As we have seen, we know why volcanoes occur, and we know where they occur. But not all volcanoes are the same. Indeed, strictly speaking, no two volcanoes are the same. But for convenience, volcanologists classify them in five main categories, even though there are really no distinct boundaries between the categories, and borderline cases are sometimes hard to pigeonhole.
Putting volcanoes in their place
The gentlest volcanic eruptions work in a way that resembles the steady release of lava from the Earth’s interior at the spreading ocean ridges. The archetypal example is the Hawaiian volcanic chain, which gives these eruptions their name: Hawaiian volcanoes. These produce very fluid flows of almost pure basalt in a runny form. Because the liquid is so runny, it is easy for any gases associated with it to bubble off, and pressure doesn’t build up to the point where the volcano blows its top in a spectacular explosion. It is on the surface of the lakes of fluid magma that fill the craters of such volcanoes that geophysicists can watch slabs of solidified material floating about – a model of the processes of plate tectonics, complete with both constructive and destructive margins and places where ‘plates’ rub side by side, like miniature San Andreas faults.
Moving up the scale of volcanic violence, the next category are the Strombolian eruptions, which get their name from the volcanic island of Stromboli, which lies between the Italian mainland and Sicily. The basalt that emerges from such volcanoes is thicker and more sticky than the lava that flows from Hawaiian volcanoes, and this allows gas to build up in large bubbles that burst out in minor explosions as often as every few minutes. In these outbursts, lumps of semi-molten lava get thrown up into the air, and sometimes lava breaks out from the crater and flows a little way downhill. But by and large Strombolian volcanoes are more noisy than dangerous, and it is a reasonable rule of thumb that the shorter the interval between explosions, the less dangerous the volcano is. On Stromboli itself, two villages are located within a couple of kilometres of the crater; Mount Etna, in Sicily, is another example. But Strombolian volcanoes are by no means restricted to the Mediterranean region, and Mount Erebus, far away in Antarctica, is another example of the type.
The first really dangerous volcanoes are located roughly in the middle category of the classification scheme, and this is where we find the volcano that gives all volcanoes their name – Vulcano itself, on a small island located to the north of Sicily. (Many volcanic archetypes are located in the Mediterranean, simply because that is where scientists first studied volcanoes; there is no other significance in this.) Vulcano, like other Vulcanian volcanoes, is only active intermittently, but when it does blow its top, it does so more spectacularly than any Strombolian volcano. A Vulcanian eruption can last for months at a time, spewing out blocks of solid material and producing great quantities of ash, carried up into the atmosphere by hot gases rising from the volcano. Although Vulcanian eruptions do often produce flows of molten lava, the ash usually does far more of the damage. The ash spreads over wide areas, and ash falls may continue for a long period.
A Vesuvian eruption, named after Mount Vesuvius, near Naples, is like a more extreme Vulcanian eruption, with a persistent plume of gas and ash rising much higher into the atmosphere. Curiously, because the force of this blast is directed upwards and the ash falls far away, Vesuvian eruptions may cause less damage in the immediate location of the volcano than Vulcanian eruptions do. But this is by no means always the case.
The fifth category of volcanic eruption has two names, or is sometimes subdivided into two essentially indistinguishable sub-categories. A Peléan eruption, named after Mount Pelée in Martinique, in the Caribbean, is a violent outburst that releases hot clouds of gas and dust, which flow down the mountainside and engulf any unfortunate towns and villages that lie in the path. These hot clouds are sometimes known as nuées ardentes, the French term for ‘glowing clouds’, but more commonly as pyroclastic flows. The other name, or sub-category, is a Plinian eruption, named after the Roman Pliny the Elder, who died during the huge eruption of Mount Vesuvius that destroyed Pompeii in AD 79. This eruption produced an enormous vertical column of dust and gas, but it also produced hot clouds of glowing material, which rolled down the mountainside. In this terminology, Plinian eruptions are the biggest volcanic eruptions of all; Peléan eruptions are simply Plinian eruptions, on a smaller scale. The only real difference is that most of the force in a Plinian eruption goes upwards, while most of the blast from a Peléan eruption goes sideways.
