the maximum plausible eruption from Mt. Ngauruhoe, this is

calculated as (3.987x109 m3) x (1,500 kg/m3), giving an erupted mass of 5.98x1012 kg.

Particle sizes for airfall from previous Mt. Ngauruhoe eruptions range from c. 6 mm

diameter (-2.5 ) to c. 7.8x103 mm (7 ), with a median diameter of c. 0.5 mm (1 ) (Moebis

2010). These values give a standard deviation of c. 3.3 mm (-1.66 ).

The maximum plume height from historic eruptions at Mt. Ngauruhoe is c. 7,000 m, which

occurred during the 1975 eruption (Hobden et al. 2002). The maximum plausible plume

height from an eruption at Mt. Ngauruhoe is 25,000 m. This was calculated using averages

for the estimated volume from the maximum plausible eruption, which correspond to

plume heights listed in the volcanic explosivity index (Newhall and Self 1982).

The number of integration steps for both the most likely eruption and the maximum

plausible eruption is 100. This gives an accurate dispersal of tephra for the given

parameters. A higher number of integration steps will give more accurate results, however

this is unnecessary as the input parameters are not precise (Connor et al. 2011).

4.4.2 Particle Parameters

Particle parameters include the fall time threshold, diffusion coefficient, lithic and pumice

densities, the number of integration steps, plume dispersal model and the plume ratio.

The fall time threshold is the maximum time allowed for particles to land. If particles have

not landed by this time, Tephra2 will stop calculations, meaning subsequent particles will

24

not be added to the accumulated mass of tephra on the ground. It is assumed that the

majority of tephra from a single eruption will have fallen to the ground within 24 hours of

that eruption, so a value of 100,000 seconds (c. 27 hours) is used (Connor et al. 2011).

The diffusion coefficient describes the advection and diffusion of particles through the

atmosphere. This parameter has little effect on the resultant tephra dispersal map. A larger

value allows for variations in the eruption column over time, as well as atmospheric

variations that occur during the time the eruption is occurring. The most likely eruption

from Mt. Ngauruhoe will occur over a short period of time, so the diffusion coefficient will

be 500. The maximum plausible eruption from Mt. Ngauruhoe will occur over a longer

period of time, giving a larger diffusion coefficient of 2,000 (Courtland 2011).

Lithic and pumice densities are taken from deposits of past eruptions at Mt. Ngauruhoe. The

density of lithics is c. 2,700 kg/m3 (Self 1975), and the density of pumice is c. 950 kg/m3

(Nairn et al. 1978) for the most likely eruption scenario. Larger, more explosive eruptions

often have a higher vesicle content, so the density of lithics will decrease to c. 2,500 kg/m3

(Nairn and Self 1978). The density of pumice for the maximum plausible eruption will

remain at c. 950 kg/m3.

The plume dispersal model for both the most likely eruption and the maximum plausible

eruption is zero. This assumes the plume is well mixed, meaning that particles of all sizes are

released from all points of the eruption column (Connor et al. 2011).

The plume ratio describes the minimum particle fallout height during the eruption. A value

of 0 indicates that particles are released from the entire eruption column, while a value of

0.9 indicates that particles are released from the upper 10% of the eruption column (Connor

et al. 2011). Plinian columns often contain umbrella regions, meaning the majority of mass

is released from the upper 10% of the column (Courtland 2011). Smaller columns, such as

those present during historic Mt. Ngauruhoe eruptions, do not contain umbrella regions.

This means the mass is released from the entire column, giving a lower plume ratio.

The maximum plausible eruption and the most likely eruption are unlikely to contain a

significant umbrella region, meaning particles will be released from the majority of the

column. This gives a plume ratio for both scenarios of 0.1 (Courtland 2011).

25

4.5 Weather Patterns Above the Central North Island

Wind direction and velocity are two of the most important factors to consider when

modelling volcanic plumes, as they can significantly alter the distribution of tephra across a

landscape (Carey and Bursik 1999).

Input Parameters for Tephra2 Modelling

Most likely eruption Maximum plausible eruption

Plume height (m) 7,000 m (Hobden et al. 2002) 25,000 m (Newhall and Self 1982)

Eruption mass (kg) 1.11x109 kg (Self 1975) 5.98x1012 kg (Gregg 1960)

Max grainsize ( ) -2.5 (Moebis 2010) -2.5 (Moebis 2010)

Min grainsize ( ) 7 (Moebis 2010) 7 (Moebis 2010)

Median grainsize ( ) 1 (Moebis 2010) 1 (Moebis 2010)

STD grainsize ( ) -1.66 (Moebis 2010) -1.66 (Moebis 2010)

Vent easting (UTM) 381803 (Google Earth 2011) 381803 (Google Earth 2011)

Vent northing (UTM) 5664923 (Google Earth 2011) 5664923 (Google Earth 2011)

Vent elevation (masl) 2,200 (Google Earth 2011) 2,200 (Google Earth 2011)

Eddy constant 0.04 (Connor et al. 2011) 0.04 (Connor et al. 2011)

Diffusion coefficient (m2/s) 500 (Courtland 2011) 2,000 (Courtland 2011)

Fall time threshold (s) 100,000 (Connor et al. 2011) 100,000 (Connor et al. 2011)

Lithic density (kg/m3) 2,700 kg/m3 (Self 1975) 2,500 kg/m3 (Self 1975)

Pumice density (kg/m3) 950 kg/m3 (Nairn et al. 1978) 950 kg/m3 (Nairn et al. 1978)

Column steps 100 (Connor et al. 2011) 100 (Connor et al. 2011)

Plume model 0 (Connor et al. 2011) 0 (Connor et al. 2011)

Plume ratio 0.1 (Courtland 2011) 0.1 (Courtland 2011)

Table 1: Input parameters for most likely eruption and maximum plausible eruption for

modelling using Tephra2.

