The recent eruption of La Soufriere volcano in Saint Vincent island reminded me of the events of 1902, of the tragedy that was unleashed by the two Caribbean volcanoes, La Soufriere and Pelée. Our current method of ranking explosivity, the VEI, doesn’t capture at all the violence of these two eruptions nor any of their kind. To avoid another disaster like this from happening it should be understood how these eruptions happen, how big they are, and how they deal their damage, and because our models and classifications do not make any justice to their real intensity, I decided to do an article that fully focuses on this style. This article is about Pelean eruptions.
The eruption of La Soufriere in Saint Vincent, 1902.
La Soufriere was a beautiful, lush, tropical mountain, topped by a large bluish-green crater lake enclosed in steep, verdant walls. But it also had a bad reputation. La Soufriere had produced major explosive eruptions in 1718 and 1812. It was probably for this reason that the local Carib people were very wary of the mountain in which they were living. When frequent earthquakes started to be felt in the month of April the people living on the western slopes of the volcano prepared for evacuation.
May 5th , the day before the climax, a column of steam rose from the crater, and soon the western slopes were being vacated at full speed, with the few available boats being sent to the northern coasts to carry the news and rescue people. It happened though that the summit was constantly shrouded in clouds, due to the trade winds, when seen from the eastern and northern slopes of the mountain, so they didn’t see any of the steaming nor the incandescence that could be clearly seen from the settlements on the other side of the volcano, and because no earthquakes or rumblings were felt either, most people in these areas had no knowledge of any unusual activity until only 3 hours before the climax, and then it was too late.
It was May 6th. At 11 AM the volcano entered a phreatomagmatic eruption with the typical ink-black jets of mud shooting water from the lake onto the ravines around the crater and pouring in powerful lahars down into the Rabacca and Wallibu rivers. Explosions were heard and light rains of pumice and ash came down to the east. Only now did much of the population on the cloudy, windward side realize an eruption was underway. Many people at this point went into hiding inside houses or cellars.
A little past 2 PM those around La Soufriere heard a loud roar and an increase in air pressure which made their ears hurt. A gigantic explosion had sent a wave rippling through the atmosphere. The huge volume of ash and gas came down in a huge pyroclastic flow that within a few minutes swept down from crater to coast, engulfing a radius of 4-7 kilometres all around the vent, and leaving death and devastation in in its way. The flow burned and uprooted trees. As it came down to the shore, where most of the people were, it had lost much of its destructive effects and it did little damage to trees or buildings, but it was hot enough to kill every person and animal who was out in the open. Most of those who were inside houses and huts died too while a few survived. Instead, those who had previously taken refuge inside cellars all survived. There was one case where a men and his wife, who were hiding in a cellar with 70 people packed together, had decided to take a breath of fresh air only to die just outside.
The 2 PM explosion and pyroclastic flows came to be known as The Great Black Cloud. This is the account of some survivors who had evacuated the area, but then returned to retrieve some of their belongings, and on their way back from their village were caught in the edge of the cloud:
“The sea was perfectly calm and the day clear, though there had been a few drops of rain in the forenoon. The boat had just rounded the point south of Richmond River and was on the north side of Chateaubelair Bay. The cloud struck them like a strong breeze, though, being under the shelter of a high spur of land, they did not feel it much. Still it came over the water with a strong ripple and a hissing sound, due to the hot sand falling into the sea and making it steam. In a moment it was pitch dark and intensely hot and stifling. The cloud was highly sulphurous, and this irritated their throats and nostrils, making them cough. The heat was terrible and the suffocating feeling very painful. They threw themselves into the sea to escape burning by the hot sand. It does not appear that the surface of the water was boiling as it was in some other cases. They all dived, and when they returned to the surface the air was still unfit to breathe and the heat intense. So they continued to dive repeatedly, but when they came up again the air was almost as bad as before. How long this lasted they cannot tell, but they thought it might have been several minutes.”
La Soufriere then entered a sustained plinian eruption that lasted for 12-15 hours. Darkness fell over the whole of Saint Vincent, and ashfall extended to neighbouring islands. As much as this stage frightened the population it was relatively harmless, and except for some of the larger raining stones which, falling at an angle due to the trade winds, broke crystal windows and injured a few people, and some lightning which set the vegetation on fire, the plinian eruption otherwise did little damage. In many locations the ash and pumice was soon washed away by the first torrential rains and everything was back to normal. It was the part of island hit by the 2 PM explosion that was destroyed, and it was because of this brief event that 1600 lives were lost. The culprit was a singular but terrifyingly powerful explosion.
