Of all the hazards associated with volcanoes, none is more terrifying or more deadly than the pyroclastic flow. A pyroclastic flow is a fast-moving current of superheated gas, ash, and rock fragments that descends a volcano's flanks at speeds of 100-700 kilometres per hour, with internal temperatures often exceeding 500-1,000 degrees Celsius. Caught in a pyroclastic flow, a person dies essentially instantly — incinerated by the heat, asphyxiated by the gas, crushed by the impact of high-velocity fragments. The body is sometimes preserved in a poses of last seconds before death, captured in the ash that envelops it. The AD 79 eruption of Vesuvius killed approximately 16,000 people, primarily through pyroclastic flows that swept through Pompeii and Herculaneum. The 1902 eruption of Mont Pelée on Martinique destroyed Saint-Pierre and killed approximately 30,000 people — essentially the entire population — in a single pyroclastic flow that took only a few minutes. Pyroclastic flows are the single deadliest volcanic hazard, and they are the reason that some volcanic eruptions become catastrophes while others remain spectacles.
What Pyroclastic Flows Are
A pyroclastic flow is a hot, dense, turbulent mixture of volcanic gases and pyroclastic material — particles of solidified or partially-molten volcanic rock ranging from microscopic ash to boulder-sized fragments. The flow is denser than the surrounding atmosphere, so it stays close to the ground and follows the topography, flowing down valleys and across plains in a way somewhat like a thick liquid. Internal temperatures are typically 200-700°C in the gas phase, with the solid fragments often even hotter. The total mass of a typical major pyroclastic flow is hundreds of millions to billions of tonnes. The flow can travel for tens of kilometres before its gas content dissipates and its energy is exhausted. Where it stops, it deposits a thick layer of ash, pumice, and other fragments — a deposit that can be hundreds of metres thick and that becomes the welded tuff or pumice fall deposits that geologists can identify in volcanic sequences.
How They Form
Pyroclastic flows form in several different ways. One common mechanism is the collapse of an eruption column. When a volcanic eruption produces an ash column rising into the atmosphere, the column is supported by hot gases rising from the vent. If the column becomes too dense (because the eruption rate increases or the magma becomes denser), it can collapse — the rising column converts into a falling column that descends back down the volcano's flanks as a pyroclastic flow. Another mechanism is the collapse of a viscous lava dome. Some volcanoes produce thick, viscous lava that extrudes slowly from the vent, building up into a dome that grows over months or years. If the dome becomes unstable (due to gas pressure building up beneath it), it can collapse, and the collapsed material — now a turbulent mixture of pieces of lava dome, magmatic gases, and air — can travel as a pyroclastic flow. A third mechanism involves directed blasts — sideways-directed explosive eruptions that produce pyroclastic surges that travel laterally rather than vertically. The 1980 eruption of Mount St. Helens involved a directed blast that produced a pyroclastic surge devastating an area of approximately 600 square kilometres.
The Speed
Pyroclastic flows travel extraordinarily fast. The slowest of them move at 30-50 km/h — fast enough that running is futile. The fastest can reach 700+ km/h on steep slopes (faster than a commercial jet aircraft). The 1902 Mont Pelée pyroclastic flow reached the city of Saint-Pierre — about 6 kilometres downslope — in approximately 1 minute. The 1991 Pinatubo pyroclastic flows reached velocities of 150-200 km/h. The 1902 Soufrière flow on Saint Vincent killed approximately 1,500 people in flows that traveled at speeds of 100+ km/h. The 1991 Unzen, Japan flow killed 43 people including several volcanologists who had been monitoring the eruption from what they thought was a safe distance. Outrunning a pyroclastic flow is impossible, even for very fit individuals in good terrain.
The Temperature
The temperatures within pyroclastic flows are extreme. The gas phase is typically 200-700°C; the largest fragments can be at 800-1,000°C. The total thermal energy in a typical pyroclastic flow is enormous — the 1991 Pinatubo flows released energy equivalent to several hundred Hiroshima atomic bombs. The flames in a forest set on fire by a pyroclastic flow reach temperatures of about 1,000°C. People killed by pyroclastic flows die from a combination of thermal injury, asphyxiation, and traumatic injury from the high-velocity impacts. Pompeii victims show evidence of being killed by all three mechanisms, with the exact cause varying with location relative to the flow. The famous Pompeii body casts — created by pouring plaster into the cavities left in volcanic ash after the bodies decayed — show people in poses of curling up against the heat, covering their faces, or trying to flee. The temperatures in the Pompeii flows were probably 300-400°C, sufficient to kill instantly and to preserve the bodies in their final positions.
Pompeii and Herculaneum
The most famous pyroclastic flow disaster in history is the AD 79 eruption of Vesuvius, which buried Pompeii and Herculaneum and killed approximately 16,000 people. The eruption began on 24 August (or possibly 24 October, with some historical evidence) of that year. The initial phase was a Plinian eruption with a 30-kilometre-high ash column that deposited several metres of pumice on the surrounding region. The pyroclastic flows began later in the eruption, when the column collapsed. Multiple flows swept through Pompeii from various directions over the course of several hours. The temperature in the flows that reached Pompeii was approximately 250-300°C — enough to kill the inhabitants almost instantly. The town was buried under 4-6 metres of ash and pumice, preserving the buildings, the household items, and even the bodies in extraordinary detail. Pompeii was rediscovered in 1748 and has been a major archaeological site ever since. The eruption is also famous because it was documented by Pliny the Younger, whose letters describing the eruption (and his uncle Pliny the Elder's death during the event) are among the earliest detailed eyewitness accounts of a major volcanic eruption.
