Pompeii: the duration of pyroclastic currents generated by the eruption of Vesuvius in 79 AD has been determined

A research on the effects of the pyroclastic flows of the 79 AD eruption on Pompeii highlighted how their duration had a tragic impact on the population

About fifteen minutes was the duration of the pyroclastic currents that hit Pompeii during the eruption of Vesuvius in 79 AD: the volcanic ashes, inhaled by the inhabitants, were fatal, causing asphyxiation.
This is what reveals the study “The impact of pyroclastic density currents duration on humans: the case of the AD 79 eruption of Vesuvius”, conducted by the University of Bari – Department of Earth and Geo-environmental Sciences, in collaboration with the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the British Geological Survey of Edinburgh. The study has just been published ‘Scientific Reports’.
“The aim of the work”, says Roberto Isaia, senior researcher of the Vesuvian Observatory of the INGV “was to develop a model to try to understand and quantify the impact of pyroclastic flows on the inhabited area of Pompeii”.

The inhabited area around Vesuvius volcano in a 3D perspective view from West; DTM overlaid with digital color orthophoto (Laboratory of Geomatics and Cartography, INGV-OV)

Pyroclastic flows, in fact, are the most devastating phenomenon of the so-called explosive eruptions. Comparable to avalanches, they are generated by the collapse of the eruptive column. The resulting dense pyroclastic flows flow along the slopes of the volcano at speeds of hundreds of kilometers per hour, at high temperatures and with a high particles concentration.
“During our research”, continues Isaia, “we carried out filed and laboratory studies of the pyroclastic deposits recognized within the archaeological excavations of Pompeii which led to the measurement and definition of the physical-mechanical parameters of the rocks. The obtained data have been used as input parameters for a mathematical model that has allowed us to carry out numerical simulations. From these we obtained the physical parameters of the pyroclastic currents and, therefore, the effects on the territory, including people, have been estimated. The main result is that the persistence of the flow of pyroclastic currents took place over a period of time between 10 and 20 minutes”.

Pyroclastic deposits within the Pompeii inhabited area including stratified layer with tractional structures formed by the Pyroclastic Density Currents

“The developed model” adds the researcher, “can also be applied to other active volcanoes around the world,. The example of Pompeii in fact, about 10 km far from Vesuvius, suggests how the use of this model could be very valuable for understanding the duration of pyroclastic flows and, therefore, the damage deriving from an eruption even at distances where the temperature and the pressure of the pyroclastic currents no longer causes harmful effects on humans and the environment. The applied methodology can therefore provide new elements of knowledge in the context of the hazard assessment of an active volcanic structure “, concludes Roberto Isaia.
“It is very important to be able to reconstruct what happened in the past eruptions of Vesuvius starting from the geological record, in order to trace the characteristics of the pyroclastic currents and the impact on population” declares Professor Pierfrancesco Dellino of the University of Bari, referent for the sector volcanic activity of the Commissione Grandi Rischi nazionale. “The adopted scientific approach in this study reveals information that are contained by the pyroclastic deposits and that clarifies new aspects of the eruption of Pompeii and provides valuable insights for interpreting the behavior of Vesuvius also in terms of civil protection”.


Who: Università degli Studi di Bari, Istituto Nazionale di Geofisica e Vulcanologia (INGV) e British Geological Survey, Edinburgh (UK)
What: A model was developed that allowed to calculate that in Pompeii the persistence of the passage of pyroclastic currents occurred in a period of time between 10 and 20 minutes, causing lethal effects on its inhabitants.
Where: The research The impact of pyroclastic density currents duration on humans: the case of the AD 79 eruption of Vesuvius in ‘Scientific Reports’.

Pyroclastic density currents are ground hugging gas-particle flows that originate from the collapse of an eruption column or lava dome. They move away from the volcano at high speed, causing devastation. The impact is generally associated with flow dynamic pressure and temperature. Little emphasis has yet been given to flow duration, although it is emerging that the survival of people engulfed in a current strongly depends on the exposure time. The AD 79 event of Somma-Vesuvius is used here to demonstrate the impact of pyroclastic density currents on humans during an historical eruption. At Herculaneum, at the foot of the volcano, the temperature and strength of the flow were so high that survival was impossible. At Pompeii, in the distal area, we use a new model indicating that the current had low strength and low temperature, which is confirmed by the absence of signs of trauma on corpses. Under such conditions, survival should have been possible if the current lasted a few minutes or less. Instead, our calculations demonstrate a flow duration of 17 min, long enough to make lethal the breathing of ash suspended in the current. We conclude that in distal areas where the mechanical and thermal effects of a pyroclastic density currents are diminished, flow duration is the key for survival.