The eruption of Mount Pelée.
It is quite surprising that barely 18 hours after the eruption of La Soufriere, and on a nearby island, Mount Pelée would bring an even bigger volcanic calamity, even though there is no apparent connection between the two events.
Before 1902 the summit of Pelée was a horseshoe shaped crater. The crater was known as Etang Sec, it was bounded by walls 300 meters high except towards the southwest where it was open, in the general direction of Saint Pierre, the city that would be destroyed. The pyroclastic flows in May 1902 advanced much further in the southwest direction, 9 kilometres, than to the other sides of the cone, 3-4 kilometres, which was perhaps due to the topography of the crater.
Unlike with La Soufriere the people around Mount Pelée did get plenty of precursory activity. The crater had been in eruption for several days, and had sent some lahars through the breach into the Riviere Blanche. However Pelée had already erupted in 1851, and this eruption was small. While many in the villages closer to the volcano did flee, mostly to the city of Saint Pierre, it was considered that Saint Pierre was at a safe distance, and this would be true if the 1851 scenario was to repeat. However volcanoes like to throw some variety, and had they known of Pelée’s previous eruptions, in the geologic past, they would have probably reconsidered their safety.
The eruption was not showing much sign of escalation either, eruptions were frequent but remained relatively small. All of a sudden, at 8 AM on the 7th of May, the top of Mount Pelée exploded with a roar that was heard all around the mountain, pyroclastic flows shot through the breach in the crater towards the city of Saint Pierre and the harbour. The blast broke away trees, blew down buildings, twisted iron beams, sent statues flying, while the temperatures of more 300-400ºC set everything combustible ablaze so that the whole city was soon enveloped in flames. The destruction was much worse on the northern side of the city, than on the southern side, where farther away from the crater the buildings were less damaged, although there was not a roof intact. Several ships that were on the harbour caught on fire and sank. After the pyroclastic flow powerful winds blew towards the destructed area and there is a description reminiscent of a fire tornado.
An observer from Morne Rouge stated that the “burst of rocks” rose no more than 50-100 meters above the crest of the mountain, a maximum of 400 meters from the crater floor, quite low, and as I will further discuss later shows how the eruption clouds of pelean blasts move low over the ground. Several witnesses of the event indicate that the pyroclastic flow travelled in 1 minute from the crater to Saint Pierre, at an speed of 400 km/hour. The destruction was terrible and practically everyone within the city lost their lives, an estimated 28,000 casualties. Because of this there are very few accounts of how it happened. This is the description from Captain Freeman of the Roddan, the only ship on the harbour that escaped destruction:
” At about 8.15 he was in the chart room; a good many of the sailors were leaning over the side of the vessel watching the distant mountain, which was emitting dense clouds of smoke and occasional flashes of light. Mr. Campbell was talking to Mr. Plissonneau on the deck. On a sudden he (the Captain) heard a tremendous noise, as though the entire land had parted asunder. Simultaneous with the noise there was a great rush of wind, which immediately agitated the sea, and tossed the shipping to and fro ; he rushed out of the chart room, and looking over the town and across the hills he saw a sight he cannot describe. He remembers calling out to Mr. Campbell, and saying : ‘ Look ! ‘ —then an avalanche of lava was upon them. It immediately caught the town afire as it passed over it, likewise the shipping. It struck his ship with the force of a mighty hammer, and the lava rained upon the deck. Everyone, as far as he could see, sought shelter at once, but the heat was so great, and the air so suffocating, that Mr. Campbell and many of the crew, among whom was the chief mate, threw themselves in despair overboard.”
Pelée continued to erupt until 1905, producing occasional explosions with pyroclastic flows, and grew lava domes including a 300 meter high spine known as the Tower of Pelée, which later crumbled into rubble.
