The August 2006 Eruption of Home Reef,
Kingdom of Tonga: Report on Activity
an d Volcanic Hazards Assessment
By
Paul W Taylor
Australian Volcanological Investigations
PO Box 291
PYMBLE NSW 2073 AUSTRALIA
AVI Occasional Report No. 06/04
(Revised 27 February 09)
Australian Volcanological Investigations
Eruption of Home Reef August 2006
ABSTRACT
Home Reef is an active submarine volcano in the central part of the Tofua
Volcanic Arc (TVA). Since it was first observed in the mid-1850s four, possibly
five periods of activity have occurred. One previously unreported period of
activity in June 1990 is also reported here. Islands have been formed during the
reported periods of activity but due to the unconsolidated nature of the
pyroclastic material that has formed the islands they have not persisted for
more than a few months following the cessation of activity.
The most recent eruption occurred in early-August 2006. The initial submarine
phase culminated in the formation of an extensive pumice raft, with strandings
of pumice occurring in northern Tonga in mid-August, much of southern and
central parts of Fiji by mid-September, southern and central Vanuatu by earlyNovember and the eastern Australian coast by early-March 2007. An island
composed of unconsolidated pyroclastic material was formed, but ocean swells
have not allowed it to persist. On formation the island measured 800 x 400 m,
but by early-November it was circular, 400 m in diameter. By early-December
there was a further reduction in size to a circular island 300 m in diameter. By
mid-February 2007 a partially submerged shoal of reworked pyroclastic material
was present at the site.
A preliminary hazard assessment has been completed and reported herein.
Due to the lack of a dedicated volcano monitoring system, local Tongan
scientists did not record an eruption. Activity was only reported some time after
the event by passing yachts. This eruption has emphasised the need for the
establishment of a dedicated volcano-monitoring program within the Kingdom of
Tonga.
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CONTENTS
Abstract
ii
Contents
iii
Introduction
1
Location
1
Form and Structure
2
Previous Volcanic Activity
2
March 1984 Activity
2
June 1990 Activity
4
August 2006 Eruptive Activity
4
Eruptive Activity
The Island
The Pumice
Volcanic Hazards
Specific Hazards
Hazards Zones
4
6
9
11
11
12
Conclusions and Recommendations
13
Acknowledgments
14
Bibliography
14
Selected Websites
15
List of Figures
Figure 1 – Locality Map
Figure 2 – The 1984 Island
Figure 3 – 1984 Pumice in Suva Harbour
Figure 4 – June 1990 Submarine Activity
Figure 5 – Initial Submarine Activity and Pumice 08 August 2006
Figure 6 – Subaerial Activity 12 August 2006
Figure 7 – Island on 04 October 2006
Figure 8 – Island Morphology 04 October and 14 November 2006
Figure 9 – Island Morphology 07 December 2006
Figure 10 – Home Reef 18 February 2007
Figure 11 – The Home Reef Pumice
Figure 12 – Hazard Map for the Home Reef Volcano
1
3
3
4
6
6
7
8
8
9
9-10
13
List of Tables
Table 1 – Volcanic Activity at Home Reef
Table 2 – Reported/Observed Activity during/after the 2006 Eruption
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Eruption of Home Reef August 2006
HOME REEF VOLCANO
Catalogue of Active Volcanoes No. 0403-08
INTRODUCTION
Numerous reports of pumice strandings along the coast of islands in the Fiji region during
late-October to November 2006 suggested the probable occurrence of volcanic activity to
the south-east (Anon. 2006a, b, c and 2007a). A report published in the local press in the
Kingdom of Tonga in early-November confirmed that an eruption at Home Reef
commenced during early-August. It was reported that submarine activity was actively
supplying pumice to the surface. Further reports indicated that cone of pyroclastic material
had formed above the surface but it was reduced in size very quickly by the ocean swell.
This report will draw together all reports and photographs of the activity. It will also provide
a preliminary volcanic hazard assessment of the activity.
LOCATION
Tonga Islands, at the location of Home Reef, midway between Metis Shoal and Late
Island, 20kms north north-east of Metis Shoal at latitude 18o59.5’South, longitude
174o46.3’West (Figure. 1). This volcanic centre forms part of the Tofua Volcanic Arc
(TVA). Richard (1962), Simkin and Seibert (1994) and Taylor and Ewart (1997) have
indicated that previous periods of activity have occurred during the past 200 years.