Press release from the Istituto Nazionale di Geofisica e Vulcanologia (INGV)

At 7:30 am of May 18th, 1980, Don Swanson of USGS claimed that “the bulge still bulges”. The weird bulge grown on the northern flank of Mount St. Helens in the past weeks was still there; since days there were warning signs of a possible eruption. It would have begun exactly one hour later.

To remember the events that brought to the eruptive event, the USGS (U.S. Geological Survey) reported an accurate day-by-day chronicle of what happened fourty years ago on its Facebook page. Pictures and fascinating memories, exclusive video recordings, curiosities and details that had gone lost through time. A brief geological history of the volcano is presented here, from its origins until the day of the eruption. For more insights, we also suggest the reading of the chronicle on Facebook and the USGS scientist Richard Waitt’s book In the Path of Destruction: Eyewitness Chronicles of Mount St. Helens.

The stratovolcano

Mount St. Helens is an active stratovolcano, part of the Cascade Range volcanic arch (Cascades, State of Washington, USA), North-American section of the Pacific Ring of Fire. A volcanic arch forms by magma rising in proximity of subduction zones, i.e. when a tectonic plate (oceanic in this case) moves under a continental one. The descend of a fluid-rich crust into the mantle reaching increasing pressure and temperature creates instability, resulting in the rise of melted material towards the surface. The tectonic movements of the Pacific and surrounding plates generated several subduction zones and volcanic arches along the so-called Ring of Fire.

The Ring of Fire surrounding the Pacific Ocean and location of Mount St. Helens, vectorial by Gringer, public domain

The magmatism by subduction usually originates at 100 km depth, since certain pressure and temperature conditions are necessary for the material to start melting and rising. However, Mount St. Helens is oddly located at only 67 km above the subduction zone. The results of the iMUSH (Imaging Magma Under St. Helens) research suggest that the volcano might seat above a cold magmatic rocks wedge (serpentinite) originating when the mantle reacts with oceanic water. The source of magmatims would be instead located eastward, under other volcanoes of the same chain. According to the research, the magma would migrate laterally feeding the Mount St. Helens eruptions.

The Geological history

The structure of a stratovolcano is originated by overlapping products emitted during its eruptive history. The shape of Mount St. Helens before the 1980 eruption was finalized during the Spirit Lake stage (Holocene, 3900 years – present day). The stage is divided in six eruptive periods: Smith Creek, Pine Creek, Castle Creek, Sugar Bowl, Kalama, Goat Rocks and the Modern one, begun with the 1980 activity. These periods were characterized mainly by explosive eruptions, proved by oldest products. The most significant event occurred during the Kalama period with the growth of the dome at the summit. This shaped the final volcano appearance. It took 100 years for the dome to grow and after the 1720 eruption it reached 1800 m above the sea level. At this altitude, an ice cap started covering the summit. The volcano remained quiet until mid-March 1980.

Mount St. Helens before the 1980 eruption. Picture from Flickr by U.S. Forest Service- Pacific Northwest Region, public domain

Precursory signs

On March 20th 1980, after sleeping 123 years, a 4.2 degree Richter earthquake was registered in the volcanic area. It happened after days of weaker tremors, its hypocenter (the origin point inside Earth) was shallow. During the following days, the instruments recorded low magnitude seismic swarms, not perceived by humans. The USGS experts struggled to define the origin of the earthquakes (tectonic or volcanic), nor they could say if or when an eruption would occur.

The volcanic activity started on March 27th: thanks to an areal view the experts identified a 70 m wide newborn crater in the ice and a plume rising up to 2 km. Moreover, they mapped two East-West fracture systems and recorded more than 50 tremors with magnitude higher than 3.5 in one day. During the following days both the eruptive and seismic activities increased, while the plume rose up to 3 km above the mountain top.