The explosion that destroyed Saint Pierre and The Great Black Cloud of La Soufriere at 2 PM cannot really be classified as plinian eruptions. The two events did not involve tall eruption columns at first, instead the material travelled horizontally from the vent over the ground in the form of pyroclastic flows, or pyroclastic density current that is how they are now being called. The volume of material that is erupted within a brief span of time in the explosion is so large that can’t possibly mix with surrounding air fast enough and become buoyant, instead the heavy, dense volume sweeps over the ground at enormous speed in a devastating blast, and it is only now as it advances over the surface and mixes with atmospheric air, which becomes heated and expands, that the pyroclastic flow inflates up into a buoyant plume known as a coignimbrite cloud.
Scientists of the time understood that the two eruptions of La Soufriere and Pelée deserved to be made into a class of their own. This is when the term pelean was coined. The eruption of Lamington in 1951 was also classified as pelean. After Lamington the term became somewhat forgotten and little used, and the type of eruptions that it had initially been created for remained little studied. Pelean was replaced with the term lateral blast, that stands for the same thing more or less, but seems to imply that the explosions come out laterally which is not true most of the times.
This is the exact way that pelean type eruptions were defined for the first time:
“Eruptions of the Pelean type are distinguished by the occurrence of one or more discharges of incandescent sand, which rush down the slopes of the mountain in the form of a hot-sand avalanche, accompanied by a great black cloud of gases charged with hot dust, which sweeps over the country with a very high velocity, mowing down everything in its path. All living beings within the zone nearest the crater are killed ; all plants reduced to charred and broken stumps. At greater distances men and animals are scorched by hot sand or mud ; plants are burnt, eroded, and stripped of leaves and branches ; but beyond the limits covered by the great black cloud no effects are produced, other than those consequent on the rain of ashes which precedes or follows the avalanche.”
This basically means eruptions with pyroclastic flows. While it is true that many types of eruptions can have them, which the authors were not aware of at the time, the 1902 eruptions of La Soufriere and Pelée do stand out for the massive scale of the flows, and the style in which they were erupted is distintive. A huge violent burst. It is similar to vulcanian explosions but much bigger. Vulcanian grades upward into pelean so it is just a spectrum between the two types.
What drives pelean eruptions? The sudden expansion of gases within the magma column or the flashing of groundwater into steam is what ejects a large amount of dense material. For this it is required that the pressure acting on the gas suddenly drops. Why this happens is not obvious in most cases. I am tempted to say that a clog seals the vent which causes magma pressure to rise in the conduit below, then when the clog is breached it explodes. In reality many volcanoes that have gone pelean were erupting just before the big blast came, so that it doesn’t look like the vent got plugged in any way, suggesting that it is more complex than this, and that such a simple reasoning would not be useful in any way.
Maar-type eruptions have a more obvious trigger. For example during the eruption of Tarawera a dyke of basalt lava intruded below Lake Rotomahana, which was a geyser field, and as the water came into contact with the magma it became superheated. When this natural, gigantic pressure cooker found a way out through the cracks, that the dyke itself was opening up, it simply blew up the whole lake in a very pelean way, and blasted away the famously beautiful Pink and White Terraces. It left behind a huge crater with a lake that was larger than before. This is called a maar crater.
Another obvious trigger is a dome collapse, or a landslide like how it happened to Saint Helens in 1980. There are suggestions that a landslide was also what started the eruption of Lamington in 1951. The lithostatic load is suddenly reduced which frees the gas to expand violently, and blows the conduit from within.
The eruption of Mount Saint Helens was closely monitored and is an insightful example. The pyroclastic flow it produced was so immense that it blasted down forests up to 20 kilometres from the volcano, which is an area far more extensive than in La Soufriere and Pelée eruptions. There are many photographs of the disaster that were taken from multiple angles around the mountain, even from a plane flying above its summit, and also the famous series of photographs taken by Gary Rosenquist from the northeast of Saint Helens.
Physical models created after the eruption together with the photographs captured during the event show how quickly the material is ejected. Saint Helens discharged 0.19 km3 of ash into the pyroclastic flow, or pyroclastic density current, that later swept northward, and this was within the span of 20 seconds This means Saint Helens reached VEI 4 in less than 20 seconds.