FIGURE 1: Locality map of the Vava’u region (after Taylor, 2002), with the Tonga Platform
(to the east) and the Tofua Volcanic Arc (to the west), showing the site of the recent
(August 2006) volcanic activity at Home Reef. The symbols indicate active centres ( - ),
ie. with recorded eruptions; inactive centres ( + ), ie. with no recorded activity, and
probable submarine centres ( . ). Bathymetric contours are in kms.
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FORM AND STRUCTURE
Home Reef is the summit of a largely submarine strato-volcano. No reports are known that
suggest that an island with longevity has ever been formed during its short observed
history. The island formed during the 1984 eruption reached a height of 50 m above sea
level. Although no accurate surveys of the shoal have been conducted, the summit may
consist of a broad platform on which eruptive activity has formed several submarine cones
and dome-like structures. During periods of quiescence, a shoal of less than 50m depth
has been present.
PREVIOUS VOLCANIC ACTIVITY
At least 3, possibly 4 periods of activity have recognised since the mid-1800s. Known
reports relating to previous activity are given in Table 1.
TABLE 1: VOLCANIC ACTIVITY AT HOME REEF
(after Taylor and Ewart, 1997)
Year
Month/Date
Character of Activity
1852
Submarine explosive activity; island formed; no other details known.
1857
Submarine explosive activity?; island formed; no other details known.
1984
Mar 2
Mar 3
Mar 4
Mar 5
mid-Mar
Mar 23
early-Apr
1990
Jun 21
Intense submarine explosive activity reported; eruption column ejected
to heights in excess of 7.5 km; pumice rafts moved 60 km to the N and
NE and to the SW; discoloured water also extended over a wide area.
Explosive activity continues; island reported; eruption column being
ejected from a submarine vent within a horse-shoe shaped island
breached to the E; eruption column ejected to a height in excess of 12
km which was dispersing to the NE.
Explosive activity continues; eruption column being ejected ''high above
the sea surface"; pumice rafts were observed but no island was present.
Explosive activity declining; activity confined to weak steam emissions;
two small islands composed entirely of pyroclastic material with a
maximum height of 20 m were present enclosing a crater 1500 x 500 m.
Pumice rafts reported to the N and NW of Tonga and in the Lau Island
group.
Mild fumarolic/steaming activity continues; an island composed of
pyroclastic material observed; dimensions estimated to be 1500 x 500
m, with cliffs 30-50 m high; discoloured water extended to the NW
indicating that further submarine activity may have been persisting;
widespread haze noted to be present in the area of the volcano.
Low level of activity was continuing; no further reports of activity are
known after this time.
Aerial photographs suggest that submarine activity may have been
occurring at Home Reef. A small submarine plume appeared to be
present at the summit region at the time when the photograph was
taken. A very diffuse plume also appears to be dispersing to the
northwest. No other reports of subaerial activity are known.
Little is known about the activity reported of 1852 and 1857 (Richard, 1962); however, this
activity was probably similar in character to recent eruptions, but the two reports given in
table 1 may have been observations of different phases of a single eruptive period.
March 1984 Activity
The eruption that occurred in early-March 1984 (Anon, 1984a, b, c), produced a small
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island (Fig. 2) composed of pumice and other volcanic debris that may have existed for
only several weeks after the cessation of the activity. It is not known how long the activity
persisted as no further observations were made following those reported in table 1.
During this eruption significant amounts of pumice were produced, being reported at
numerous locations along the Fiji coast. Pumice was reported to have accumulated in
Suva harbour (Fig. 3) and along southern the coast of Viti Levu (Ryan, 1986).
Figure 2: The island formed during the March 1984 eruption of Home Reef. Note the significant
amount of discoloured water around the island and the still steaming crater lake (photo by P.
Shepherd, RNZAF).
Figure 3: Pumice from the 1984 eruption of Home Reef accumulating in Nabukalou Creek in
central Suva (photo by P. Ryan).
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JUNE 1990 ACTIVITY
Examination of an aerial photograph taken on 21 June 1990 (Fig. 4) suggested that
submarine activity was occurring at Home Reef. A small submarine plume appears to be
present around the summit region at the time when the photograph was taken. A very
diffuse plume also appears to be dispersing from the summit in a north to northwesterly
direction. No further reports of this period of activity are known.