In view of their place in popular mythology, it may be a surprise to learn that today there are only about five hundred active volcanoes scattered around the Earth, and probably no more than twenty-five of them will erupt in any one year. But the dramatic impact of even a modest volcanic outburst ensures that these geological actors continue to grab our attention. Geologists have accordingly coined a variety of suitably dramatic names to describe the material thrown out by volcanoes. ‘Bombs’ are lumps of solid or semi-solid lava that may be bigger than a human being, thrown out hundreds of metres from the centre of activity; if a solid crust surrounds a sticky interior that bursts open when the bomb hits the ground, it is called a ‘breadcrust bomb’. Ribbons of lava that twist and solidify as they fly through the air are called, logically enough, ribbon bombs; round lumps are dubbed spherical bombs; long spindly pieces are known as fusiform bombs; and material that makes a splatter on landing is called a cow-dung bomb (the children’s favourite).
Smaller pieces of ejecta, like grit and pebbles, are called lapilli, and the even smaller bits are simply called ash and dust. The overall name for all the solid material ejected from a volcano is tephra. But it is the smallest pieces of solid material that can do the most damage, as the inhabitants of Pompeii found out.
Vesuvius AD 79
Present-day Vesuvius is the modest remnant of a site of ancient volcanic activity. There, we see a huge crater, part of which can be traced along the semi-circular ridge known today as Monte Somma. The geological activity of the region is a consequence of the collision between the African and Eurasian plates, in which the African plate is being pushed under the Eurasian plate to the north.
Two thousand years ago, the region was geologically quiet and Vesuvius itself had not erupted in living memory. The slopes of the mountain were cultivated with vineyards, which flourished in the fertile soil, and the region was, by the standards of the day, densely populated. In AD 63, there were signs that this period of volcanic inactivity was coming to an end, since Seneca that year recorded a violent earthquake occurring on the mountain and damage to the surrounding towns. Over the next decade and a half, Romans became used to minor earthquakes in the region, regarding them as normal – nothing to worry about. Then, in AD 79, in possibly the most famous volcanic eruption in history, the mountain exploded, burying the nearby cities of Pompeii, Stabiae, and Herculaneum in ash. An eyewitness account of the event has come down to us from Pliny the Younger, the nephew of the elder Pliny. Since then, more modest eruptions have become a regular feature of Vesuvius, changing the size and shape of the mountain dramatically over the centuries, as recorded by many artists. The most recent eruption was in 1944 – and Vesuvius is the only volcano on the European mainland to have erupted since 1900. Naples and its three million inhabitants, just nine kilometres (5.5 miles) from the volcano, can only wait and wonder when, and how big, the next eruption will be.
The younger Pliny, then just seventeen, was staying with his uncle (who was also his adoptive father), Pliny the Elder, at Misenum, thirty-three kilometres (twenty miles) away from the volcano across the Bay of Naples, when the eruption began. He later wrote to Tacitus that:
On the ninth of the calends of September, about the seventh hour, my mother informed [my uncle] that a cloud appeared of unusual size and shape ... the cloud (the spectators could not distinguish at a distance from what mountain it arose, but it was afterwards found to be Vesuvius) advanced in height; nor can I give you a more just representation of it than the form of a pine-tree, for springing up in a direct line, like a tall trunk, the branches were widely distended ... It sometimes appeared bright, and sometimes black, or spotted, according to the quantities of earth and ashes mixed with it. This was a surprising circumstance, and it deserved, in the opinion of that learned man, to be inquired into more exactly.
The elder Pliny, who was in charge of the Roman fleet at Misenum, then took several boats and set off ‘to inquire more closely’ into the phenomenon, and help the citizens of Stabiae, about 4.5 kilometres (2.8 miles) from Pompeii. Hampered by the wind, showers of hot cinders, and pumice covering the water, Pliny was forced to stay overnight in Stabiae with a friend. In the morning, the family fled to the boat, heads covered ‘with pillows bound with napkins; this was their only defence against the shower of stones. And now, when it was day everywhere else, they were surrounded with darkness, blacker and more dismal than night, which however was sometimes dispersed by several flashes and eruptions from the mountain’.
