Hazards to ships from volcanoes

In the first of an occasional series, we look at a very real risk to ships and seafarers about which there is surprisingly little information

Captain L P Cragg, BSc (Hons)

Over time, hundreds of man hours and thousands of dollars have been devoted to researching volcanic hazards to aircraft – and yet, I could find no evidence that any research has ever been done on volcanic hazards to ships. Digging deeper revealed that, as far as I could find, neither an aircraft crash nor fatality in an airplane had ever been caused by volcanic activity. Conversely, hundreds of seafarers have been killed and dozens of ships affected by volcanic eruptions. As well as encountering volcanic activity by chance, vessels may be called to assist in providing humanitarian support or evacuation in the event of an eruption. While both international maritime law and tradition oblige Masters to render any assistance they can to any vessel or seafarer in distress at sea, there is no obligation to assist, at any time, during a crisis on land except when the vessel has been hired to render such assistance. Where this is the case the ship would normally receive detailed instruction on the scope of work, any hazards that may be encountered and the possible detrimental effects to the vessel and crew. However, for a ship caught up in, or assisting during, a volcanic crisis this hazard information is not readily available. Even pilot books from areas of the world with a substantial number of active volcanoes offer little advice other than to take care and/or avoid the area. I know for sure that I was not instructed in any dangers to vessels or seafarers from volcanoes during the years I spent studying for my Master’s Certificate of Competency. Asking around my compatriots reveals this lack of knowledge pervades the whole industry. Having carried out several years of research into volcanic hazards to ships (see left), I now hope to fill this gap. For this first article I will focus on submarine eruptions as they hold a special place in my heart. Submarine and barely emergent/just emergent volcanic eruptions are manifested in a variety of ways, and I will deal with each separately, starting with deeper volcanoes and shoaling as I progress.

Submarine eruptions
The pressures and water temperatures at depth do not easily allow for the violently explosive type of activity often associated with volcanic eruptions on land. Deep submarine eruptions will often have no surface expression, at least in the vicinity of the volcanic edifice, and I could find no record indicating any significant expression further away. A few years ago, a remote submersible exploration of a mid-ocean ridge eruption identified gases being vigorously emitted but these would be substantially disbursed as they rose through the water column. Explosive activity was limited to the immediate area of the vent and no pumice or buoyant material was observed.

Floating debris
As the depth of water above the volcanic edifice reduces, so too does the suppressive pressure, and the observed violence of the explosive element of the eruption increases. Again there is little surface evidence of this, but, depending on the magma type involved, either basaltic balloons (gas-filled bubbles of lava that can be up to several metres in size) or pumaceous rock can form. Both are less dense than seawater and will float toward the sea surface, although some may break apart and lose buoyancy on the way up as water pressure decreases. Basalt balloons will not generally affect surface shipping. Like hot air balloons, they tend to consist of only one chamber full of gas and the outer skin is very brittle. This usually fractures soon after reaching the surface and the fragments then sink relatively quickly. Pumice rock, on the other hand, is made up of many small separate chambers, each holding some gas and making the whole rock less dense than water. Pumice pieces can range from small pebbles to larger than a football. When a large volume of pumice is erupted in a short time it forms pumice rafts which can be anything from millimetres to tens of metres thick. A pumice raft can be deep enough and have sufficient buoyancy to allow people to walk on it. Ships entering these pumice rafts will lose speed due to both increased resistance and reduction in the efficiency of the propellers. Where the pumice raft is deep enough the pumices will get entrained into the sea suctions, and will eventually block the strainers and lead to engine shutdown due to overheating. A Chief Engineer I once sailed with recounted a tale where his vessel got stuck in the middle of a raft of pumice as the engines overheated from pumice blocking the sea suctions. An attempt to measure the depth of the pumice failed to find the bottom of the raft. By cleaning the strainers and running the engine in short bursts they eventually managed to get to a point where they could determine the bottom of the raft and record it thinning as they progressed until eventually the raft was thin enough to clear the sea suction. Individual pieces of pumice have been found floating freely within and around pumice rafts at different depths related to their own neutral buoyancy. It is therefore possible, but less likely, that lumps of pumice could be sucked into the cooling water systems even if the ship does not enter the raft. Pumice is also extremely abrasive and will at the very least damage the hull paint as the vessel pushes through the raft, if not remove it altogether. The pumice’s impinging of the bottom plating of the vessel will exert an upward force similar to taking the blocks in dry dock. This will have a negative effect on the vessel’s stability but, as far as I can determine, not enough to normally cause a problem. At the time of writing there is a large raft of pumice drifting around the South Pacific Ocean which started life over a submarine volcano near Tonga. At the time of eruption some areas of this raft were thick enough to support a human being standing upright and some of the pumice stones measured more than 300mm in diameter. These rafts tend to thin and spread out over time, often splitting into narrow streams of pumice as they drift across the oceans. As they travel they become attractive to marine fauna and flora and by the time they make landfall are often so overgrown with marine organisms they are difficult to identify as pumice. In a year or two, much of the pumice in the present raft will find a final resting place on the beaches of north eastern Australia.