Diffuse
plume
Summit of Home Reef
Figure 4: Aerial photograph of Home Reef taken on 21 June 1990 (photo
Government of Tonga, 1990). Note the plume-like feature that may be a submarine
eruption plume. A diffuse plume appears to be dispersing to the north and
northwest of the summit region.
AUGUST 2006 ERUPTIVE ACTIVITY
Information on the activity that occurred at Home Reef during August 2006 has been
drawn from a number of sources. Reports of activity and the associated phenomena have
been summarised in Table 2.
It is unknown when activity commenced, but based on reports activity may have
commenced sometime prior to 08 August when the initial phenomena was observed. No
eruptive phenomena, submarine or subaerial, was noted on MODIS images acquired on
05 August. A MODIS image taken at 10:20 (local) on 08 August (Fig. 5) indicated that an
eruption column was present, and pumice was being actively produced at the surface.
Although activity may have reached the surface during the morning of 08 August,
submarine activity may have been occurring for sometime prior to the initial observations.
Eruptive Activity
Initial reports of eruptive phenomena were received from Allan Bowe from Vava’u. He
reported that during the afternoon of 08 August, he heard a “continuous thunder rumbling”
and on the morning of 09 August observed “a huge plume of smoke and cloud” being
ejected. Bowe’s reports suggest that strong subaerial activity was occurring during the
morning of 09 August.
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TABLE 2: REPORTED/OBSERVED EVENTS DURING AND FOLLOWING THE 2006
ERUPTION OF HOME REEF VOLCANO
Year
Month/Date
Activity/Phenomena
2006
Mar 12
Seismic activity; Low to medium T-wave activity was recorded during a 12 hour period beginning
on 07:00 (local). 52 events were recorded during this period.
Aug 05
No activity observed on MODIS image
Aug 08
Submarine explosive activity: At 10:20 Local time (21:20UT 07 Aug) eruption plume and pumice
raft being produced from a submarine vent on MODIS image. Pumice raft was roughly circular,
2
about 10km in diameter and covering an area of about 80km . The eruption column was being
dispersed toward the SSW. Late in the afternoon a continuous rumbling like thunder was heard
from Vava’u. During the evening a red glow could be seen on the horizon.
Aug 9
Submarine/subaerial? explosive activity: continuous rumbling like thunder to the south (of Vava’u),
large mushroom shaped eruption plume was being ejected above the vent in the morning.
Aug 10
Submarine/subaerial? explosive activity: continuous rumbling like thunder to the south (of Vava’u),
large eruption plume being ejected above the vent.
Aug 11
Submarine/subaerial? explosive activity: continuous rumbling like thunder to the south (of Vava’u),
large eruption plume being ejected above the vent. Pumice rafts reported to the north of Late island
by the yacht Maiken on the afternoon of Aug 11. Pumice rafts/hydrothermal plumes observed on
MODIS images.
Aug 12
Subaerial explosive activity: eruption column moving to the northwest reported by crew of the yacht
Maiken. Surtseyan explosive jets emitting projectiles that fell to the sea around the vent area.
Multiple peaks had formed around a central crater that was open to the sea on one side. The island
was composed of “black coal” and was about 1 mile in diameter. Pumice rafts were moving away
from the island heading north and northwest. Pumice rafts observed on MODIS images.
Aug 13-22
Aug 26
Aug 30
Pumice reported in the vacinity of the Lau Islands, Fiji
Pumice strandings reported on Mounu Island, Vava’u.
Sept 14
Pumice strandings reported on Tavenui, Fiji.
mid-Sept
Island reported present at the site of Home Reef by Tongan fishing vessel.
Sept 20
A “strong sulphur odour” noted by a yacht passing to the west of the volcano.
mid-Sept
Pumice strandings reported in various locations in northern and southern Fiji.
early-Nov
2008
Island present with hydrothermal plumes being dispersed to the north and then to the northwest on
MODIS image. Pumice rafts had dispersed to the north and north west towards Fiji.
Sept 02
Oct 04
2007
Pumice rafts to the north and northwest of Late Island observed on MODIS image.
Minor submarine hydrothermal activity: several submarine hydrothermal plumes observed on
ASTER images. The island present was oval in shape, with the long axis orientated in a NE-SW
2
direction. The island measured about 800 x 400m and covered an area of 0.23km . Several small
lakes were present within the island.
Pumice strandings reported on Efate, Vanuatu.
