When the news reported multiple volcanoes erupting in Vanuatu – when new volcanoes appeared near Tonga Islands – when earthquakes shook most of Melanesia left right and center – I have often wondered if, or why, there are no volcanoes on the Fiji Islands? Well, you probably guess the answer – of course there are! They are just not very busy at the moment, so they may escape our attention, and may lull the residents in false security, too. The three that are mentioned by the GVP are Taveuni and Koro Islands, and Nabukelevu which occupies the SW end of Kadavu Island.
Now why are the Fiji volcanoes so quiet when they are sitting in such a seismically active place?
The answer lies in their complex tectonic history, and here comes – in a tiny nutshell:
For orientation: The Fiji Islands sit on the northern end of the Lau Ridge, which looks like the handle of a crooked walking stick, curling around the extending Koro Sea basin. The island group is surrounded by the North Fiji basin in the N and W; the South Fiji Basin in the S; and the Lau Basin in the E.
The Fiji Islands represent a portion of the old Vityaz Arc which was split up and rotated to its present position. They were then on a micro-plate – the Fiji plate. The time 12 to 7 Ma was a period of plenty volcanic activity in the Fiji region. The Fiji plate has since fused with the Australian plate and is now called Fiji platform.
The Fiji platform is located between two opposite-facing subduction zones at the indo-Australian and Pacific plate boundary. The stresses created by the opposing plate movements have resulted in the formation of transform faults such as the Fiji Fracture Zone to the N and the Hunter Fracture Zone to the S.
About 3 Ma sea floor spreading began and led to the opening up of the North Fiji Basin to the W and the Lau Basin to the E. Fiji and the Lau Ridge are no longer active volcanic island arcs, as subduction has ceased. Today, active sea floor spreading is still happening in the Lau Basin and in the North Fiji Basin. There, Tonga and Vanuatu are still active volcanic islands arcs, as subduction is still occurring under them.
(And this explains why we hear so much of erupting volcanoes to the right and to the left – while the Fiji volcanoes…)
However, in the context of the crustal extension regime, several volcanic centers did form around the margins of the Koro Sea, and they are still the source of active volcanism in the Fijis. Volcanic activity taking place on the Fiji platform is seen now as related to intra-plate volcanism.
(…. are just taking a deserved nap!)
Taveuni is the third-largest island in Fiji. It consists of an oceanic intraplate volcano in the northeastern Fiji Group and has been active throughout the Holocene. The entire island is a broad, elongated basaltic shield massif. The highest point is Mt. Uluiqalau with 1241 m a.s.l..
Alkali basalt magmas were erupted through some 150 monogenetic cones and vents at constantly shifting locations along a SW-striking rift zone that extends the entire length of the 40-km-long island. The eruption style was mostly effusive Hawaiian and cone-building Strombolian, but phreatic eruptions have also occurred. Taveuni can be considered a monogenetic volcanic field.
Most Holocene volcanism was confined to the southern two-thirds of the island; however, the locations of activity have constantly shifted in an apparently random fashion. In many areas lava deltas formed where the lava covered former coastal reef platforms. The longest lava flow travelled 8 km to the coast.
In a period of voluminous eruptions between about 300 and 500 AD settlers abandoned either the southern part of, or even the entire island, until ~1100 AD. The latest known eruption produced a lava flow at the southern tip, sometime between about 1450 and 1650 AD.
Can Taveuni erupt again?
Up to the end of the 20th century, the time scientists took a close look at the history and behaviour of Taveuni, it was generally believed that the volcano was utterly extinct. The surprise must have been great when it turned out that, with the last eruption probably less than 400 years back, Taveuni has all reason to be called an active volcano.
A lot of research has been done on Taveuni (by S. Cronin and colleagues) to determine the historic and possible future impact of eruptions on the local population. They found that in those most active times during the first millennium AD, lavas and tephra must have buried much of the people’s food and water sources.
At least 58 eruptions have occurred on Taveuni since the first known human settlement of the Fiji Islands about 950-750 BC. There are no eruptions recorded after Europeans arrived first in these islands, but up to 25 probably affected earlier settlements badly. At least four former villages are known to have been affected by volcanic products. Even though eruptions were comparatively small – mostly at VEI 2 – the villages and water sources were destroyed, household possessions, crops and gardens were buried.
A probabilistic prediction of future eruptions on the island estimates that the SW end of Taveuni Island has the greatest potential for further eruptions. Not least because an overall shift of volcanism toward the south along the Taveuni rift axis, perhaps even to offshore, is very likely. Unfortunately, this is also one of the more densely inhabited areas of the island with important infrastructure.
The thing with this volcano is, though, that it is nearly impossible to forecast the most likely next eruption site. Apart from the high chance that it will occur somewhere along the central rift axis, there is no way to know where another crater or rift may open.