26

Climate data in New Zealand is accessed through the NIWA CLIFLO system, which maintains

the New Zealand National Climate Database. This contains climate records taken throughout

the country and can be accessed, at no cost, online (CLIFLO 2011). Wind data over the

central North Island is collected using three different methods; forecasting, surface wind

observations and upper air observations.

Forecasting is carried out by the New Zealand MetService by using daily radiosonde readings

taken between 20 March 2000 and 20 March 2003 at the windrun forecast location sites

(Figure 7). This creates windrun files, which are text files containing over 9,000 pairs of

modelled data each. In the central North Island these files are available at six locations; Mt

Ruapehu, New Plymouth, Okataina, Taupo, Whenuapai and White Island.

Legend NIWA CLIFLO Upper Air Stations

NZ Volcanoes

Windrun Forecast Locations

Hamilton Airport Radiosonde

Rotorua Airport

Kaitaia Observatory

Kaikohe

Whangarei

Auckland North Whenuapai

Mt Tamahunga

Auckland Field Auckland South

Hamilton White Island

Mayor Island

Rotorua

Paraparaumu

Taupo

Ohakea

New Plymouth

Taranaki Ruapehu

Ngauruhoe Tongariro

Wellington Nelson

Gisborne

Figure 7: Locations of North Island volcanoes, as well as selected NIWA climate data observation sites.

Okataina

27

Surface wind observations have been made at the Rotorua airport every three hours from

1981 onwards. The weather station used is automated, and is located 283 metres above sea

level.

Upper-air observations are made from Hamilton airport using radiosondes, released at 172

metres above sea level. Upper-air wind data has been collected from this location from 1

August 2001 onwards. Wind direction and velocity are measured by radiosondes at ground

level, 1,828 metres, 3,657 metres, 5,486 metres, 7,315 metres and 9,144 metres above

ground level (Figures 8 to 13).

Figure 8: Frequency distribution of annual surface winds in azimuth bins, from selected Windrun and

CLIFLO datasets.

4.5.1 Average Wind Velocity and Direction

Figures 9 to 14 show the frequency distribution of selected Windrun and CLIFLO upper air

data in elevation subsets. Sites used are Ruapehu and Taupo for the Windrun dataset, and

Hamilton for the CLIFLO dataset. These sites are used because of their location in relation

to Mt. Ngauruhoe, and the likelihood that wind at these points will influence the

movements of ash plumes from Mt. Ngauruhoe. These graphs are shown with wind

directions “blowing toward”, rather than the more conventional “blowing from”. This is

because the direction of tephra dispersal is in the direction the wind is blowing, rather than

the direction the wind is coming from.

0

5

10

15

20

25

30

N NE E SE S SW W NW Calm

Freq

uen

cy

Azimuth Bin (Wind Blowing Towards)

Annual Surface Winds

Ruapehu, 0 m

Taupo, 0 m

Hamilton, 0-2500 m

28

Figure 9: Frequency distribution of annual winds at approximately 2,000 m in height in azimuth bins, from selected Windrun and CLIFLO datasets.

Figure 10: Frequency distribution of annual winds at approximately 4,000 m in height in azimuth bins, from selected Windrun and CLIFLO datasets.

Figure 11: Frequency distribution of annual winds at approximately 5,500 m in height in azimuth

bins, from selected Windrun and CLIFLO datasets.

0

5

10

15

20

25

30

35


Freq

uen

cy


Annual ~2,000 m Winds

Ruapehu, 1828 m

Taupo, 1828 m

Hamilton, 2501-5000 m

0

5

10

15

20

25

30

35


Freq

uen

cy



Ruapehu, 3657 m

Taupo, 3657 m


0

10

20

30

40


Freq

uen

cy



Ruapehu, 5486 m

Taupo, 5486 m


29

Figure 12: Frequency distribution of annual winds at approximately 7,000 m in height in azimuth

bins, from selected Windrun and CLIFLO datasets.

Figure 13: Frequency distributions of annual winds at above 9,000 m in height in azimuth bins, from

selected Windrun and CLIFLO datasets.

The wind values from figures 8 to 13 were averaged, producing a series of resultant vector

(RV) values. These represent the vector mean direction the wind is blowing towards within

each dataset. The RV value gives the direction that has the greatest likelihood at any one

time during the year. This also gives the direction that tephra is most likely to be carried

towards (see Figure 14). The Hamilton CLIFLO site values were adjusted because the

elevation of the measuring location is different to that of the Windrun sites. This simplifies

the process of creating an average value at each height for the three sites.

0

5

10

15

20

25

30

35

40


Freq

uen

cy



Ruapehu, 7315 m

Taupo, 7315 m


0

10

20

30

40

50


Freq

uen

cy


Annual >9,000 m Winds

Ruapehu, 9144 m

Taupo, 9144 m

Hamilton, 7501 - 10000 m

30

Figure 14: Annual resultant vector values versus elevation for the three observation sites most

relevant to Mt. Ngauruhoe (Ruapehu, Taupo and Hamilton CLIFLO), as well as the average of these

three sites.

