Hazards to ships from volcanoes (part 2)

Subaerial eruptions, the risk posed by tephra fallout – and what to do about it

Captain L P Cragg, BSc (Hons)

Submarine volcanoes tend to become more explosive as they shoal and the overlying water pressure decreases. Maintaining the theme from my earlier article, I will start at sea level and gradually increase altitude.
Surtseyan eruptions (see Seaways, February 2020) can be extremely explosive and deadly, but they are generally very local in their effect. The December 2019 eruption on White Island, New Zealand, affected only the people on the island. One tourist boat had just left, but although it was still very close the crew and passengers were unscathed; they were able to return immediately to rescue survivors.
As the volcanic edifice grows and solidifies the interaction between the magma and surrounding water decreases and the styles of eruption change. The Aeolian islands north of Sicily, Italy, can be considered examples of the next stages of evolution of a volcanic edifice, named after islands in this group: Stromboli and Vulcano.
In a Strombolian eruption, regular, small releases of accumulated gas throw incandescent magma a few tens of metres into the air. The volcano on the island of Stromboli has been erupting for thousands of years and normally has a minor ‘burp’ about every 20 minutes. Nicknamed ‘The Lighthouse of the Sea’ due to its visibility from afar, this site is often visited by cruise vessels and leisure craft. Although mainly benign, it has been known to have more violent eruptions which can be a danger to anyone close to the shore.
A Vulcanian eruption can be considered a direct progression of the Surtseyan eruption. The volume of erupted material is usually larger, but as there is often less interaction with water the eruption itself is not as violent. A vessel close to a Vulcanian eruption is likely to experience only a minor amount of tephra (volcanic ash) fallout.
This type of eruption is often associated with the growth of a volcanic dome in the crater. These domes are mechanically weak and may collapse due to instability or reaction to rainfall. If this happens, the debris from the dome collapse can run away down the volcano slopes in a hot avalanche – a pyroclastic density current (PDC). Dome collapse was a contributing cause of the hazards during the catastrophic eruption of Mt Pelée, on the Caribbean island of Martinique. The volcano has given its name to Peléan eruptions.

PDC – a deadly hazard
As the edifice and islands continue to grow so too does the size of the eruption. Peléan eruptions are the first where we see sustained eruption columns. The eruption of Mt Pelée was the first time PDC was described and studied. A PDC can hurtle down the side of the volcano at hundreds of kilometres an hour and be extremely hot (>500°C). They destroy everything in their path, including killing all living things exposed to them. These hot tephra avalanches are formed by the collapse of a dome or an eruption column, the former tending to be smaller in volume and destructiveness.
There is very little footage of a PDC flowing over water, but one from Montserrat, in th Caribbean, can be seen below...

This only travelled a few hundred metres over the sea but the one from Mt Pelée travelled at least a couple of miles. When a PDC enters the sea, it creates a further hazard, as the mass of rock displaces a substantial amount of water and sets up a tsunami wave. If the PDC is very hot it will flash off a large quantity of water, and the vast increase in volume from water to steam further accelerates the PDC. In this case the PDC will travel faster than the tsunami wave and hit any nearby vessel first.

If a vessel finds itself in an area where a PDC might occur, the crew should shut down all deadlights and any non-essential openings into the accommodation. If a PDC is seen approaching the vessel then the best option will be to head toward it at full speed with all crew inside the accommodation; perhaps another good use for the citadel. Just before the PDC hits, abandon the bridge. It is likely the bridge windows will be blown in and anyone still there will probably die.

Reports from some of the surviving crew from the Mt Pelée eruption mention whirlpools and strong currents affecting the ship immediately before the PDC and tsunami hit. I have not been able to re-create these in simulation, but a vessel farther away from Martinique also reported strange currents, so there may have been some seabed disturbance happening around the same time.

When the catastrophic PDC from Mt Pelée entered the bay at St Pierre on 8 May 1902 the first vessel it met was the cable ship SS Grappler. The force of the blast capsized her and the heat of the ash cloud set the remains alight; all hands were lost. It is believed that 18 ships of varying sizes were at anchor, and the remains of all but one of them are still at the bottom of the bay. The vessel that escaped, SS Roddam, managed to steam to the nearby island of St Lucia although 17 of the crew were either dead or dying. Soufrier volcano on St Lucia was also in eruption at the time; the Captain must have thought he had got out of the frying pan only to sail into the fire. Although the ships and the seafarers who died are poorly documented, a few hundred seamen and passengers were probably killed that day. This is the largest documented marine loss directly related to a single volcanic event.

Perhaps the best known type of the eruption is the Plinian eruption, named after Pliny the Younger – the author of the account of the AD 79 eruption of Mt Vesuvius, Italy, which buried the towns of Pompeii and Herculaneum. In this type of eruption, PDCs are still a major hazard, but vessels are more likely to be affected by tephra fallout. Although a vessel may experience fallout from eruptions of lower magnitude it is only as the eruption size increases that the hazards from the tephra start to cause serious concern.