Even in the boat the family was far from safe. Because of continuing earthquakes that violently disturbed the water, they could not move out into the bay but had to shelter as best they could by the shore until the ash fall ended. There, the elder Pliny died, ‘stifled’, his nephew surmised, ‘by the sulphur and grossness of the air’. But since the rest of the party suffered no such extreme ill effects and there were no marks on the body, it seems more likely that the old and rather corpulent man was overcome by his exertions and suffered a heart attack.
In Misenum, at six o’clock the next morning:
On the land side a dark and horrible cloud, charged with combustible matter, suddenly broke and shot forth a long trail of fire, in the nature of lightning but in larger flashes ... Not long after the cloud descending covered the whole bay, and we could no longer see the island of Caprea or the promontory of Misenum ...
We had scarce considered what was to be done, when we were surrounded with darkness, not like the darkness of a cloudy night or when the moon disappears, but such as is in a closed room when all light is excluded. You might then have heard the shrieks of women, the moans of infants, and the outcries of men ...
A little gleam of light now appeared. It was not daylight, but a forewarning of the approach of some fiery vapour – which, however, discharged itself at a distance from us. Darkness immediately succeeded. Then ashes poured down upon us in large quantities, and heavy, which obliged us frequently to rise and brush them off, otherwise we had been smothered [or] pressed to death by their weight ...
When the cloud settled and daylight shone on the scene, everything around them was ‘covered with ashes as thick as snow’.
The inhabitants of Pompeii were less fortunate. There, the weight of ashes did smother them to death, entombing the whole city and preserving the remains until their discovery in the eighteenth century. It is estimated that about four cubic kilometres – a cubic mile – of ash and rock was spread over the area to the south and south-east of Mount Vesuvius by the eruption, which lasted for about nineteen hours.
Krakatau 1883
The other candidate for ‘the most famous volcano of all time’ has to be Krakatau, sometimes known by the westernized version of its name, Krakatoa. The violent explosion of Krakatau in 1883 is now seen as an almost perfect example of activity associated with an island arc, in this case a site near Java in Indonesia, on the western side of the famous ‘Ring of Fire’, which forms the rim of the Pacific plate. It is part of the same region of tectonic activity responsible for the Boxing Day tsunami.
After being quiet for a couple of hundred years, Krakatau showed distinct signs of life on 20 May 1883, erupting with a noise described as being like distant artillery fire and producing ash which fell gently on the island of Java, more than 150 kilometres (100 miles) away. Over the next few days, ships passing through the busy twenty-four-kilometre- (fifteen-mile-) wide strait between Sumatra and Java reported pumice floating on the surface of the sea, and a column of dust and smoke rising above the volcano. On 26 May an expedition to the island found it covered in white ash, with clouds of material still being spewed high into the air from time to time. The activity was coming from a vent on the side of the mountain, just 120 metres (about 400 feet) from the shore, on a mountain that rose 812 metres (2664 feet) above sea level. With hindsight, it seems likely that the proximity of this active vent to the shoreline contributed significantly to the violence of the explosion that eventually followed.
But the mountain rumbled on alarmingly throughout June, July, and most of August before something even worse than anyone had feared happened. By the afternoon of 26 August 1883, violent eruptions were shaking the mountain, with explosions so loud that they were rattling windows and shaking pictures hanging on the walls of houses in Batavia, more than 160 kilometres – one hundred miles – from Krakatau. There was already now little hope for the few people living on islands near the volcano. But the worst was yet to come.
The explosions which made Krakatau a household name for volcanic aggression occurred on 27 August, at 5.30 a.m., 6.44 a.m., 10.02 a.m., and 10.52 a.m. The third explosion was by far the biggest, according to the distant observers who survived to share their observations. This explosion in particular was so huge that for almost a century it defied explanation. But the best modern supposition – it can never be more than an educated guess – is that the explosions that occurred on 26 August weakened the rock between at least one volcanic magma chamber and the sea, and this collapsed completely on that day, allowing water to flood in and mix with the magma. You might think that water would quench the fire – but where there is so much molten rock and it is so hot, there is no chance that this will happen. Instead, conditions are ripe for what is known as a fuel-coolant interaction, or FCI.