Wind speed varies with elevation above ground level (figure 15). These values were

calculated using two selected windrun datasets (Ruapehu and Taupo) as well as the

Hamilton CLIFLO dataset. Although values from the Hamilton CLIFLO dataset vary

significantly from both the Ruapehu and Taupo Windrun sites, they are retained in the

graphs and calculations. The Hamilton CLIFLO dataset is important because it involves the

physical collection of data. Sites at Ruapehu and Taupo use forecasts, which decrease the

accuracy of the data (CLIFLO 2011).

Figure 15: Annual average wind speed versus elevation for the three observation sites most relevant

to Mt. Ngauruhoe (Ruapehu, Taupo and Hamilton CLIFLO) as well as the average of these three sites.

0100020003000400050006000700080009000

10000

45 67.5 90 112.5 135

Elev

atio

n (

m)

RV Azimuth (Wind Blowing Towards)

Annual Resultant Vector Values

Ruapehu

Taupo

Hamilton adjusted

Average of three sites

0100020003000400050006000700080009000

10000

0 5 10 15 20 25

Elev

atio

n (

m)

Wind speed (ms-1)

Annual Average Wind Speed vs Elevation

Ruapehu

Taupo

Hamilton adjusted

Average of three sites

31

The information in Figures 14 and 15 has been used to evaluate the most likely wind

direction and velocity at different elevations above the central North Island. These values

are required as an input for Tephra2 (Table 2).

Table 2: Average wind speed and direction at different elevations around the central North Island,

using data taken from sites in Ruapehu, Taupo and Hamilton.

No wind data for altitudes >9,144 m is available from the CLIFLO website. As the

temperature in the stratosphere (>10,000 m above sea level) remains relatively constant

(Gaffen 1994), it means that the wind speed and direction remain relatively constant in the

stratosphere, as there are no convective cells to create variation. The data used for the

9,144 m altitude wind speed and direction has therefore been continued to 25,000 m above

sea level, to allow for the maximum plausible eruption plume height of 25,000 m. These

values are shown below, and are required as an input for Tephra2 (Table 3).

Elevation (m)

Wind speed (ms-1)

Wind direction (° from north)

0 7.96 81.78

1,828 9.45 81.07

3,657 11.86 83.47

5,486 15.00 87.13

7,315 17.83 88.86

9,144 19.28 92.07

32

Elevation (m) Wind speed (ms-1) Wind direction (° from north)

0 7.96 81.78

1,828 9.45 81.07

3,657 11.86 83.47

5,486 15.00 87.13

7,315 17.83 88.86

9,144 19.28 92.07

10,973 19.28 92.07

12,802 19.28 92.07

14,631 19.28 92.07

16,460 19.28 92.07

18,289 19.28 92.07

20,118 19.28 92.07

21,947 19.28 92.07

23,776 19.28 92.07

25,605 19.28 92.07

Table 3: Average wind speed and direction at different elevations around the central North Island,

using data taken from sites in Ruapehu, Taupo and Hamilton, including estimations for elevations

above 9,114 m.

4.6 Results from Tephra2 Modelling

The isopach map created using Tephra2 (Figures 16 and 17), can be overlain on a

geographical map to allow visualisation of affected areas (Figures 19 and 20). The isopach

map produced by Tephra2 uses lines of equal mass per unit area measured in kgm-2. This

can be converted to thickness by dividing the mass per unit area by the average density of

the tephra deposit (Table 4).

33

Tephra mass per unit area

Average tephra density

Tephra thickness

1 kgm-2 1,500 kgm-3 6.6x10-4 m (0.66 mm)

10 kgm-2 1,500 kgm-3 6.6x10-3 m (6.6 mm)

100 kgm-2 1,500 kgm-3 0.066 m (6.6 cm)

1,000 kgm-2 1,500 kgm-3 0.66 m (66 cm)

Table 4: Conversion between mass per unit area of tephra and tephra thickness, using an average

tephra density of 1,500 kgm-3.

34

10 1

Figure 16: Map of the North Island of New Zealand, showing isopachs measured in kgm-2, modelled

using Tephra2 for most likely eruption scenario. 1 kgm-2 is equal to 0.66 mm, and 10 kgm-2 is equal to

6.6 mm.

100 km

N

Auckland

New Plymouth

Tauranga

Gisborne

Palmerston North

Napier

Taupo

Waiouru

35

Figure 17: Map of the North Island of New Zealand, showing isopachs measured in kgm-2, modelled

using Tephra2 for maximum plausible eruption. 1 kgm-2 is equal to 0.66 mm, 10 kgm-2 is equal to 6.6

mm, 100 kgm-2 is equal to 6.6 cm and 1,000 kgm-2 is equal to 66.6 cm.

100 km

N

Auckland

New Plymouth

Tauranga

Gisborne

Palmerston North

Napier

Taupo

1 10

100

1000

Waiouru

36

Chapter Five – Potential Impacts from Volcanic Activity at Mt. Ngauruhoe

Coloured isopach maps showing approximate tephra thicknesses for each eruption scenario

were created in order to simplify the results from Tephra2 (Figures 19 and 20). These maps

include the location of major infrastructure that may be affected by an eruption from Mt.

Ngauruhoe.