VEI means Volcanic Explosivity Index, and while it is meant to measure explosivity it would still make no distinction between Saint Helens, and any other eruption that took hours or days to produce the equivalent volume, because VEI considers only the total amount of ejecta, whatever the eruption rate or style. Seems absurd doesn’t it? But it goes beyond the VEI, because the overreliance on this way of classification also reflects the aspects that we give most importance to, the total volume, and the column height, both very poor indicators of the destructive consequences that powerful brief explosions unleash. It does turn out that the vast majority of pelean eruptions are VEI 3. The eruption of Lamington in 1951, which flattened the tropical forest over a radius of 12 kilometres around the volcano, had a volume of “only” 0.025 km3, and the same is true for the eruption of Taal in 1911 and many others. Often a pelean event reaches VEI 4 or more only when there are other stages in the eruption that produce a greater volume. The Volcanic Explosivity Index is so flawed that there are fire fountains which can be watched safely from up close, in the same VEI 3 slot as eruptions that could blast away whole cities.
Sparsely a single short explosion can have a volume of >0.1 km3, like with Mount Saint Helens, and even more rarely they may reach a much greater scale during caldera-forming eruptions. Krakatau volcano erupted in 1883 with an initial plinian stage during the afternoon of August 26. The following morning came a series of 4 powerful explosions, I’d say pelean type, that produced massive pyroclastic flows, tsunamis, and killed 36,000 people. The largest of Krakatau’s explosions was heard 4800 kilometres away, and is estimated to have been 8 times more energetic than the blast of Saint Helens.
Pelean eruptions the most dangerous for many reasons, mainly: pyroclastic flows, lahars and tsunamis. A crater lake can be ejected into lahars during the initial explosion. The massive pyroclastic flows that ensue can generate lahars when they move over a glacier, like with Nevado del Ruiz, and can displace the water of a lake or the sea into a tsunami, like with Krakatau. Tsunamis and lahars extend the damage of the pyroclastic flows to greater distances.
The coignimbrite cloud
Initially the pyroclastic cloud is way too dense to rise so instead it flows laterally over the ground like a sheet. As it advances it mixes with external air increasing the temperature of the later, which becomes buoyant, so that the top of the flow inflates with hot air and rises in a powerful updraft, known as a coignimbrite cloud or plume. This creates an area of low pressure, drawing air inward, and often this inflow is strong enough to reach hurricane wind speeds that can deal damage on their own.
The eruption of Pinatubo in 1991 had a series of precursory eruptions before the climax, the first four of which were plinian, which were followed by thirteen pelean explosions with shockwaves and giant pyroclastic flows. The following image shows the coignimbrite cloud of one of the explosions, 8 hours before the climax. The line of mountains is 1 kilometre tall as a reference. The photo was taken only 6 minutes after the eruption started, which shows how fast the pyroclastic flows and coignimbrite cloud grow.
The shockwaves allowed the precursory explosions of Pinatubo to be identified. Pelean erutions make powerful shockwaves. This is why measuring the infrasound waves generated by volcanic eruptions might be the only way to accurately detect the individual pelean explosions, which otherwise are very sneaky, they happen quickly, often shrouded in the darkness of ashfall, and moving very close to the ground, as shown by people that were caught in the edges of Saint Helens’ pyroclastic flow, who often didn’t see it coming until it was upon them because it would be hidden behind trees or hills. The coignimbrite cloud then shoots upwards from the pyroclastic flow and would be detected by planes or satellites, but even then it can’t be distinguished from a typical plinian column unless you have a visual of the broad base, which probably isn’t the case.
There is too much attention being paid to the plinian style, a sustained jet of pyroclasts, gas, and hot entrained air that rises up into the air into a lofty umbrella. The VEI is appropriate to measure the impact of a plinian eruption in such aspects like aviation or climate. However pelean explosions, also known as lateral blasts, are far more dangerous to the immediate surroundings of the volcano, as well as very important to understand how volcanoes work, and to how ignimbrite deposits are formed, so should be given as much attention as plinian and the VEI doesn’t rank them properly.
Pelean explosions are brief but powerful and they may occur isolated, intercalated in plinian or phreatomagmatic eruptions, or at the onset or the end. They generate massive pyroclastic flows that will most living beings in their path, so the area should have been evacuated by the time they happen or else the loss of life can be terrible. And lets hope that no large city is ever found in the path of a large pelean blast, again.