Nov 12
No activity observed on ASTER images. Island was still present but with no crater lakes. The island
2
was triangular in shape and covered an area of 0.146km .
Nov 14
Mild subaerial explosive activity; an island was observed by a Tongan Defence Force patrol boat at
a location 12nm south of Late. The island was reported to be approx 40m high and roughly 300m
square. The island looked quite consolidated with pumice on the outside while the centre looked
“more substantial”. Although not in eruption, a plume of smoke/steam was being emitted.
Dec 07
Minor hydrothermal activity: An overflight by a RNZAF Orion reported that a roughly circular island,
o
o
450m in diameter was present at Home Reef. The position was reported as 18 59.4’S 174 45.4’W.
The crew estimated the height as “up to 240ft” (75m). Minor fumarolic activity was occurring in the
crater and there was a noticeable smell of sulphur. Sediment/hydrothermal plumes were present in
the surrounding waters.
Feb 18
No activity observed; Home Reef visited by scientists from the UK and Australia. They noted that
only a small “pumice-mound” was present. The mound measured 75m in diameter with a maximum
height of 5m and was composed of reworked pyroclastic material and was commonly awash with
the prevailing waves. Large blocks >1m were on the southern side of the mound. No hydrothermal
was observed but a strong sulfur was noted downwind (to the NW). Discoloured water (translucent
milky green in colour) was present around the mound and out to in excess of 500m from the
shoreline.
early-Mar
Tongan Defence Forces patrol passed the site of Home Reef and reported that the island had been
eroded significantly from last reports. A small shoal with a max height of 2m above sea level was
observed.
Mar-Apr
Pumice reached the eastern Australian coastline by Mar. By mid-Apr pumice strandings were
reported along the northern NSW and Queensland. The strandings extended for more than 1,300
km and clasts ranged in size from 1 – 4 cm, with the largest clasts up to 17 cm in diameter.
Nov 20
Minor hydrothermal activity: Visited by scientists from the UK. Summit of the seamount was about
9-10m below the surface, swell “lipping” over the top of the seamount. Hydrothermal plume present
2
over an area of 1-2 km , with a smell of sulphur was present in the area.
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The yacht Maiken sailing out of Vava’u bound for Fiji on 12 August, having passed the
volcanic island of Late reported a volcanic island south of there position, coinciding to the
location of Home Reef. The island was in full eruption, emitting a voluminous column of
steam and Surtseyan (cocks tail) explosive eruption plumes (Fig. 6). The prevailing winds
were dispersing the eruption plumes to the west and north-west. Crew on the Maiken who
were observing the eruption from a distance of 2.8km, suggested that there were multiple
Vava’u
Late Island
Home Reef
Pumice
N
Eruption column
10kms
FIGURE 5: MODIS image 10:20 (Local time) on 08 August (21:20 UT 07 Aug) shows
an eruption plume and pumice being actively produced at the surface (image
NASA/USGS LP DAAC).
A
B
FIGURE 6: A. Surtseyan explosions from the crater 12 August (photo F Fransson); B. Island formed
during the eruption, viewed from the northwest 12 August. Note the voluminous column of steam
produced from the crater (photo F Fransson).
explosions occurring within a crater region. From the reports it appears that explosive
activity was vigorous during the initial phases of the eruption. However, when the island
had stabilised and the vent was closed off from the open ocean, activity changed to mild to
vigorous steaming. Activity during November to early-December was confined to mild
steaming and light hydrothermal activity.
The Island
Within 24 hours of the onset of activity, sufficient pyroclastic material had accumulated
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around the vent to form an island that was located about 12nm south, south west of Late
Island (Fig. 7). The island was roughly oval in shape 800 x 400 m with the long axis
orientated in a north, north east to south, south west direction. The island was composed
FIGURE 7: ASTER image taken on 04 October of Late and the island formed at Home Reef.
Submarine hydrothermal plumes can be seen dispersing to the north and east of the island
(image by NASA/USGS LP DAAC).
entirely of pyroclastic material, which the crew of the Maiken referred to as “black coal”
and consisted of several hills (Fig. 6B) along the western and southern side. A steaming
lake formed at the north end of the island when the crater area was closed off from the
sea. Observations of the initial activity (Fig. 6A) suggested there was at least 2 active
vents present in the crater region.
As the island was composed of pyroclastic material and it was exposed to the open ocean
swells the morphology was prone to change once the level of activity had reduced to a
mild steaming. The early morphology was oval in shape, measuring about 800 x 400 m.