5.1 Proximal Impacts

Proximal deposits are unlikely to have any long term effects on surrounding infrastructure,

however the surrounding area will be subjected to tephra fall. The most significant danger

to proximal areas in historic records occurred during the 1974 – 1975 eruption sequence,

when metre-sized ballistics were ejected as part of a block and ash flow as far as 2.8 km

from the vent (Hobden et al. 2002). This is far enough from the vent to potentially impact

the Tongariro Crossing, which follows the northern flank of Mt. Ngauruhoe. No buildings or

infrastructure are located on the flanks of Mt. Ngauruhoe, with the exception of the

Tongariro Crossing. GeoNet maintains a number of seismographs surrounding Mt.

Ngauruhoe, however the most proximal to the vent is c. 3 km away, so is not classed as

being in the proximal zone (GeoNet 2011).

5.2 Distal Impacts

5.2.1 Energy Infrastructure

Volcanic tephra has the potential to damage both electricity production systems and

electricity distribution systems. Hydro-generation facilities which are part of the Tongariro

Power Scheme are particularly susceptible to tephra fall from Mt. Ngauruhoe due to their

close proximity. Three 220 kV electricity transmission lines are located c. 12 km east of Mt.

Ngauruhoe (Figure 18), and may also be damaged during an eruption from Mt. Ngauruhoe

(Figures 19 and 20).

5.2.1.1 Electricity Distribution Systems

Electricity distribution systems are susceptible to a number of impacts from tephra fall

including insulator flashover, controlled outages during cleaning of tephra, line breakages or

line tower collapse due to excess weight, and the breakdown of air conditioning systems in

37

substations, caused by the blockage of air intakes. The most common volcanic impacts to

electrical distribution systems are insulator flashover and the breakdown of air conditioning

systems in electricity substations (Wardman et al. 2010).

A number of factors contribute to insulator flashover potential. These include the adherence

of tephra to insulators, the conductivity of the tephra and the insulator dimensions. Tephra

adherence is dependent on insulator orientation and condition, electrostatic charge,

grainsize and moisture content of the tephra, in addition to the weather conditions at the

time of eruption. The conductivity of tephra depends on its moisture content and grainsize,

as well as the concentration of soluble components within the tephra (Wilson et al. 2009). In

general, fine grained, moist or wet tephra falling at thicknesses greater than 5 mm causes

the highest flashover potential. Flashover causes the failure of insulators, which leads to

transmission line failures (Amarh 2001). Three 220 kV transmission lines are located c. 12

km to the east of Mt. Ngauruhoe (Transpower 2010). These contain a number of insulators

which are susceptible to flashover, potentially causing electricity transmission failure to

large areas of the North Island during both the most likely eruption and the maximum

plausible eruption.

Figure 18: 220kV transmission lines to the east of Mt. Ngauruhoe, with Mt. Tongariro to the right of

photo.

N

38

Figu

re 1

9: M

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are

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39

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100

km

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ap o

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40

Based on modelling using Tephra2, the most likely eruption scenario will coat c. 7 km of

transmission lines with more than 5 mm of tephra, affecting c. 23 insulator sites. The

maximum plausible eruption will coat c. 25 km with more than 5 mm of tephra, affecting c.

83 insulator sites.

Air conditioning systems are used extensively to keep electrical equipment cool at electricity

distribution points such as substations. These systems are vulnerable to damage during

tephra fall, through corrosion and the blockage of air filters. Increased air speed through the

condenser reduces susceptibility to blockage from tephra, however modern systems use

slower air speed in order to reduce noise pollution, which increases the potential for

blockage (Wilson et al. 2009). The most likely eruption scenario will cause tephra to fall on

at least 200 km2 of land, and the maximum plausible eruption at least 5,000 km2 of land,

both affecting large numbers of substations in the rural central North Island.

5.2.1.2 Electricity Production Systems

The Tongariro Power Scheme uses a series of lakes, canals and tunnels to bring water from

the catchment area to the Tongariro River, which provides flow for the Rangipo and

Tokaanu Power Stations, before discharging into Lake Taupo. The Tongariro Power Scheme

supplies c. 4% of the electricity production capacity of New Zealand (Genesis Energy 2010).

During the 1995-1996 eruption from Mt. Ruapehu, which is located 16 km SSW of Mt.

Ngauruhoe, c. 7.6 million cubic metres of tephra was deposited on the Tongariro Power

Scheme catchment. It has been estimated that 40% of the minerals from these eruptions

had a hardness between 5 and 6.5 on the Mohs scale of hardness, which is equivalent to

between glass and a steel working file. The tephra moved down the catchment, and the

grain size was small enough to become suspended sediment within the Tongariro River.

Because of its hard nature, this caused substantial abrasion to components of the Tongariro

Power Scheme, in particular the Rangipo Power Station.

In the seven months between September 1995 and April 1996, c. 15 years of normal life was

worn from the two 60 megawatt turbines in the Rangipo Power Station. A number of other

components, including penstock priming butterfly valves, were also damaged following this

eruption as a result of abrasion by tephra suspended in water (Figures 21 and 22).

41

Similar volumes of tephra can be expected from a large eruption from Mt. Ngauruhoe,

which may cause damage to the Tongariro Power Scheme similar to that seen during the

Mt. Ruapehu eruption. The most likely eruption modelled using Tephra2 will deposit c.