Mount St. Helens
Phreatic eruption at the summit of Mount St. Helens. Picture taken on April 4th 1980. Picture by D.A. Swanson from USGS, public domain

In the first days of April the instruments recorded some low-frequency harmonic tremors. Compared to usual earthquakes caused by sudden energy release from the rocks, the harmonic tremors were associated to volcanic activity. They are usually caused by magma movements under the surface or by gas release from magma. Therefore, they can indicate an imminent eruption. Nevertheless, the wait continued.

On April 3rd the experts noticed a 450 m wide and 90 m deep crater on the northern flank, some fractures visible in the snow and an odd area of exposed terrain, probably the first signs of the bulge. The scientists collected eruptive samples and measured low concentration of sulfide dioxide (SO2), which can indicate sudden magma rise, if found in high concentrations. Water and ice surrounding the crater had formed muddy lakes on the crater floor. When new fluids entered the crater from outside, they warmed up and were ejected as steam and ashes in the atmosphere.






The bulge

The activity seemed to settle in the first half of April: moderate explosions generated small plumes, some harmonic tremors were recorded, fractures on the northern flank extend and the bulge swells. The latter attracted the attention of the experts: at the end of the month it was 1.5 kilometer long, 2.5 kilometers wide and 100 m high. Its origin was a magmatic intrusion into the volcano (cryptodome), that occurred at the end of March. At the end of April its growth rate was 1.5 meter a day. The danger its existence and its fast growth posed was a material (snow, ice and rock) detachment from the flank and a rapid descent along the slope.

Despite the fact the scientists agreed about the danger on the northern flank, nobody knew exactly when it would have happened: the yield of the underlying rocks was unknown as well as the deformation depth. On May 12th a 5.0 magnitude earthquake caused an ice and rock avalanche, validating experts’ concerns.

The bulge on the northern flank of Mount St. Helens on April 27th 1980. Picture by Peter Lipman from CVO Photo Archives, public domain


May 18th, 1980

Few minutes after 8:30 am, a 1.5 East-West fracture opened north of the main crater. The resulting seism caused a detachment of 2.5 cubic kilometers of material from the northern flank and its descent for 14 kilometers. The volume was equal to the sum of a million of olympic swimming pools.

Mount St. Helens
Mount St. Helens eruptive plume on May 18th 1980. Picture by Austin Post from USGS, public domain

The volume detached from the flank was maintaining pressure on the magmatic system. After the cryptodome removal, the boiling water in the system was transformed into steam and triggered lateral hydrothermal explosions from the fracture exposed by the bulge. The explosion generated an eruptive plume that reached the height of 24 kilometers in less than 15 minutes and a pyroclastic flow that travelled for 8 kilometers at 130 km/h. The flow teared down trees in a 10 km radius.

Trees felled by the horizontal explosion on May 18th 1980. Two USGS geologists at bottom right set the scale. The alignment of the trees indicates the explosion direction (left to right). Picture by Lyn Topinka, shot on September 24th 1980. From USGS Cascade Volcano Observatory, public domain

Moreover, the pyroclastic flow melted part of the ice cap generating several lahars, mud rivers fast flowing towards the valley. The most destructive one travelled up to 80 km from the volcano. The lahars destroyed 27 bridges and almost 200 households.

St. Helens bridge on the Highway State 54 after being devastated by the lahar on May 18th 1980. The structure was dragged for half kilometer and partially buried by the mud. Picture by R.L. Schuster from USGS, public domain

The total amount of victims killed by May 18th 1980 explosions were 57. Most of the population and tourists had been previously evacuated. Because of its magnitude, the eruption has been classified as plinian on the scale of eruption types.

The eruptive event continued until October 1981, characterized by small explosive events with plumes up to 15 km high and pyroclasitc flows from the northern flank. Their products reached metropolitan areas in the states of Washington and Oregon that had not been affected by May 18th eruption.


View of Mount St. Helens from Johnson Ridge before and after May 18th 1980 eruption. The red dotted line shows the volume removed by the explosion. Picture by Harry Glicken, USGS/CVO and Gripso_banana_prune; graphics by Gryphonis, public domain


Mount St. Helens
An USGS geologist observing Mount St. Helens from Coldwater II observation point on May 1st 1980. Unknown author, picture by USGS, public domain