Satellite images of the island obtained on 04 October (Fig. 8A) suggest that little change
had occurred within the initial month of activity with the island covering an area of 0.23km2.
However, by 12 November (Fig. 8B) shape of the island had changed considerably, now
having a triangular shape, 300m across and covering an area of 0.146km2. A survey by
the Tongan Defence Force on 14 November suggested that the morphology of the island
had stabilised. The island was roughly square about 300m across with an estimated height
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of 40m asl. A further aerial survey by the Royal New Zealand Air Force on 07 December
indicated that the island was roughly circular, 450m in diameter with a height of 75m (est)
(Fig. 9A and B). The remains of the central crater being protected from the open ocean by
a low mound of pyroclastic material (Fig. 9C) and a small well-formed cone persisted at
the south western end of the crater region (Fig. 9D).
FIGURE 8: ASTER images taken on 04 October and 12 November
showing the change in morphology of the island. The area of the
island has halved during this period (images NASA/USGS LP DAAC).
A
B
C
D
Figure 9: The Home Reef island on 07 December 2006. A. The island from the south, with Late,
some 12nm distant in the background. B. The island is roughly circular with a diameter of about 400m,
note the discoloured water around the island. C. Part of the central crater region of the island still
persists, as it remains protected from the ocean swell by a low mound of pyroclastic material. D. Detail
of the crater region, a well-formed crater persists at the base of the cliff (all images by RNZAF).
Due to its exposure to the open ocean swell, the island produced during the August 2006
eruption of Home Reef will not persist for long before it is eroded to a shoal.
Home Reef was visited by a group of UK and Australian scientists on 18 February 2007.
They reported that only a partially submerged “pumice mound” was present at the
southern end of the shoal at a location of 18.993oS 174.758oW (Fig. 10). The mound
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measured 50-75m in diameter and had a maximum height of <5m and was generally
awash in swells of greater than 2m. The mound was composed of reworked pyroclastic
material, although large blocks (<1m in diameter) were scattered on the surface of the
mound. No hydrothermal or fumarolic activity was reported, although a strong smell of
sulfur was noted downwind (to the NW) indicating mild degassing may still be occurring at
shallow depths around the shoal. Surface water temperatures around the shoal were
reported to be ambient (ie. 28-29oC). The surrounding water was discoloured, commonly a
translucent milky green colour, generally extending to more than 500m in a northerly
direction from the mound.
Figure 10: The “pumice mound” present at the southern end of the Home
Reef shoal on 18 February 2007 (image by S. Bryan).
Observations by the Tonga Defence Services patrol boat Voea Savea in early-March 2007
confirmed that the island had almost disappeared.
Home Reef was again visited by UK scientists on 20 November 2008. They reported that
the island that had formed during the eruption was no longer present. The summit of the
seamount was at 18.990oS, 17.769oW and was about 9-10m below the surface. The top of
the seamount was relatively smooth and had an area of about 500 x 500m. A
hydrothermal plume was present in the area producing an area of discoloured water 12km2. There was also the smell of H2S in the area.
The Pumice
The eruption produced an extensive pumice raft. The pumice was produced during the
initial submarine phase of the eruption during the morning of 08 August (Fig. 5). Pumice
initially covered an estimated area of 80km2, being centred above the active vent. The
MODIS image (Fig. 11A) taken on 10 August showed that the pumice raft had reached the
northern side of Late covering an estimated area of 440km2 with the majority dispersing to
the north and to the north-east toward Vava’u. Following its departure from Vava’u on 11
August, the yacht Maiken sailed through the pumice raft, then located to the south west of
Vava’u (Fig. 11B and C). From this time the pumice dispersed in a westerly and northwesterly direction.
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A
N
Vava’u
10kms
B
C
D
E
F
FIGURE 11: The Pumice: A. MODIS image taken 10 August showing the initial dispersal of the pumice raft
to the north of Late Island (image NASA/USGS LP DAAC); B. Pumice raft on 11 August South-west of
Vava’u (image F. Fransson); C. Travelling through the pumice on 11 August (image F. Fransson); D. Pumice
strandings on Indigo Swan Beach, Naitauba Island, Northern Lau Islands, Fiji on 19 September (image D.