0.62x106 m3 of tephra on the Tongariro Power Scheme catchment, and the maximum

plausible eruption will deposit c. 9.97x108 m3 of tephra on the Tongariro Power Scheme

catchment. These were calculated using the Rangipo catchment area (Figures 19 and 20),

and by determining the volume of tephra deposited in this area (Section 4.4.1). Either of

these deposits have the potential to cause damage to a similar scale as seen following the

1995 – 1996 Mt. Ruapehu eruption.

Following the 1995-1996 eruption from Mt. Ruapehu, Genesis Energy initiated a number of

mitigation measures to protect against damage from further eruptions. These include

shutting down power generation when the amount of suspended volcanic tephra in the

Tongariro River causes excessive wear to components, retaining the tephra close to source

by using sediment traps and retention dams, diverting the water sources which are carrying

suspended volcanic material, separating the tephra before it reaches the Rangipo Dam, and

creating a sediment trap in the Waihaha pipe bridge (Malcolm et al. 1997). Damage caused

by potential further eruptions from either Mt. Ngauruhoe or Mt. Ruapehu will take longer to

occur because of the permanent mitigation measures carried out by Genesis Energy.

5.2.2 Transport Infrastructure

A number of transport networks including roads, railways and air transport may be

adversely affected by distal impacts from an eruption at Mt. Ngauruhoe.

The road network through the central North Island provides important transport links for

both economic and social activities. During natural disasters, such as volcanic eruptions,

road networks are vital for allowing rapid evacuation, access to important infrastructure and

the movement of emergency services. State Highway (SH) 1 travels the length of New

Zealand. The Desert Road is the section of SH1 between Rangipo and Waiouru, which is

located approximately ten kilometres east of the Tongariro Volcanic Complex (Figure 23).

42

Figure 22: Damaged cheek plate from

the Rangipo Power Station after seven

months exposure to suspended volcanic

tephra within the Tongariro River

(Meredith 2002).

A C

Figure 21: (A and B) Penstock priming

butterfly valve taken from the Rangipo

Power Station after seven months exposure

to suspended volcanic tephra within the

Tongariro River (Kieran Devine pers. comm.

2011). (C) New penstock priming butterfly

valve (Casting Quality 2011).

B

43

The Desert Road has an average daily traffic volume of approximately 3,429 vehicles. This

section of SH1 contains an average heavy traffic volume of 17.1% and is a critical freight link

(New Zealand Transport Agency 2011). It is estimated that the closure of the Desert Road

costs the New Zealand economy c. $11,000 per hour, and if both SH1 and SH47 (on the

opposite side of the Tongariro Volcanic Complex) are closed, c. $32,000 per hour (Dalziell

1998, modified to account for inflation). The modified values are likely to be

underestimated, as reliance on SH1 has increased (New Zealand Transport Agency 2011).

Tephra thicknesses greater than 5 mm cause dangerous driving conditions such as reduced

visibility and significant loss of traction (Barnard 2003, Blong 1984), while thinner deposits

cause minor disruption, making road markings difficult to see and lowering traction. Ash

deposits as thin as 1 mm can obscure or completely cover road markings, making driving

hazardous. Constant traffic flow would stir up the ash, eventually allowing it to move to the

sides of the road (Johnston et al. 2000).

The most likely eruption scenario modelled using Tephra2 does not cover any roads with

more than 5 mm of tephra, so will have minimal impact on safety. Approximately 8 km of

Figure 23: Map of Tongariro Volcanic Complex showing proximity to State

Highway 1 (east) and State Highway 47 (northwest).

10 km

N

44

State Highway 1 will be covered by <5 mm of tephra, which may obscure road markings

along this section. The thin deposit means this will be easily cleared, causing only minor

disruption to motorists. The maximum plausible eruption scenario modelled using Tephra2

has a more significant impact to roads. Approximately 14 km of SH 1 will be covered by

more than 5 mm of tephra, reducing visibility and causing loss of traction, as well as

covering road markings. This is the only major road which will be affected to this level

following an eruption from Mt. Ngauruhoe under the conditions used in modelling.

Approximately 300 km of major roads, including SH1, SH2 and SH5 will be covered with

<5mm of tephra, which may obscure road markings.

Volcanic tephra can also cause physical damage to vehicles. Because it is often very fine,

tephra is able to infiltrate into vehicles, causing abrasion and scratching of surfaces and

moving parts. Air filters become clogged easily, leading to overheating of engines and

eventually engine failure. Regular oil changes, cleaning of filters and removal of tephra helps

protect vehicles from long term damage (Federal Emergency Management Agency 2011).

Rail networks are less susceptible to volcanic tephra fall than road networks. No significant

hazard exists, however poor visibility and breathing problems for train crews can occur.

Tephra can also cause damage to train engines and other equipment, leading to increased

wear, and can occasionally cause the shutdown of rail services. Volcanic tephra can also lead

to short circuiting of signal equipment, causing additional danger. Following the Mount St.

Helens eruption in 1980, ten trains in western Montana, 625 km north of Mt St. Helens,

were shut down for nearly 24 hours due to tephra fall on the railway tracks (Blong 1984).

Major rail lines in the central North Island run to the west of Mt. Ngauruhoe, following SH4

(Figure 23). Under the conditions used for modelling with Tephra2, this section of rail will

not receive any tephra fall. Minor rail lines are located on the eastern coast of the central

North Island, however these will receive <1 mm of tephra so are unlikely to be impacted.

Air transport networks are at significant risk from volcanic eruptions. Potential effects

include disruption of flights, physical damage to aircraft and damage to facilities. The most

common effect is temporary shutdown of operations while the ash cloud disperses.