If an eruption were to happen today, Taveuni’s economy could suffer considerably. Unlike with many other volcanoes, the largest hazard from Taveuni is actually its lava flows. Because Taveuni is heavily forested, fires can spread fast and easily – any farms or holiday resorts in the path of lava could catch fire. Also, about every port in Taveuni is in a bay where lava flows might arrive and come in contact with sea water. The valleys and bays are also hazard zones for lahars or pyroclastic flows.
The population of Taveuni is presently some 19000, probably much higher than ever before, and growing rapidly. So, the investigation of the impact of past eruptions is a strong reminder to the present people to consider and design strategies for protecting their population and infrastructure.
YT Video by travel-the-world-with-us.de. Natural water slide on Taveuni – I had fun just watching this! 🙂
I have found nothing in the way of recent research on this volcanic island other than the overview by the GVP (below). However, its location next to Taveuni to the SW may suggest that it could belong to the same rift system. Therefore, similar conclusions than those drawn for Taveuni Island could apply for Koro, especially as the latter has “young” lava flows as well.
“The 16 x 9 km, shark-tooth-shaped island, located between Fiji’s Viti Levu and Vanua Levu Islands, is part of the volcanic Lomaiviti Islands. A chain of basaltic cinder cones of upper Pleistocene or possibly Holocene age extends from north to south along the crest of the island. With the exception of one location on the west coast where young lava flows reached the sea, the youngest lava flows, erupted from the NNE-SSW-trending cinder cone chain, are confined to the central plateau, where they form a flat, undissected lava field that extends primarily to the east.”
YT promotional video by Hilton & Hyland, shows good aerial views over Koro Island:
Nabukelevu is an andesitic-to-dacitic lava-dome complex on Kadavu Island at the southern end of the Fiji archipelago. The Kadavu Island Group is an emergent portion of the eastern Hunter Ridge. The high point of the complex is Mt. Washington, 805 m, an andesitic lava dome. At least two important NNE-trending faults cut the complex in several locations.
Flat-lying dacitic lava flows are found at Cape Washington along the W coast and at Talaulia Bay on the NE coast. Onshore and offshore deposits indicate that several eruptions have occurred during the Holocene. There is also a native legend that strongly appears to describe a volcanic eruption on Kadavu. Block-and-ash flows related to dome growth have occurred within the past few hundred years. They were probably generated by gravitational collapse and mass-wasting of the growing lava dome, with little explosive energy (comparable to Merapi’s behaviour).
The block-and-ash flows on the NW flank seem to be the youngest on Nabukelevu. Radiocarbon age has been determined from charcoal flakes within the deposits at two locations: One of them has probably been laid down in the 17th century, while the other result is not quite as reliable but the probe may be even younger.
Lavas flows on Nabukelevu are usually ~10m thick and stubby. The summit area and upper eastern flanks of the edfice are formed by a prominent dome or coulée. A minimum age estimate of 2420 ± 90 years BP is derived from dating a peaty sediment within a small swamp developed on this dome near its summit. Pale-grey high-silica andesites and dacites are the most common lavas.
The Nabukelevuira scoria deposits occur in two lobes, to the E and SW of the main Nabukelevu edifice. The generally high content of bubbles in the rocks of these deposits suggest an open-vent eruption, with pyroclastic flows generated by collapses. A large dome flow or coulée underlies the present summit, and the scoria deposit overlies this dome. A shallow crater-like feature immediately west of the summit dome is considered the most likely vent area.
The dome complex also features several collapse scarps, which were the source of debris avalanches that have incorporated human artefacts and remains. Debris avalanches have entered the sea on the both the N and S sides of the volcano.
Recent studies corroborate this widespread local myth about how catastrophic events in Nabukelevu over the last 2000 years destroyed an entire settlement on Mt. Washington. Earthquakes, induced by the ill-placed fault lines, combined with cyclone activity in the region have a potential to cause a large-scale edifice failure, which would send large chunks of Nabukelevu Volcano into the ocean, causing a tsunami that could reach Suva, Fiji’s capital city, which is located 110 km (68 miles) north of Kadavu Island.
An assessment of the risk for further eruptions places the extreme western end of Kadavu Island as the most likely danger zone. Apart from possible renewed eruptive activity, this geological-fault-ridden island has the additional hazard of flank collapses during eruptions or strong earthquakes. These in turn could entail the generation of tsunamis, which, in the worst case, would affect surrounding islands up to Viti Levu with the Fiji capital Suva.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP: Taveuni, Nabukelevu, Koro
– The Tectonic History of Fiji
– Arc dynamics and tectonic history of Fiji based on[…] (2002)
– Impacts of volcanism on pre-European inhabitants of Taveuni (2000, paywalled)
– Probabilistic Assessment of Eruption Recurrence on Taveuni (2001, paywalled)
– Nabukelevu volcano (Mt. Washington), Kadavu – a source of hitherto unknown volcanic hazard in Fiji (2004, paywalled)
– Geological map of the Fiji Islands (1965)