In such an interaction, instead of the coolant quenching the fuel, the fuel is so hot and there is so much of it that as the coolant mixes in to it, the coolant is surrounded and turned into vapour very quickly. Being ‘turned into vapour very quickly’ is exactly what happens when a conventional explosive is triggered, and the result is the same for an FCI – an explosion. Under the right conditions, a bubble of expanding coolant will fling the surrounding hot fuel, and anything else around it, far and wide. When one cubic centimetre of lava is cooled from 1100 degrees to 100 degrees Celsius (2000 degrees to 212 degrees Fahrenheit), the energy liberated is nearly three hundred joules, meaning that five grams of lava involved in a violent cooling release as much energy as the explosion of one gram of TNT. With tens, or even hundreds, of millions of tonnes of lava, even hotter than 1100 Celsius, present in the magma chambers beneath Krakatau, there is no difficulty comprehending how the mountain blew apart, with a violence estimated as equivalent to the explosion of at least fifty megatonnes of TNT; some estimates suggest it may have been equivalent to a two hundred megatonne bomb. The explosions were so loud that they were heard more than three thousand kilometres away in Australia, and nearly five thousand kilometres away in Mauritius – the latter a distance of three thousand miles.
There were no survivors in the immediate vicinity of Krakatau, but the scale of the disaster that hit Batavia, more than 150 kilometres (100 miles) away, gives a graphic picture of the power of the event. At first, when the mountain began its series of explosions on Sunday, 26 August, the inhabitants of Batavia thought that they were hearing the eruption of another volcano, much closer than Krakatau. It hardly seemed credible that such a distant volcano could literally be rattling their windows. Their alarm increased when the gas supply to much of the island was cut off at about 2 a.m. on 27 August, and although many people did set off to begin the working week, they did so under an overcast that reduced the daytime temperature to as low as seventeen degrees Celsius (sixty-three Fahrenheit) – unprecedented for the location. Fine ash soon began to fall from an increasingly dark sky. As panic set in, thousands of people began to flee from the city, with no clear idea of where to go, but feeling the same urge that the younger Pliny and his companions had – to do something. Then, a tsunami, triggered by the mountain’s explosion, struck. Batavia city itself, three kilometres, or two miles, from the shore, was flooded to a depth of well over a metre, or about three feet, and the coastal regions of Java and Sumatra were swept by a series of waves, the largest at least thirty metres (one hundred feet) high, that destroyed three hundred towns and villages, killing most of the thirty-six thousand victims of the disaster.
But it wasn’t just water that was a killer on this occasion. Hot avalanches of fine ash and volcanic gases spread out from the island of Krakatau as pyroclastic flows. These hot avalanches are bad enough on land, where they can travel at speeds of more than a hundred kilometres, or sixty miles, per hour, engulfing anything in their path. But they can also travel over water. Instead of being quenched, the heat from the pyroclastic flow, typically around 700 degrees Celsius (1300 degrees Fahrenheit), flash-vaporizes the surface of the water, making a layer of steam on which the hot material rides, like a hovercraft riding on a cushion of air. That enabled the hot ash flows from Krakatau to travel up to forty kilometres, or twenty-five miles, across the sea, engulfing ships on the way and causing still more death and destruction when they hit land. Arriving just ahead of the tsunamis, these flows are estimated to have killed about 4500 of the people who died as a result of the 1883 eruption.
When it was safe for boats to venture back into the strait, they found that the geography of this vital link between the Indian Ocean and the South China Sea had been altered. Two-thirds of the island of Krakatau had gone completely, and there were new small islands where none had been before. From a height of 450 metres (1500 feet) above sea level, Krakatau mountain had collapsed into a crater more than 300 metres below sea level; the volcano remains active today. In 1928, a new island dubbed Anak Krakatau (‘Child of Krakatau’) poked above the surface, and has continued to grow ever since.
It is estimated that the explosion, again, the equivalent of between fifty and two hundred megatonnes of TNT, blasted twenty cubic kilometres (five cubic miles) of rock into fine ash and sent it high into the atmosphere, where it reached heights in excess of twenty-five kilometres, or fifteen miles. There, it spread around the world, producing beautifully coloured sunsets for several years and acting as a sunshield that cooled the entire globe by about half a degree Celsius in the 1880s. Locally, it was so dark that in Djakarta, on Java, people had to use torches and lanterns for the whole day, while in Sumatra they needed artificial light for two days before the skies cleared. The area directly around Krakatau that was affected by falling ash, the fraction of the debris that did not get into the stratosphere, covered four million square kilometres, or 1.5 million square miles, about eight times the area of Spain.