45

Disruption to flights may occur either because of tephra on the runway and surrounding

area, or because of the location of the ash plume in the atmosphere. Tephra on the runway

may cause difficult landings due to reduced runway friction, as well as loss of visibility when

engines stir up the tephra (Barnard 2009). Aircraft are unable to fly through ash

concentrations greater than 2x10-3 g/m3, as it begins to melt in the combustor and redeposit

in the high pressure turbine. This increases pressure on the engine, leading to an engine

surge or blockage (Sixsmith 2010). As a result of the potential danger to aircraft flying

through ash clouds, most airlines choose not to fly in the vicinity of volcanic ash.

During the 1995 – 1996 Mt. Ruapehu eruption at least thirteen airports were disrupted,

even though only eight of these received any tephra fall. This was because of the closure of

airspace by the Wellington Volcanic Ash Advisory Centre, causing the cancellation or re-

routing of a number of flights (Barnard 2009).

The only airport which will be exposed to tephra fall during an eruption from Mt. Ngauruhoe

is Napier airport, which has an average daily schedule exceeding 40 aircraft movements.

Airspace across the central western North Island will be restricted by both the most likely

eruption and the maximum plausible eruption under the wind conditions used for modelling

with Tephra2.

5.2.3 Water Supply Infrastructure

Volcanic tephra has three main impacts on water supply infrastructure. Firstly, tephra can

physically disrupt or damage water sources, as well as treatment and distribution systems.

Secondly, tephra can change the physical and chemical characteristics of surface water. And

thirdly, a high demand for water resources for clean-up following an ash fall may lead to

water shortages (Wilson et al. 2011).

Tephra can block filters at water intakes, reducing the supply of water to affected

communities. During the 1945 eruption from Mt. Ruapehu, several millimetres of tephra

was deposited across the North Island. The Whanganui River is the source of drinking water

in the town of Taumarunui. Filters at the water supply intakes became blocked, reducing

pumping rates from 90,000 litres per hour to 31,500 litres per hour (Johnston 1997). Ash can

46

also damage components of water distribution systems such as pumps, switchboards, filters

and motors (Wilson et al. 2011).

The contamination of water supplies by tephra generally breaches drinking New Zealand

drinking water standards for aesthetic determinands before breaching the equivalent

standards for health determinands (Johnston et al. 2004). This means that water will

become undrinkable because of its taste prior to becoming unsafe to drink. Volcanic tephra

is known to leach over 55 soluble components into surface waters (Delmelle et al. 2007). A

number of leachates can cause health issues, however the main toxic element adsorbed on

volcanic ash is fluoride. Fluoride levels in water higher than the recommended guidelines of

1.5 mg/l are associated with an increased risk of dental fluorosis, and levels greater than 10

mg/l can lead to skeletal fluorosis (Ministry of Health 2005; Witham et al. 2005).

Acidification of water may also occur, due to the presence of acids such as H2SO4, HCl and

HF. Most surface waters contain sufficient alkalinity to buffer against significant pH change

so acidification does not often occur (Wilson et al. 2011).

Water resources are often placed under heavy demand for clean-up purposes following an

ash fall. This has follow on effects on services such as fire fighting, and can also lead to

shortages of water for hygiene, sanitation and drinking (Wilson et al. 2011).

The most likely eruption scenario will deposit tephra across at least 200 km2 of land, and the

maximum plausible eruption at least 5,000 km2 of land. These events could potentially

impact water supply infrastructure in a number of small towns, each of which have fewer

than 100 residents. The quantity of tephra deposited is not likely to be great enough to lead

to significant health issues in most areas. Towns which source drinking water from the

surface may be susceptible to increased contamination, especially in areas with significant

tephra fall, such as Te Haroto (population 60), Putorino (population 57) and Tutira

(population 87) (Statistics New Zealand 2006). It is not known whether these areas source

drinking water from the surface or from ground water.

5.2.4 Telecommunication Infrastructure

In the event of a volcanic eruption causing tephra fall, the ability to communicate is

extremely important to risk managers, such as Civil Defence and GNS Science. The

47

telecommunication network in New Zealand includes radio, television, mobile phones and

landlines for both voice and data. Methods of transmission include hard wiring, radio waves,

microwaves, fibre optics, the internet and satellites.

The most common impact of volcanic tephra on telecommunications is overloading of the

network. This is because of the large number of people trying to contact each other at the

same time due to widespread alarm. Physical damage to infrastructure through the

overloading of tephra is uncommon, with the exception of telephone lines which may

collapse under excess weight. All telecommunications equipment has the potential to be

damaged by falling trees and branches due to tephra overloading (Barnard 2009).

Roadside exchange cabinets and cellphone exchanges are unlikely to be damaged by tephra

overloading, however the attached air conditioning units are susceptible to damage by

tephra fall, which may lead to failure of the exchange cabinet. This occurs when the

compressor of the air conditioning unit fails, causing the internal temperature of cabinets to

rise above 30°C within minutes. Once 30°C is reached, automatic fans begin drawing in

cooler air from outside. These fans cannot be controlled remotely, so must be switched off

on site. The outside air which is sucked into the cabinets brings fresh tephra, which can

cause bridged connections, short circuits and the eventual corrosion of internal

componentry (Barnard 2009).

