Pyroclastic Flows

Pyroclastic flows pose a great threat to human life due to their high velocities and considerable temperatures. Pyroclastic flows have been the most common form of direct volcanic-related death in the last 400 years (Table 6 on p. 14). In many historical eruptions pyroclastic flows have been the major cause of death (Table 7).

Kilauea, 1790

The following extract (from Stearns, 1925) describes the effects of a pyroclastic flow from Kilauea in 1790:

"The army of Keoua a Hawaiian chief, being pursued by Kamehameha, were at the time near Kilauea. For two preceding nights there had been eruptions with ejections of stones and cinders. The army of Keoua set out on their way in three different companies. The company in advance had not proceeded long before the ground began to shake and rock beneath their feet, and it became quite impossible to stand. Soon a dense cloud of darkness was seen to rise out of the crater, and almost at the same instant, the thunder began to roar in the heavens and the lightning to flash. It continued to ascend and spread around till the whole region was enveloped, and the light of day was entirely excluded.

The darkness was the more terrific, being made visible by an awful glare from streams of red and blue light variously combined through the action of fires of the pit and the flashes of lightening above. Soon followed an immense volume of sand and cinders, which were thrown to a great height and came down in a destructive shower for many miles around. A few of the forward company were burned to death by sand, and all of them experienced a suffocating sensation.

The rear company, which was nearest the volcano at the time, suffered little injury; and after the earthquake and shower of sand had passed over, hastened on to greet their comrades ahead on their escape from so imminent peril. But what was their surprise and consternation to find the center company a collection of corpses. Some were lying down, and others were sitting upright, clasping with dying grasp their wives and children, and joining noses as in the act of taking leave. So much like life they looked that at first they supposed then merely at rest, and it was not until they had come up to them and handled them that they could detect their mistake."

Some of the narrators, who saw the corpses, affirm that though in no place deeply burnt, yet they were thoroughly scorched.

Bandai, 1888

In 1888 at Bandai (Japan), a debris avalanche buried 461 people but only 117 bodies were recovered, some battered by falling rocks. A hurricane of hot ashes and falling stones injured a further 70 people. The following extract describes the scene (from, Wilcoxson, 1966):

"The wounded are receiving treatment in the schoolhouse at Inawishiro, but their condition is terrible. Some have fractured skulls, the majority broken limbs, while others are fearfully burned. Five villages have been totally buried. The dead appear as the victims of a huge boiler explosion. Many are cut to pieces, and others parboiled, so that it is difficult to distinguish sex. But the most ghastly sights which met the eyes of the helpers were bodies dangling on the branches of trees. Thrown into the air by the awful violence of the eruption, their descent had, in many cases, been arrested by the trees. And there the victims hung, their bodies exposed to the cruel and well nigh ceaseless rain of red hot embers and burning ashes. From appearances death speedily relieved them from their agony, yet, short as the time was, their suffering must have been past belief. In other places flesh hangs from the branches of trees as paper from London telegraph wires."

Table 7. Eruptions with lethal pyroclastic flows, adapted from Blong (1984).

Soufriére, 1902

Figure 11.

Following the Soufriére (St, Vincent - West Indies) eruption of 1902, the death toll was between an estimated 1565 to >2000. The deaths were attributed mainly to asphyxiation by a hot mixture of air, steam and dust, but also due to burns by lapilli and dust, blows by falling stones, nervous shock, burning by steam and lightning strikes. The gases SO₂ and H₂S also probably killed some of the people. Death by asphyxiation was indicated by the positions in which many of the bodies were found. Steam burns occurred under undamaged clothing and most of the dead (as well as the survivors) were severely burned on any exposed areas of their bodies. A survivor that suffered severe burns to his face, arms and neck gave the following account (from Will, 1903):