Pelée 1902
Pyroclastic flows played a part in the destruction caused by the eruption of Krakatau in 1883, but only a relatively minor part. When the volcanic Mount Pelée erupted on the French Caribbean island of Martinique in 1902, however, it was pyroclastic flows that did most of the damage. Martinique is part of an island arc, a curved chain of volcanic islands about 850 kilometres, or 525 miles, long, stretching from Puerto Rico to Venezuela. The arc appears where the South American plate is pushing under the piece of crust known as the Caribbean plate, at a rate of a couple of centimetres per year.
Known as ‘the Paris of the West Indies’, the city of St Pierre was completely destroyed at 7.59 a.m. on 8 May 1902, when Pelée, seven kilometres (four miles) to the north-east, was ripped apart in four deafening explosions, described as being like the cracking of an enormous whip. The explosions had been preceded by activity on the volcano for the previous few days. Nobody had taken this as a sign that anything dramatic was amiss, and no evacuation of the city was attempted, partly because an important political election was imminent. People were also lulled into a false sense of security when they heard that Soufrière, on the nearby island of St Vincent, was erupting; they wrongly thought that this would relieve the pressure on Pelée. In a classic example of bureaucratic complacency, a ‘volcano committee’ summoned by the governor assessed the situation as follows:
This phenomenon is normal and commonly observed on all volcanoes around the world. The craters are open so the expansion of the vapours will continue without earthquakes or rock projection. Based on the location of the craters and the valleys leading to the sea, St Pierre is perfectly safe.
But the activity on Pelée did mean that a few observers were out of the city keeping an eye on the volcano, and some of them were lucky enough to survive, as they were not in the path of the pyroclastic flow. These eyewitnesses described a red cloud licking out from the mountain like a gas jet towards St Pierre, with a sound like ‘a continuous roar blending with staccato beats like the throbbing of a Gatling gun’. Observers on a passing ship saw the mountainside rip open and a dense black cloud shooting out horizontally.
When it was safe to return to what remained of the city, people saw wood turned to charcoal, iron bars bent into fantastic shapes, and all glass melted. One wrote of a ‘desert of desolation, encompassed by appalling silence ... a world beyond the grave’. A French scientist was horrified by the ‘pulverized, formless, putrid things which are all that is left of St Pierre’. The damage had mostly been done by hot gas; the flow left behind only a thin layer of white ash, which covered the ruined city.
In the space of a few seconds, the town and its thirty thousand or so inhabitants had been destroyed by a hot, glowing cloud of gas and ash hurtling down the mountain at a speed of several hundred kilometres per hour, with temperatures in excess of 1000 degrees Celsius (about 1800 Fahrenheit), producing devastation which can only be compared to that produced by the explosion of a nuclear bomb. Incredibly, there were two survivors in the city itself. One, Louis-Auguste Cyparis, was a prisoner being held in an underground cell – essentially a dungeon – in the local jail. The other, Léon Compère-Léandre, lived right on the edge of the city, which was only brushed by the pyroclastic flow. Although badly burned, he lived to tell his tale:
I felt a terrible wind blowing, the earth began to tremble, and the sky suddenly became dark. I turned to go into the house, with great difficulty climbed the three or four steps that separated me from my room, and felt my arms and legs burning, also my body. I dropped upon a table. At this moment four others sought refuge in my room, crying and writhing with pain, although their garments showed no sign of having been touched by flame. At the end of ten minutes one of these, the young Delavaud girl, aged about ten years, fell dead; the others left. I got up and went to another room, where I found the father Delavaud, still clothed and lying on the bed, dead. He was purple and inflated, but the clothing was intact. Crazed and almost overcome, I threw myself on a bed, inert and awaiting death. My senses returned to me in perhaps an hour, when I beheld the roof burning. With sufficient strength left, my legs bleeding and covered with burns, I ran to Fonds-Saint-Denis, six kilometres from Saint-Pierre.