Forsythe); E. Characteristics of the pumice seen near Matava Resort on the South cost of Kadavu, Fiji
(image R. Matthews); F. Pumice strandings On Tanna Is, Vanuatu in early-December (image G. Kourounis).
The pumice rafts have continued to disperse in a westerly and north-westerly direction
reaching the Lau Islands in Fiji in late-August, Taveuni, Fiji in mid-September, the
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remainder of central and northern Fiji by mid to late-September (Fig. 11D and E). In earlyNovember pumice strandings were reported on Efate, Vanuatu and on Tanna in eralyDecember (Fig. 11F). Pumice reached the Australian coast in March 2007. By mid April
2007, extensive pumice strandings were reported along a 1,300 km stretch of the northern
NSW and Queensland coastline.
Full details of the characteristics and movement of the pumice can be found in the Bulletin
of the Global Volcanism Network published for September, October, December 2006 and
April 2007 (Anon. 2006a, b, c; 2007) and the weekly Report from the USGS/SI volcano
alerts for 16-22 May 2007).
VOLCANIC HAZARDS
Submarine volcanic eruptions are the most common forms of activity that have occurred
along the TVA during historic times. Recent eruptions have occurred at Hunga Tonga and
Hunga Ha’apai, Falcon Island, Metis Shoal, Home Reef, Submarine Volcano III, a
previously unknown centre west of Vava’u and in the region of Curacoa Reef.
Assessments of the volcanic hazards that occur at submarine volcanic centres are
sometimes difficult to accomplish due to the remoteness of many of the centres and the
incomplete record of activity. It is common that only a small number of eruptions that occur
at the centres are detected, and even fewer are actually observed. It is only those
eruptions that produce a structure that actually reaches the surface are observed and can
be assessed.
Two types of submarine eruptions have occurred at centres along the TVA during recent
times:
i)
Explosive eruptions, often resulting in the production of pyroclastic cones around the
vent regions, that are quickly eroded by the ocean. Subaerial lava flows or lava
domes are not formed during this type of eruption. This type of eruption is caused by
the exsolution of magmatic gases from the magma within the conduit below the vent.
As these eruptions are volatile-rich they commonly produce highly vesicular pumice,
which when it reaches the surface forms extensive rafts that move away from the
vent under the influence of wind and ocean currents. In some cases where the
eruption does not reach the surface, pumice rafts are commonly the only phenomena
that may be observed. Examples of this type of activity include recent eruptions at
Curacoa Reef in 1973 (Simkin and Onyeagocha, 1973), Home Reef in 1984 (Anon.
1984 a, b, c), Metis Shoal in 1979 (Woodhall, 1979), Falcon Island in 1927-36 (Taylor
and Ewart, 1997), Submarine Volcano III in 1999 (Taylor, 1999) and the Unnamed
Submarine Volcano to the west of Vava’u in 2001 (Taylor, 2002; Bryan et.al., 2004).
ii)
Effusive eruptions, that result in the formation of lava domes or rocky shoals. Phases
of explosive activity may occur during this type of eruption. They are, however,
caused by the explosive fragmentation of lava during its interaction with the
seawater. Islands that are formed during this type of activity commonly remain above
or near the surface for long periods after activity has ceased. As these eruptions are
relatively volatile-poor, pumice is not produced. Examples of this type of eruption
include recent eruptions at Metis Shoal in 1967-68 and 1995 (Melson et.al., 1970;
Scott, 1995) and at a vent near Hunga Tonga and Hunga Ha’apai in 1988 (Taylor and
Ewart, 1997).
Based on the reports of activity, the recent eruption that occurred at Home Reef appears
to be entirely explosive in character, ie. eruptive activity type (i), with an extensive pumice
raft being produced.
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Specific Hazards
There are a number of specific hazard types that are observed during submarine volcanic
activity, or may occur as a result of the activity. These hazards have the potential to affect
the immediate area surrounding the centre or even islands several tens of kilometres away
or the islands of neighbouring countries. The hazards of concern are listed in figure 12.
Volcanic explosions from the vent region will produce ballistic ejecta which may be
propelled several hundred metres. Larger explosions may eject blocks up to 1 - 1.5 kms
from the vent. Shock waves from the explosions may also be experienced during activity.
The yacht Maiken observed volcanic explosions, however, ballistic ejecta were not
observed during the eruption.