Gas emissions
There has been quite a lot of research and discussion into whether gas emissions from submarine sources might lead to a vessel losing buoyancy and sinking. Much of this research was done while looking at the possibility of harvesting methane clathrates (methane trapped in the crystal structure of water and deposited in submarine sediments) and the risks posed by extraction of the methane. The majority of the research indicates that a well-found large commercial vessel is unlikely to be affected by this gas release as the concentration of rising bubbles, whether from methane clathrates or volcanoes, is unlikely to be sufficient to overcome the buoyancy of the surrounding water. If you search YouTube you will find clips purporting to show bubbles sinking ships but the majority are either flawed or do not relate to larger commercial seagoing vessels.

Shoaling
As the water above the edifice gets shallower, so the eruption becomes more explosive due to both the interaction between the hot rock and water and the increasingly violent release of entrained gases. Indeed, shallow marine and just emergent volcanoes tend to be the most violent eruptions with regard to explosivity compared to volume of magma erupted. In 1952 a Japanese hydrographic department research ship, the Kaiyo Maru No.5, was blown apart with the loss of all 31 personnel on board by an eruption of the shallow submarine volcano Myojin Sho. This is still an active volcano and the Japanese pilot book warns vessels to stay away from the area. Eruptions of this type are typically called ‘cockscomb’ due to the shape of the eruption cloud immediately before dispersal by the winds. This type of eruption is quite prevalent in the SW Pacific and the darker explosive debris takes on this shape about 50 seconds into the following video...

This type of activity is named Surtseyan after the initial eruptions that eventually resulted in the formation of Surtsey Island. The recent White Island, New Zealand eruption was of this type although it is difficult to see the cockscomb shape of the explosion due to the amount of steam masking the eruption column – see the below video...

As the volcano emerges from the water the volume of water / magma interaction often decreases until the volcano starts to erupt in a fashion more in keeping with the magma type and subaerial edifice. This leads us nicely on to the next article, which will cover the effects that a vessel might experience from volcanic eruptions on land.

A volcanic eruption can take on many aspects, each of which may have an adverse effect on a vessel to a varying degree. A slow moving lava flow trundling down the side of a nearby volcano may cause some effect by distracting the crew from their work, perhaps allowing a minor injury to happen, but is unlikely to be catastrophic. Conversely, a 500°C cloud of tephra (volcanic ash) and large rocks travelling at 250 knots across the surface of the sea (pyroclastic flow) will kill all exposed to it, smash parts of the vessel to pieces and set fire to exposed combustible material to name just a few of the effects that might be expected. It was one of these pyroclastic flows that devastated around 18 ships at anchor off St Pierre, Martinique in 1902. It hit the cable ship Grappler beam on and capsized her, before setting fire to the remains, with the loss of all hands. Only one vessel managed to limp clear of the harbour but with about half the crew dead or dying. The pyroclastic flow also wiped out the city of St Pierre, killing around 28,000 people. A few seafarers from different ships survived although, somewhat surprisingly perhaps, it is often said that the only survivor from the direct blast was a prisoner in one of the jail cells.