Tephra fallout – navigation

Tephra fallout can extend many miles from the volcano. Navigation in areas of tephra fallout will be affected by both reduction in visibility and degradation of the radar signal. Volcanic ash is opaque to radar and the emitted signal will suffer attenuation while the radar screen will be ‘whited out’ by excess signal returns from back scatter. In case of light fallout, when visibility is still more than a mile, this should not be much worse than experienced during moderate to heavy rain showers, well within the experience of the average seafarer. As the magnitude of the tephra fallout increases, so do the adverse effects. Eventually, the radar will be ineffective, for either collision avoidance or navigation, and visibility will quickly reduce to almost zero. Footage of an eruption in Japan (below)shows how far visibility can be reduced by what is effectively a very light dusting of ash.

 Dry tephra is transparent to GNSS signals, so the vessel position should be reasonably accurately depicted on the GPSs and ECDIS – although it will be virtually impossible to verify the GNSS position by sight or radar. As large volcanic eruptions can have an effect on the atmosphere, tephra will often fall through clouds and land on the vessel wet. If this damp tephra accumulates on the GNSS aerial the received signal will slowly become weaker until positional accuracy is lost. During my research I did not have access to other electronic position fixing equipment so do not know how these will be affected. However, a deduction can be made by the fact that UHF/VHF seem to be unaffected whereas both X and S band radars are badly affected.

Stability concerns
The density of tephra varies from volcano to volcano, and even between different eruptions from the same volcano, but it is frequently quoted at approximately 1t/m3 . This figure can be used to quickly assess the effects on the vessel stability from tephra accumulation. Ships with low freeboards such as loaded tankers or bulk carriers and/or with large metacentric heights (GM) should remain stable despite heavy accumulation of fallout on the decks. However, vessels with high freeboard and/or small GM may become unstable and in danger of capsize. Pumice rafts are also closely associated with Plinian eruptions: the reduction in GM from upthrust of pumice on the ship bottom, which could be quite substantial, added to loss of GM caused by accumulated tephra on the topside of the vessel could see stability quickly become a major problem.
All vessels should re-assess their stability before entering an area where an explosive eruption is either in progress or probable. As a minimum, all vessels should apply the ‘icing condition’ formula to their stability calculations. It is relatively straightforward to produce a graph of GM against thickness of tephra accumulation for the stability condition of the vessel. This will act as a rough guide for immediate decision-making. The Master can use the graph to assess the degree of danger posed to the vessel and whether the crew can remain in the accommodation until the vessel is clear of the worst areas of fallout.

Table of dynamic pressures measured or estimated from some eruptions and the equivalent heeling GZ a vessel might experience if hit broadside. The high value for Mt St Helens was due to this PDC starting from a lateral blast rather than column or dome collapse. b)

The heeling curves superimposed on a typical GZ curve taken from a container vessel’s stability book. Note that the PDCs from both Mt Unzen and Mt St Helens would have capsized the vessel.
Trans-Paci­fic liner trades frequently pass close to Alaska, the Aleutian arc and the Kamchatka peninsula. In winter they may be affected by icing as well as tephra fallout. This could double the problem of added weight. Any tephra fallout occurring at the same time as ice accretion will become frozen to the vessel within the ice and dif­ficult to remove.

Clearing tephra
If it is decided that the crew need to clear the tephra off the vessel then it is wise to take a few precautions before entering the area of fallout. Check that scuppers and freeing ports are clear and any measures available for clearing decks of ash and pumice – such as brooms and shovels – should be made ready before going outside. Crew working to clear decks will require some sort of lung protection. Inhalation of minor amounts of tephra over a short timescale should not pose a health threat to the average seafarer, but when brooms and shovels are used the volume of tephra in the air will increase signi­ficantly.
Fire hoses may be used to wash fallout off the tops of containers and upper decks, provided this is started soon after the tephra begins to accumulate. When using water to wash tephra off the tops of containers bear in mind that tephra can absorb about 30% by weight of water before it becomes mobile. The added weight may lead to collapse of the container roof, and the wet tephra will now be caught inside the container. This is almost certain to happen with open-top containers that are protected only with a tarpaulin. Even steel-topped containers may suffer. They are load-tested by applying 300kg to an area 600mm × 300mm (IMO, 1992), but a thick layer of wet tephra may well exceed this load.
Consideration should be given to shutting down non-essential services to ease the burden on the crew. You should also consider shutting down the air conditioning system, if it cannot be run on 100% recycle mode. This will reduce exposure to ­ ne ash particles and reduce the risk of sensitive electronic equipment being damaged.

Tsunamis and other phenomena
Tsunamis can be formed during volcanic eruptions. The large tsunami created when Krakatau erupted in 1883 was more than 30m high when it made landfall. The SS Berouw was carried more than 2km inland and all hands were lost.
Another hazard associated with volcanoes is the formation of lahars – a ‑ ow of mud-rock slurry, which can persist for decades after an eruption. Lahars are formed where heavy rainfall or snowmelt mobilise unconsolidated tephra. As the density of the liquid increases so does the danger to anything in its path. I could ­find no documented evidence of damage to maritime seagoing vessels, but many river craft have been sunk or broken adrift as a result of these.