The eruption column produced is composed of a mixture of tephra and volcanic gases,
including SO2. The tephra produced consists of small fragments of rock produced during
the explosive disruption of the magma. Volcanic gases, steam and boiling seawater can
combine to form volcanic fog or vog. Both tephra and vog are dispersed under the
influence of the prevailing winds, to be deposited at localities sometimes tens to hundreds
of kms downwind of the vent. Vog can include highly corrosive acids that may affect many
structures encountered downwind. Prevailing winds in the region of Home Reef blow
generally from the east or south-east, being more consistent during the dry season (AprilOctober). Tephra-laden eruption columns, if injected to upper tropospheric levels, have the
potential to affect aircraft transiting the region. Although an eruption column was produced
during the eruption, a Notice to Airman (NOTAM) was not issued to warn aircraft transiting
the area to exercise caution.
Because of the entirely volatile-rich explosive character of the activity, pumice was
produced forming rafts that may drift great distances from the vent under the influence of
the prevailing ocean currents and winds. Pumice rafts are commonly washed up on the
shore of neighbouring islands or disrupt shipping transiting the region. During this eruptive
period large amounts of pumice were reported on the surface of the sea in the vacinity of
the vent. Strandings along shorelines at numerous locations in Fiji, were reported from
early-September 2006 and Vanuatu from early-November.
Large amounts of pyroclastic material may produce shoals or in some cases islands.
Most of the shoals formed in such a fashion are highly unstable and quickly eroded by the
sea. However, during their brief presence they can cause considerable hazard for shipping
transiting the area. An island was reported during this eruption, but it will probably be
reduced in size quickly by the ocean swell and disappear under the surface within several
months to form a shoal. Although an island was formed during the eruption, a Notice to
Mariners was not issued to warn shipping transiting the area to exercise caution.
Due to the unstable nature of the structures formed during pyroclastic eruptions, large
explosions or seismic activity may initiate volcanic edifice collapse. This results in the
movement of large amounts of material down the submarine slopes of the volcano, which
may cause volcanogenic tsunamis. Small tsunamis may also be produced by shock
waves of the submarine explosions. Tsunamis produced by either of these mechanisms
have the potential to affect regionally extensive areas. Although volcanogenic tsunamis
have not been reported during the current eruption, continual erosion of the island has
added a considerable amount of suspended sediment to the surrounding ocean currents.
Hazard Zones
The volcanic hazards map shown in figure 11 indicates three hazard zones around the
volcano. The boundaries between the zones are based on assessments conducted at the
other submarine volcanoes along the TVA, eg. Scott (1995) and Taylor (1999, 2002), and
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Eruption of Home Reef August 2006
elsewhere in the world. The zones indicate the relative degree of risk from specific
volcanic hazards around the volcano. In general terms the degree of overall risk decreases
with distance from the volcano, this is the case for risk from explosions, ballistic
FIGURE 12: Volcanic Hazards map for the Home Reef Volcano showing the three zones of relative risk
around the centre and the hazards that are likely to affect each zone (after Scott, 1995 and Taylor, 1999).
It also indicates the recommended restrictions on access to each zone.
ejecta and the presence of shoals. The risk from some hazards, ie. volcanogenic
tsunamis, may increase with distance from the volcano.
For other hazards, including vog, tephra and pumice, areas at risk will be located
downwind/downcurrent of the volcano.
Due to the nature of volcanic hazards that may occur on and around this volcano it is
strongly recommended that no access be permitted to zone 1 (Figure 11), ie. within 2 km
of the volcano. Furthermore, it is also strongly recommended that access to zone 2
(Figure 11), ie. between 2 km and 4 km of the volcano, be restricted. Because of the
unpredicability of many volcanic phenomena it is also recommended that extreme care be
taken when approaching the volcano within distances of less than 5 km.
CONCLUSIONS AND RECOMMENDATIONS
This period of activity resulted in the production of an island and an extensive pumice raft
that affected coastlines in northern Tonga, as well as Fiji and Vanuatu.
The eruption was noteworthy in that it was not recorded by the Tongan Ministry of Lands,
Survey and Mineral Resource seismic network. Initial reports of probable activity in the
Tongan region could be inferred by the numerous pumice strandings that were reported
along the coast of Fiji from mid-September. Further reports of activity became known
through reports from several passing yachts and the analysis of satellite (MODIS and
ASTER) satellite images.