Mont Pelée and Saint-Pierre
The most catastrophic single pyroclastic flow in modern times struck Saint-Pierre on Martinique on 8 May 1902. Mont Pelée had been showing increased activity for weeks beforehand — ashfall, gas emissions, small earthquakes — but the urgency of evacuation was not appreciated. On the morning of 8 May, a major pyroclastic surge swept down the southwest flank of Mont Pelée toward Saint-Pierre, the city below. The surge reached the city in approximately 1-2 minutes and killed essentially everyone in the city — approximately 28,000-30,000 people, with only a few survivors. The most famous survivor was Auguste Cyparis, a prisoner in the city's solitary confinement cell, who survived because the cell walls (partly underground) protected him from the worst of the surge. Another survivor was a shoemaker named Léon Compère-Léandre, who survived because his home was somewhat sheltered. The city of Saint-Pierre was rebuilt but has never recovered its pre-1902 population or importance. The disaster substantially advanced volcanological understanding of pyroclastic flows.
Mount St. Helens 1980
The 1980 eruption of Mount St. Helens in Washington State, USA was the first major pyroclastic flow event to be observed by modern volcanologists with full scientific instrumentation. The eruption began with a directed blast — the north flank of the volcano collapsed in a massive landslide, exposing the magma chamber to atmospheric pressure and causing the magma to flash to gas and fragments. The resulting pyroclastic surge swept north from the volcano at speeds of approximately 480 km/h, devastating an area of approximately 600 square kilometres. Fifty-seven people were killed, most of them within the most heavily affected zone (which had been thought to be outside the dangerous area). The volcanologist David Johnston, who was monitoring the volcano from a ridge approximately 10 kilometres away, was killed by the pyroclastic surge that devastated his post. His last transmission ("Vancouver! Vancouver! This is it!") is one of the most famous moments in volcanological history. The 1980 St. Helens eruption transformed scientific understanding of directed blasts and the size of pyroclastic surge hazards.
Pinatubo 1991
The 1991 eruption of Mount Pinatubo in the Philippines was the second-largest volcanic eruption of the 20th century. The eruption produced massive pyroclastic flows that descended the volcano's flanks for tens of kilometres, depositing ash and fragments as thick as 200 metres in valleys near the volcano. The eruption was accompanied by Typhoon Yunya, which dropped heavy rainfall and produced lahars (mudflows mixing pyroclastic material with water) that continued to kill people and destroy infrastructure for years after the initial eruption. The combination of pyroclastic flows from the eruption and lahars from the rain caused approximately 800 deaths directly and tens of thousands of additional deaths over subsequent years from displacement, disease, and lahar-related hazards. The eruption was successfully forecast in advance, however, and approximately 60,000 people were evacuated from the highest-risk zones before the eruption peaked — saving many thousands of additional lives.
Monitoring and Warning
Modern volcanology has developed sophisticated techniques for monitoring volcanoes capable of producing pyroclastic flows. Seismic monitoring detects the small earthquakes produced by magma intrusion. Gas measurements at volcanic vents track the composition and rate of gas emission, providing clues to the proximity of magma to the surface. Ground deformation measurements detect the swelling that often precedes major eruptions. Satellite imagery can detect heat anomalies, gas plumes, and surface changes. Combined, these techniques can provide hours to weeks of advance warning before a major eruption. The 1991 Pinatubo eruption was forecast accurately enough to save tens of thousands of lives. Mount St. Helens in 1980 was less well-forecast, but the broad signs of imminent eruption were recognised and most of the high-risk zone was evacuated. The 1985 eruption of Nevado del Ruiz that killed 23,000 in Armero is the type case of failed risk communication — the signs of impending eruption were recognised by scientists but were not effectively conveyed to local authorities and the population.
Living with the Risk
Many millions of people live within potential range of pyroclastic flows from active volcanoes. The Bay of Naples region around Vesuvius has 3 million inhabitants. The Manila metropolitan area is within reach of various Filipino volcanoes. The Cascadia Subduction Zone region has substantial populations near various Cascades volcanoes. Living with the risk requires understanding the hazard, having effective monitoring and warning systems, having pre-planned evacuation routes and procedures, and being prepared to actually evacuate when warnings are issued. The challenge is that pyroclastic flow events are rare — many decades may pass between major events at a given volcano — but they are catastrophic when they happen. Maintaining the readiness for evacuation during long quiet periods between events, and convincing populations to actually evacuate when warnings are issued, are continuing challenges of volcanic risk management. The continued possibility of pyroclastic flow disasters is one of the persistent hazards of life in geologically active regions, and it will continue to be so for as long as the underlying volcanism continues.