St Helens 1980
The description of ‘dense black cloud shooting out horizontally’ will have struck a chord with anyone who has seen pictures of the eruption of Mount St Helens, in Washington State. The volcano is located 154 kilometres (ninety-seven miles) south of Seattle and eighty-five kilometres (fifty-three miles) north-east of Portland, Oregon, in the Cascade volcanic arc, which is part of the Pacific Ring of Fire. The trigger for the volcanic activity in this arc is the same set of plate movements that triggered the March 1964 earthquake near Anchorage, Alaska. On a geological timescale, earthquakes and volcanic eruptions are common in the Pacific Northwest of what is now the United States; but the eruption of Mount St Helens that occurred at 8.32 a.m. local time on 18 May 1980, after a hundred years of quiet, was the most lethal and economically disruptive volcanic outburst in the country’s history.
Fortunately, in terms of the death toll, that is not saying very much. Only fifty-seven people were killed, thanks to the remote location of the mountain, the fact that it had been showing signs of increasing activity for long enough for people at risk to be evacuated, and because the eruption occurred on a Sunday, when there were no loggers working in the surrounding forest. But 250 homes, forty-seven bridges, twenty-five kilometres (fifteen miles) of railway, and 300 kilometres (175 miles) of highway were destroyed. With about three cubic kilometres of debris – three-quarters of a cubic mile – spewed out in the explosion, the entire top of the mountain disappeared, reducing its height from 2950 metres (9680 feet) to 2550 metres (8370 feet). This left a horseshoe-shaped crater 3000 metres (1800 miles) wide and 800 metres (2600 feet) deep, open at the northern side. Before the eruption, the beautiful symmetry of the peak had often been compared to Japan’s Mount Fuji; any similarity was destroyed in the blast. The eruption did not have the global impact of a volcano like Krakatau, however, because most of the ash from the blast came out sideways, instead of being ejected vertically into the stratosphere. About six square kilometres, or 2.5 square miles, of forest containing more than ten million trees was laid flat by the blast, and much of it covered by ash.
In spite of this, as the eruption continued, some of the ash was carried upwards to form a mushroom-shaped cloud, which darkened the sky as it spread downwind in the stratosphere and drifted over eastern Washington State. Travelling at an average speed of one hundred kilometres (sixty miles) per hour, the cloud reached Idaho by noon. The darkness of what became known as ‘Black Sunday’ eventually covered 57,000 square kilometres – 22,000 square miles – of eastern Washington, northern Idaho, and western Montana. Overall, the eruption lasted for nine hours, but most of the damage occurred in the first seconds, in the initial explosion.
One of the people who died in that blast was David Johnston, a thirty-year-old volcanologist who was monitoring the activity of the mountain from what seemed like a relatively safe position on a nearby ridge. But the sideways explosion sent the pyroclastic flow straight for him at a speed of about three hundred kilometres – nearly two hundred miles – per hour. He was in touch with his colleagues by radio at the time the explosion occurred, so that his last words were recorded for posterity. They were: ‘Vancouver! Vancouver! This is it!’ No trace of his body was ever found.
This sideways eruption produced the largest avalanche of debris ever recorded, mixing the ash with ice, snow, and water to form mudflows known as lahars. It was these lahars, travelling down the Toutle and Cowlitz rivers at speeds of about seventy-five metres per second – the equivalent of 170 miles per hour – that swept away bridges and did other damage. An estimated three million cubic metres, or 700 cubic miles, of debris was carried all the way into the Columbia River, twenty-seven kilometres (seventeen miles) to the south, by the lahars.
In terms of energy, the explosion of Mount St Helens was equivalent to the blast from twenty-four megatonnes of TNT. It was heard as what people described as ‘a thunderous roar’ more than three hundred kilometres away. The volcano has remained active since 1980, and there is geological evidence that it has a history of large, explosive eruptions, making it the most active volcano in the entire Cascade Range over the past ten thousand years.
One eyewitness to the eruption was Rowe Findley, a writer for National Geographic. In an article for the magazine, he recounted:
More than fear for personal safety, I felt a growing apprehension for all of us living on a planetary crust so fragilely afloat atop such terrible heats and pressures. Never again, it came to me then and remains with me to this day, would I regain my former complacency about this world we live on.
That seems like a suitable note on which to end this roll call of disasters.