The Bay of Plenty is an evocative name. The Bay is sandwiched between the two northerly peninsulas of The North Island. The name includes the adjacent land region, so this is a Bay you can live in. The geyser of Rotorua is world-renowned. So are the adjacent Taupo volcanoes, although not quite as widely publicised. Too many of the Earth’s largest volcanic eruptions of the past million years have come from here, the most recent one around 100 AD. Since humanity arrived here there has been one notably eruption. The region is quiet now without any sign of impending activity. GPS measurements show deflation across the region which is a positive sign for those who like their volcanoes scenically quiet.
But there is also activity in the Bay itself, and it is centred on its main volcano, White Island. This volcano seems designed for tourism. It is partly submerged, so you don’t have to climb the summit – the sea carries you. The crater rim is breached on one side not far above water level – so a short hike brings you into the caldera. And there are wonders to be seen: steam rises from a crater lake, and yellow sulfur is deposited all around you. This is Dante’s Hell without the inconveniences. But this volcano has two faces, and this week we saw its unpredictability.
East of The North Island, the Pacific Plate is being subducted underneath the continental crust of New Zealand. This happens over a 3000 kilometer zone extending to Tonga, but only here is the subduction underneath a continent. In New Zealand, it has given rise to the Taupo Volcanic Zone. North of New Zealand, in oceanic crust, this becomes the Havre Trough. The two are offset by some 50 kilometers, and this offset is accommodated by echellon faults. These echelon faults are about 20 kilometers north of White Island. South of there, the Taupo zone has formed a graben in the Bay of Plenty. It is about 40 kilometer wide, and is bounded by two faults: on the west side runs the Tauranga Fault Zone, and on the east side of the graben is the White Island Fault Zone.
Volcanic activity is centred on the eastern fault, and his fault runs all the way to the legendary volcanoes of Taupo itself. This chain of volcanoes runs from Whakatane seamount on the edge of the continental shelf to Ruapehu, with the Taupo volcanoes located in the middle. There is an interesting change in the magma properties: while the central part has rhyolitic eruptions, the volcanoes on either end do andesitic magma.
Under water, there are a number of small sea mounts near and beyond White Island. The mounts are associated with the Ngatoro and White Island ridges. White Island itself is flanked by two sea stacks, on either side, Club Rocks and Volckner Rocks.
The Māori name is Te Puia o Whakaari. I will be using the English name as being more recognizable, but the Māori name should probably have precedence. White Island is the emergent summit of a submarine mountain covering an area of 16 km by 18 km. The total volume is at least 78 km3. The base of the volcano is 300 meters below sea level, and the highest point is 321 meters above sea level. The island is about 2km in diameter. The two flanking sea stacks are part of the same submarine mountain.
There are two overlapping stratocones in White Island. On the west side is the extinct Ngatoro Cone, 310 meters tall. Most of the rest of the island is taken up by the younger Central Cone, which is active and which has a number of vents and craters. Ngataro is heavily eroded and may have been lost it summit before the Central Cone developed. The main crater consists three separate structures, one of which has broken through the crater rim and now allows easy access to the crater. The crater floor is only about 30 meter above sea level. All current activity happens on the northwestern end, furthest from the sea.
The crater rims shows a succession of lava layers alternating with pyroclastics. It appears that this volcano goes through cycles, where a phase of lava eruptions is followed by a phase of explosive eruptions, both phreatomagmatic and strombolian eruptions. The cycle has been suggested to last some 1000 years.
The magma chamber is probably located around at 5 kilometer depth. The composition is mainly mantle derived, and is magnesium-rich, but there is a component from crustal melt. It has been compared to that in southwest Japan. The melt probably originates at the subducted oceanic crust which here is some 150 kilometers deep and is strongly hydrated. It rises rapidly to the 5-km deep reservoir, where some mixing with crustal melt takes place. Prehistoric lavas seem to have a bit more crustal melt than the youngest (1977) lavas.
The conduit to the surface is not open. It seems blocked by a viscous plug in the upper kilometer. Small plugs of magma can occasionally rise rapidly to the plug but remain there, until eventually the plug breaks and a phase of effusive eruptions begins. While the plug exists, the eruptions are mainly phraetomagmatic.
There is no evidence for major eruptions from this site. It would be rare for significant ash to reach The North Island: this may be expected perhaps once every 1000 years. There is also no evidence that event here have caused tsunamis in the Bay.