"I lived at Overland (Fig. 11). On the morning of the eruption I was cutting canes on Tourama Estate (around 5.5 km east of the volcano). About 9 o’clock there was a drizzle of rain with ashes falling, but I still goes on work, then ashes fall heavier. About 12 o’clock, I start to go home, the ashes still fall and small stones, and when I go home I hear a great noise in the direction of the mountain like a rushing river. All the people then start to run from the village, Tourama way, but when we get half part of the way we turned back, because we meet the Tourama people coming to Overland. As soon as we reach Overland again large stones begin to drop, and this cause all the people to run into the houses. With many others I run into Victor Sutherland’s shop. It have a strong galvanised roof, but soon some large stones fall through the roof. About 2 o’clock great darkness come on, and we shut the doors and windows. After this a great heat come with hot ashes through the chinks in the doors and windows, and through the holes in the roof. The hot ashes get into our mouths, and stop our mouths as fast as we try and breathe. We toss backwards and forwards for about two seconds and then everyone fall down. I did not lose my senses, but cannot tell exactly what happened after I fall. I feel choked with the hot stuff going down my belly, and smell plenty sulphur. This did not last too long, only two or three minutes, then I try to get up, but two people both dead lie across me, and after a struggle enough I get on my feet. I then know I was burned, I think I was burned when lying on the floor. I now open a window a little to get air, this relieved me a little, but stones still dropped, and more ashes come in, I shut it again, I watched the great black cloud for about two minutes, while the window was open, it was full of fire like lightning, and I see stones drop on the ground break in pieces and glow hot. At this time many people in the shop done dead, and some of the houses outside were burning.
Stones now ceased to drop, and there was a calm - this would be about 4 o’clock, and it was still dark. We now open the windows and doors, and sit up till day clean, then I walked to the Hospital at Georgetown. I know of 6 people being alive in the shop next morning, they were all burned. I think about 80 people die in the shop, most after we fall, but some during the night."

Another group of survivors were caught on the edge of a pyroclastic flow while off the west coast of St. Vincent in a rowing boat. They described a 1 to 2 minute period of a hot, highly suffocating gas cloud that irritated their throat and nostrils, causing them to cough. Other survivors reported the following (from Will, 1903):

"Further evidence showed that every person who left shelter during the wave of hot ashes was killed in a few minutes, that inhaling the ashes, etc., caused a feeling as if the windpipe was being compressed, and that this feeling was less acute while the sufferers held their breath…

The burns seem to have been entirely caused by the hot dust falling on exposed parts at a temperature high enough to cause vesication and destruction of the skin, but not at a sufficiently high temperature to ignite the clothing or the thatched roofs of the houses. Many houses were burned, but they seem to have been ignited by the “fire-stones”. None of the burns presented the appearance characteristic of those inflicted by lightning and the depth of tissues destroyed was greater than that usually found in burns by steam.

The large number of deaths caused by the eruption appear to have been almost entirely due to asphyxia by the hot ashes and heated air, the latter being probably somewhat deficient of oxygen. The cloud of dust was highly charged with electricity, and it is likely that some of the deaths outside the houses were cause by lightning, also that a few, especially children, may have been killed by the falling stones."

After the May 7 eruption, 224 people were hospitalised, 221 with burns, from which 79 later died. The Governor described tending the wounded, (from Will, 1903):

"The 191 cases of burns admitted were characterised more by the extent of the areas implicated than by the depth of destruction of the affected surfaces. The sites were chiefly the face, ears, neck, forearms, and backs of the hands, the legs and feet, and about 20% had body burns as well, situated in most of these cases on the shoulders and buttocks. In one case only were there as few as 2 burns, the majority had at least 4, and some as many as 8 or 10.

The degree of burn varied from destruction of the cuticle to that of the true skin and subcutaneous tissues. The latter degree occurred in 5 cases only ….. In most of the cases several of the degrees were combined in the surface affected ….. In consequence of the degrees and extent of the injuries, pain was intense."