The most likely eruption scenario modelled using Tephra2 is unlikely to have any impact on

telecommunications, with the exception of overloading of the network because of

widespread panic. The maximum plausible eruption scenario modelled using Tephra2 is

likely to deposit tephra to a sufficient thickness to cause air conditioning units to fail.

5.2.5 Tourism

Impacts to tourism from a volcanic eruption are wide ranging. Tourists are unlikely to want

to travel to an area which is being subjected to a volcanic eruption, meaning a lower

number of tourists will visit. Even if tourists still chose to travel, the presence of an ash

plume in the air and tephra on the ground may limit the transport options available. Impacts

to actual tourist destinations may also occur.

48

The most significant tourist destination which is likely to be affected following an eruption

from Mt. Ngauruhoe is the Tongariro Crossing. Up to 12,000 people walk this track each

month (Martelli 2007). A number of other tourist activities are located in the vicinity of Mt.

Ngauruhoe including fishing, jet boating, hunting, mountain biking and rafting.

Modelling carried out using Tephra2 shows that for the most likely eruption scenario, parts

of the Tongariro Crossing will receive less than 1 cm of tephra. For the maximum plausible

eruption scenario, some parts of the Tongariro Crossing will receive between 5 cm and 50

cm of tephra while others will receive only trace levels.

5.2.6 Residential, Commercial and Industrial Property

No significant residential, commercial or industrial property exists to the east of Mt.

Ngauruhoe. Several small townships (<100 residents) are located on SH 5, c. 80 km east of

Mt. Ngauruhoe.

Roof collapse occurs when the weight of tephra is great enough to cause structural failure.

Flat, long span roofs are most vulnerable to collapse through tephra overloading. Weak

structures such as barns and sheds may collapse under less than 100 mm of tephra,

especially if the tephra becomes moist during or following deposition (GNS Science 2010).

Between 100 mm and 300 mm of moist tephra may cause the buckling or collapse of flat

roofed buildings (Wilson et al. 2009). Major roof collapse occurs with the deposition of over

300 mm of moist tephra (GNS Science 2010).

The modelling carried out using Tephra2 shows that the most likely eruption scenario will

not deposit a sufficient thickness of tephra to cause roof collapse. The maximum plausible

eruption will deposit a tephra thickness greater than 100 mm across c. 1,000 km2, however

no buildings of any significance exist in the area which will receive this thickness of tephra.

Clean-up of urban areas following tephra fall is important in order to minimise damage to

infrastructure and property. Tephra must be removed from roofs as soon as possible to

avoid the potential for roof collapse. By moistening thick tephra deposits, the likelihood of

remobilisation by wind reduces, decreasing the time required for clean-up.

49

5.2.7 Other Infrastructure

Other areas of infrastructure include recreational, social, economic, solid waste

management and governance. An eruption from Mt. Ngauruhoe is likely to have little

impact on the majority of these facilities, with the exception of the Rangipo Prison.

The Rangipo Prison is located c. 18 km northeast of Mt. Ngauruhoe, and has a capacity of

600 minimum to low-medium security prisoners as well as 251 staff members. During a

significant eruption from Mt. Ngauruhoe this facility may need to be evacuated, as there is

potential for roof collapse if more than 100 mm of moist tephra is deposited.

The maximum plausible eruption will deposit between 1 cm and 0.5 cm of tephra on the

Rangipo Prison, which is not a great enough thickness to cause roof collapse. If the wind

direction used for modelling was towards the north-east rather than towards the east, the

Rangipo Prison would receive between 5 cm and 50 cm of tephra. If enough water was

contained within the tephra, the thickness would be great enough to potentially cause roof

collapse. Details on the engineering of the Rangipo Prison are unavailable, however it is

assumed in this study that it was engineered to a higher level than normal residential

buildings to assist in the prevention of prisoners escaping.

50

Type of

infrastructure

Most likely eruption scenario Maximum plausible eruption

scenario

Electricity

distribution systems

c. 7 km of 220 kV transmission lines

will be coated with more than 5 mm

of tephra, potentially causing insulator

flashover at 23 insulator sites. c. 200

km2 of land will be covered by tephra,

causing air conditioning in numerous

substations to fail.

c. 25 km of 220 kV transmission lines

will be coated with more than 5 mm of

tephra, potentially causing insulator

flashover at 83 insulator sites. c. 5,000

km2 of land will be covered by tephra,

causing air conditioning in numerous

substations to fail.

Electricity

production systems

Damage to components of the

Tongariro Power Scheme will be

caused by tephra falling in the

catchment area. c. 0.62x106 m3 of

tephra will fall on the catchment area.

Significant damage to components of

the Tongariro Power Scheme will be

caused by tephra falling in the

catchment area. c. 9.97x108 m3 of

tephra will fall on the catchment area.

Transport c. 8 km of SH1 will be covered with

less than 5 mm tephra, obscuring road

markings. Rail networks are unlikely to

experience any impacts. Air transport

networks will not experience direct

impacts, however airspace will be

affected causing disruption to air

traffic.

c. 14 km of SH1 will be covered with

more than 5 mm tephra, causing

dangerous driving conditions. c. 300 km

of major roads including SH1, SH2 and

SH5 will be covered with less than 5 mm

tephra, obscuring road markings. Rail

networks are unlikely to experience any

impacts. Air transport networks will

experience few direct impacts, with the

exception being Napier airport. Airspace

will be affected, causing disruption to

air traffic.

Water supply Minimal impacts to drinking water will

occur.