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Eruption of Home Reef August 2006
It is also significant to note that a dedicated volcano-monitoring program has not yet been
established within the Kingdom of Tonga. Given the frequency of activity that has occurred
from centres within the Kingdom and the potential effects of hazards that would occur
during future eruptions, it is strongly recommended that a volcano-monitoring program
be established for the Kingdom of Tonga. This program should include:
a.
periodic visits to and observations of all the known volcanoes in the Kingdom;
b.
the use of seismic and other instrumental monitoring techniques to enable
forecasting of future activity; and
c.
the development of awareness programs to educate Tongans on the effects of
hazards during future eruptions.
ACKNOWLEDGMENTS
The following people are thanked for providing information on the eruption: Mary Fonua of
Matanagi Tonga for initially bringing the activity to the author’s attention, LCDR Bill Trifitt,
RAN (MSA to Tonga) for providing information on the eruption and copies of several
images taken by the RNZAF, and Allan Bowe of Mounu Island, Vava’u, Tonga for
providing descriptions of the initial activity. FLTLT Catherine McGowan and SGT Daniel
Coe of Headquarters Joint Forces, New Zealand are thanked for providing a copy of the
entire set of images taken by the RNZAF on 07 December 2006. Scott Bryan (Kingston
Univ, UK) is also thanked for the photo of Home Reef taken in February 2007. Much of the
information on the eruptive process and the dispersal of the pumice was extracted from
the monthly reports of the Global Volcanism Network, Smithsonian Institution, Washington.
BIBLIOGRAPHY
Anonymous (1984a) Home Reef Volcano, Tonga Islands, S Pacific. Scientific Event Alert
Network Bulletin, Vol.9, No.2, p2.
Anonymous (1984b) Home Reef Volcano, Tonga Islands, S Pacific. Scientific Event Alert
Network Bulletin, Vol.9, No.4, p7-8.
Anonymous (1984c) Off-lying Islands and Reefs: Home Reef. Pacific Islands Pilot, vol.II,
p345.
Anonymous (2006a) Home Reef, Tonga Islands, SW Pacific. Bulletin of the Global
Volcanism Network, vol.31, No.9, p2-5.
Anonymous (2006b) Home Reef, Tonga Islands, SW Pacific. Bulletin of the Global
Volcanism Network, vol.31, No.10, p12-15.
Anonymous (2006c) Home Reef, Tonga Islands, SW Pacific. Bulletin of the Global
Volcanism Network, vol.31, No.12, p6-10.
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Volcanism Network, Vol.32, No.4, p11-13.
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Geological Survey Weekly Report 16-22 May 2007.
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Natural Resources and Environment, Tonga, p9.
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Greig, A. and Leslie, R (2004) Pumice Rafting and Faunal Dispersion During 200127 February 2009
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2002 in the Southwest Pacific: Record of a Dacitic Submarine Explosive Eruption
from Tonga. Earth and Planetary Science Letters, vol.227, p135-54.
Government of Tonga (1990) Aerial Photograph of Home Reef Volcano, AUSC 250, Run 1
- 56-57, 21 June 1990.
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North Tonga Islands. Unpublished manuscript.
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Taylor, P. W. (1999) A Volcanic Hazards Assessment Following the January 1999
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Occasional Report No. 99/01.
Taylor, P.W. (2002) Volcanic Hazards Assessment Following the September-October
2001 Eruption of a Previously Unrecognised Submarine Volcano West of Vava’u,
Kingdom of Tonga. AVI Occasional Report No.02/01.
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von Wolff, F. (1929) Der Vulkanismus. Vol.II, Stuttgart, p276.
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SELECTED WEBSITES
Global Volcanism Program – Smithsonian Institution, Washington USA:
www.volcano.si.edu/world/volcano.cfm?vnum=0403-08=
National Aeronautics and Space Administration, USA:
http://www.nasa.gov/multimedia/imagegallery/image_feature_696.html
Université Libre de Bruxelles, Brussels, Belgium:
http://www.ulb.ac.be/sciences/cvl/homereef/homereef.html
GEONET, New Zealand:
http://data.geonet.org.nz/geonews/sab/
Matangi Tonga Online, Nuku’alofa, Kingdom of Tonga:
http://www.matangitonga.to/article/tonganews/natural_events/new_island061106.shtml
http://www.matangitonga.to/article/tonganews/natural_events/tonga_eruption_081106.shtml
Images from the Yacht “Maiken”
http://yacht-maiken.blogspot.com/2006/08/stone-sea-and-volcano.html
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