Mt. Pelée, 1902

The 1902 eruption of Mont Pelée (Martinique) caused between 28 000 and 30 000 deaths in St. Pierre. The deaths were attributed to the same causes as those at Soufriére on the previous day, but with some due to crushing beneath falling walls and other objects and cremation in burning buildings. However, the lack of respirable air was probably the main factor. Witnesses on the margins of the flow reported a lack of air to breathe, although they were not suffocated by ashes or detectable gases. Many people and animals in St. Pierre were found dead in frozen positions including a man holding a struggling frightened horse, but not all were killed instantly. Many other bodies were found in positions of flight and people in boats offshore witnessed people running along the waters edge for several minutes. A witness to the aftermath of the event reported the following (from Will, 1903):

"At every turn scenes of indescribable horror met the eye. Dead bodies lay scattered about, singly and in groups, most with face to the ground, and many with the face resting on the forearms or covered with the hands. With one exception all the victims were devoid of clothing. This exception appeared to be a white man, probably a sailor, who had escaped from one of the burning ships. He was lying on the face with his jersey pulled over the head.

A brief examination was made of a number of the victims. In many there were no signs of actual burning, they appeared shrunken, dried up, and mummified. On the other hand some were terribly charred, the flesh being in parts entirely consumed, exposing the bones of the limbs and skull. There was no stench, only a faint odour of burning flesh.

For a town of 26 000 inhabitants of whom not a single soul escaped, the number of bodies seen in the streets was comparatively small, and even in the houses where, no doubt the majority had sought refuge from the burning ashes, few bodies were seen. It seemed evident that the majority of the victims had been buried under the fallen and burning roofs, and that large numbers must have been entirely incinerated."

An earlier visitor to the ruins of St. Pierre had also noted that the bodies of victims were greatly reduced in size, as though the moisture had been extracted from them. However, they were not burned and trees and other pieces of wood were barely charred. He concluded that the duration of the heat must have been extremely brief, or because of a lack of oxygen had failed to ignite anything. The few survivors in the city were severely burned as were almost all survivors on the ships in the harbour, although most of these were first degree burns.

The varying nature of the appearance and apparent cause of death of the inhabitants of St. Pierre indicate a variable intensity of the pyroclastic flow destruction in different parts of the city.

Taal, 1911

Figure 12.

Following the eruption of Taal (Philippines) in 1911, around 1335 people were killed by pyroclastic flows, but the bodies of only 732 were found. On the island upon which Taal volcano is located (Fig. 12), 12-13 people survived although being badly wounded.

Some villages on Volcano Island were marked only by charred and blackened bodies, everything else was swept away by tsunami waves. There were no survivors at Gelud and Bilibinang villages (Fig. 12), within a zone of complete obliteration. Outside this zone in places such as Subig and Bayauyunggan very few people were killed. Around 200 people were injured - mostly burned by hot sand and ash. Survivors escaped by staying under mats and reported that people died by suffocation. This corresponds with the face-downward crouching position of many of the corpses, apparently trying to escape from suffocating gases.

The following account describes the scene after the eruption (from Heiser, 1936):

"The sight was horrible. Not only had all life been wiped out on Volcano Island itself, but also for miles away from the shore of the lake. The dead could wait; our first care was for the wounded and the homeless strays who were left without food, drink, and shelter. Temporary hospitals were set up; acres of gauze were used to dress the frightful burns, bruises and fractures. Many women and animals were found to have aborted, doubtless from the terrific concussion.

The number of wounded was in remarkably small proportion to that of the dead, and the evidence showed that the majority of the fatalities had been caused by the explosion of gases. When nature runs amok, she often does so in a ghastly way. The Filipinos are normally so modest that they are never caught with their clothes off. Even in bathing they are well covered. But among the ruins we found Filipinos stark naked, standing upright and embedded in ashes to their knees, still holding protecting umbrellas over their heads. They had apparently been attempting to get away when the ashes rained over them and the vacuum caused by the exploding gases had torn off their clothes."

Lamington, 1953

During the 1953 eruptions of Mount Lamington in Papua New Guinea, 2942 people were killed by pyroclastic flows. All of the 35 Europeans living at Higaturu and Sangara Mission died (Fig. 13). At a school between Higaturu and Sangara, around 40 people crowded into a small house to escape. The house was destroyed by the pyroclastic flow, leaving the dead occupants piled on the floor.