Some impacts to drinking water may

occur, particularly where water is

sourced from the surface rather than

from the ground.

Telecommunications Telecommunications will not be

significantly impacted. The most likely

impact will be overloading of the

network due to widespread alarm.

Telecommunications may be impacted

where tephra is thick enough to cause

air conditioning units to become

blocked. This occurs somewhat by

chance, as factors such as the direction

the unit is facing relative to the eruption

will determine the accumulation of

tephra. Overloading of the network due

to widespread panic will also occur.

51

Tourism The Tongariro Crossing will have up to

1 cm of tephra fall on some sections. It

is difficult to assess other areas of

tourism, as they are dependent on

other factors such as transport

availability.

The Tongariro Crossing will have

between 0.5 cm and 50 cm of tephra

along its entire length. Other areas of

tourism will be significantly impacted,

as transport around the region will be

substantially decreased.

Residential,

commercial and

industrial

Tephra thickness will not be great

enough to cause roof collapse. Clean-

up of tephra from urban areas will be

required in some cases.

No buildings of any significance are

located within the area receiving the

greatest thickness of tephra. Buildings

in a 1,000 km2 area will receive tephra

thicknesses greater than 100 mm,

which, when moist, is thick enough to

cause roof collapse in poorly

constructed buildings.

Other Other areas of infrastructure will

receive little impact from tephra fall.

The Rangipo Prison will receive

between 0.5 cm and 1 cm of tephra,

which is not a great enough thickness to

cause roof collapse. A change in wind

direction may significantly alter the

dispersal of tephra over the Rangipo

Prison. Up to 50 cm of tephra may be

deposited if the wind direction was

towards the north-east rather than

east.

Table 5: Overview of risk to each type of infrastructure from tephra fall.

52

Chapter Six – Discussion and Conclusions

Impacts from a future volcanic eruption at Mt. Ngauruhoe have the potential to affect a

widespread area and a number of corridor infrastructure networks and urban areas across

the central North Island. The most likely eruption scenario modelled using Tephra2 will

deposit up to 1 cm of tephra over an area of >200 km2. This will have a noticeable impact on

electricity production, electricity transmission and transport. There will be lesser effects on

the tourism industry and water supply, as well as residential, commercial and industrial

property including agriculture and forestry. The maximum plausible eruption scenario

modelled using Tephra2 will have a more widespread impact on a number of different

areas. An area of >5,000 km2 will receive between 0.5 mm and 50 cm of tephra. This will

have a large impact on electricity production, electricity transmission, transport,

telecommunications and tourism. There will be smaller but still noticeable impacts on water

supplies, as well as residential, commercial and industrial property.

Modelling using Tephra2 was carried out using the most likely wind direction and speed. The

values used occur on average between 26% and 48% of the time, based on data collected at

the Ruapehu, Taupo and Hamilton weather monitoring sites. If an eruption from Mt.

Ngauruhoe occurred under different weather conditions, results from Tephra2 would

change. The plume would remain of a similar size, however the direction of the plume from

the vent would vary along with areas affected by ash fall.

The most likely eruption scenario has the potential to distribute >0.5 mm tephra thickness

up to 50 km from the vent in any direction. This would impact a number of towns including

Waiouru (population 1,383), Ohakune (population 1,101), Taumarunui (population 5,055)

and Turangi (population 3,240) (Statistics New Zealand 2006). Trace levels of tephra would

be distributed up to 100 km from the vent. Taupo (population 22,300) (Statistics New

Zealand 2006) is the only main centre that may receive trace levels of tephra from the most

likely eruption scenario under different weather conditions to those modelled.

The maximum plausible eruption scenario has the potential to distribute tephra at

thicknesses between 0.5 cm and 0.5 m up to 250 km from the vent in any direction. This

would impact several major towns and cities, including Taupo (population 22,300), Gisborne

(population 34,000), Rotorua (population 65,900), Hamilton (population 143,000), Tauranga

53

(population 120,000), New Plymouth (population 69,000), and Wellington (population

195,000) (Statistics New Zealand 2006).

Historic records from Mt. Ngauruhoe are only c. 170 years old, out of a total age of c. 7,000

years. Prehistoric records from Mt. Ngauruhoe are poorly constrained because of its recent

formation and lack of deeply incised drainage channels. Some deposits from prehistoric

eruptions have been identified in road cuttings, although many of these also contain

deposits from other volcanoes. The Mangatawai Formation is one of these deposits, and is

thought to have occurred during the period of most vigorous growth in the history of Mt.

Ngauruhoe. This deposit was used as a basis for the maximum plausible eruption from Mt.

Ngauruhoe. The Mangatawai Formation was deposited between 2,568 ± 508 cal. years BP

(Fergusson and Rafter 1959) and 1,717 ± 13 cal. years BP (Lowe et al. 2008). The Papakai

Formation was formed from by both Mt. Ruapehu and Taupo Caldera, and the Mangamate

is older than the first deposits from Mt. Ngauruhoe.

Future research on hazards from Mt. Ngauruhoe should look at increasing the number of

wind directions and other variables used for modelling, perhaps by using the Monte Carlo

method. This will give a more accurate representation of the eruption which is most likely to

occur. The limited time available meant only two eruption scenarios could be modelled.

Future work should expand on this, with a particular focus on better understanding the

potential dispersal of tephra in relation to wind speed and direction. This study was only

able to use the most common wind values, rather than looking at a number of different

wind speed and directions.

54

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