The crater is currently in a phase of phraetomagmatic eruptions. Water circulates efficiently through the volcano, perhaps not a surprise seeing the low level of the crater floor, in the midst of the Bay. This water means that any rising magma would cause such explosions. Effusive eruptions would require the volcano to dry out, and this hasn’t happened yet.
In the early 20th century, a sulfur mine operated at White Island. This was a risky operation and there were several fatal accidents. In September 1914, a piece of the crater wall collapsed and formed a lahar in the central crater which reached the sea. All 10 people present and four of the five camp cats died. The floor of the crater remains shaped by the debris from this collapse. The ruins visible on some of the recent photos come from that mine.
Since the first exploration in 1826, there have been regular small explosions in the westernmost crater. A lake is often present (it is a very wet volcano) which was temporarily drained for the sulfur mining. The explosions often leave small vents. There were larger explosions in 1933 and (probably) in 1946 which both caused small craters. Explosions between 1962 and 1971 formed three craters, which reached a depth of over 100 meters. After that there was a quiet interlude, until activity resumed in 1977 and tapered off to the 1990’s. The ash from the explosions was mixed with lava bombs: magma had intruded close to the surface. The lava bombs only fell in the crater: the explosions remained small on the VEI scale. After each explosion, the crater would be off-limits for a few days.
It is a very gassy volcano, and the sulfur and water combine to give a very acidic environment. Gas masks are recommended.
Small explosions have continued, most recently in 2016. The 2016 event excavated 10 meters from the crater floor and destroyed the crater lake. But the lake quickly reformed and grew quite deep during 2018.
Over the past decades all explosions were at a time no one was at the island. Until this week.
Was there any warning? There was an M5.8 earthquake on Nov 23, but at a depth of 120 km, this is unlikely to have affected the volcano. Going back longer in time, there had been a slow increase in activity over the past 20 years. A sudden increase to recent levels would have been a clear warning sign and would probably have closed the crater to visitors. A very slow increase is more likely to be ignored. There isn’t a clear cut-off point where a volcano becomes too dangerous, even when the warning level is raised – as recently had happened.
White Island is not particularly dangerous as volcanoes go. But being in a crater of an volcano with occasional phraetomagmatic eruptions, which happen without warning, is never safe. Volcano tourism should be done carefully. Carl has written about such tourism. And on the list of volcanoes best seen from a safe distance, perhaps this one should now be on it too.
As a final remark, Kilauea has also recently obtained a crater lake. It is quiescent at the moment, but once activity resumes, the water will likely make it explosive. HVO has a strong safety culture. Don’t be surprised if they start moving people back from the rim. It is for good reason.
And I’ll end by reproducing Mike Ross’ list of essential equipment for the serious volcano tourist.
I’ll give a little safety advice… my ‘gear list’ for fieldwork – may be added to or subtracted from depending on the volcano – and the rationale…
– Hard hat. Should go without saying! When to wear it at all times and when to merely carry it ready for instant use in case things go pear-shaped is left to the judgement of the individual.
– Satellite phone. Bit pricey but no excuse for not having one if you’re serious about this.
– Radio receiver/scanner. Can be useful to tune in to emergency service broadcasts etc. as well as normal broadcast news and weather.
– Gas meter. Doesn’t need to be precisely calibrated at vast expense so long as you’re sure it work. I recommend MultiRAE meters; they can be found on eBay and are good reliable instruments – we used them in hazmat work at the fire department. The ideal sensor combination for volcanology would be H2S, SO2, and O2. CO is not a significant volcanic hazard. CO2 is – but your alarm sensor for that gas is the O2 sensor as CO2 is an O2-displacing asphyxiant. The meter should always be worn on the waist belt in case you blunder into a CO2 pocket; it’s heavier than air!
– Thermal Imager of some kind. It’s good to know when something is hot! It can be hard to tell fresh and still very hot lava flows from old cold ones – until you get uncomfortably close! These days I would suggest the CAT S60 phone which has a built in TIC. It’s also a very rugged phone; important on volcanoes!
– Gas masks (plural). You need a range – different masks for different conditions. If there’s just a little ash blowing around a nuisance dust mask may suffice. If there’s significant SO2 around you may want to break out the proper respirator style make fitted with the correct ‘acid gas’ filters. And if things get really nasty you may need a firefighter-style full-face mask equipped with the same filters; it covers your eyes too – there’s no point in being able to breath if your eyes are so irritated you can’t *see*!
These are the more volcano-specific add-ons to the general outdoors/mountain gear you’ll need anyway.