Figure 13.

In the zone of partial destruction (Fig. 13) some villagers were killed, while survivors were severely burned, some over their entire bodies, leaving skin peeling off in strips. Forty survivors from the inner zone of total devastation (Fig. 13) reached the mission hospital at Issivita, but 22 died in the following 24 hours. Seventy casualties were airlifted to more distant hospitals and only three of these perished. The experience of the survivors was firstly, pains in the mouth, throat and eyes, followed by burning sensations in the chest and abdomen, and then a feeling of rapid suffocation. They also had severe burns to uncovered parts of their body and internal burns. They had excessive thirst but could not swallow with internal damage causing a mucus or blood discharge from the nose and mouth.

The following observations made after the event discuss the causes of death (from Taylor, 1958):

"The exact cause of death in the glowing cloud was, unfortunately, not clarified by autopsy; putrefaction was too advanced when the medical services were free from their urgent obligations to the living. The appearance of the bodies in the devastated area at Lamington suggested that rapid damage to the respiratory system might have been an important factor in the cause of death. It was extremely difficult to distinguish between Europeans and natives 48 hours after the eruption. This may have been partly due to an intense post-mortem lividity which is characteristic of death from asphyxia. In this form of death the blood does not readily coagulate, but migrates to the extremities of the body. Severe internal burns were suggested by the presence of dried blood around the nose and mouth of some victims. Rigidity was a notable feature of many bodies and was mostly due to the well known effect of heat stiffening brought about by coagulation of the albuminous material in the muscles. In some cases, however, sudden death had cause the instantaneous rigidity characteristic of cadaveric spasm and the bodies remained, after death, in sitting or kneeling positions. Lacroix attributed similar occurrences in St Pierre to inhibition, a term which describes the depression of the heart's action by stimulation of the vagus nerves. Few people appeared to have been killed by flying debris or by being crushed by falling trees or buildings. There were no dismemberments such as were common in St. Pierre."

St Helens, 1980

Following the May 18, 1980 Mt. St Helens eruption 57 people were killed, although the bodies of only 35 were recovered. In the few days following the eruption, 128 survivors were found, 53 within 1.6 km of the zone where trees were destroyed by the flow. Many of the other survivors were found south of the volcano, the blast having occurred to the north. On the edge of the zone of searing from the pyroclastic flow/lateral blast there were 11 survivors; four suffered severe burns (including two who were inside a car).

Autopsies revealed that asphyxiation by tephra was the primary cause of death, with most victims having a plug of ash and mucus blocking the upper airways. Death probably occurred within a few minutes; this included victims within vehicles. A few deaths were caused by physical injuries, from falling trees and large blocks. In many cases skin was tanned or mummified, portions of bodies within flow deposits were skeletonised. Victims within 18 km of the volcano had singed hair indicating temperatures of >120^oC. Severely burned bodies appeared dried and baked, rather than the usual charred appearance of fire victims.

Summary

The detailed observations of victims following the Mt. St. Helens eruption were similar to those reported from earlier pyroclastic flows, such as those at Mt. Pelée and Soufriére. Asphyxiation from inhalation of hot ash and gas was probably the most important cause of death in those victims recovered. However, many victims in these types of events are not recovered at all, being buried within pyroclastic flow deposits.

In addition to asphyxia, burns or scalds can be an important cause of death. This is probably more so in wet pyroclastic flows because water conducts heat more rapidly than dry air, thus markedly decreasing the exposure time necessary to cause scalding.

To increase chances of survival in a pyroclastic flow, shelter in sturdy, earthed (against lightning strike) buildings with heavy protective clothing and breathing masks is probably the only measure possible. Masks will help to slow the asphyxiation process, perhaps enabling survival through the typically few-minute duration of most pyroclastic flows. Clothing helps against burn injuries, although it can be ineffective if the pyroclastic flow contains steam. In addition, small pyroclastic flows tend to follow topographic lows, hence the safest areas to shelter are on the lee slopes of ridges away from stream valleys. However, large flows tend to be less affected by topography.