What volcanic hazard do we face?
Studying volcanic hazards, understanding how they are generated, and knowing their consequences is essential to being well-prepared and taking preventive measures. Some effects can last for years after an eruption and even impact the entire planet! In this blog you will discover that there are many volcanic hazards associated with an eruption, which are classified into two categories: DIRECT and INDIRECT. Let’s explore them together!
DIRECT HAZARDS
Direct volcanic hazards are those that are directly related to the materials and processes generated by volcanic activity. They generally occur during the course of an eruption, although some of them may also manifest before and shortly after the eruption. Three main types can be distinguished:
(i) FALL PROCESSES, when materials fall by gravity,
(ii) FLOWAGE PROCESSES, when materials move as if they were liquids,
(iii) and other processes, such as EMISSION OF VOLCANIC GASES.
FALL PROCESSES are the most common direct volcanic hazard, although not necessarily the most severe. These processes occur mainly as eruption column, eruption cloud, ash fall, and volcanic bombs.
The volcanic ash and gases emitted during an explosive eruption form an eruptive column that can reach great heights and spread laterally, depending on the prevailing winds, generating huge eruptive clouds that can travel long distances (Figure 1a). they can even circle the Earth! In the atmosphere, the gases form tiny droplets (aerosols), and the ash blocks sunlight, causing darkness and a drop in temperatures. Ash also poses a serious risk to aviation (Figure 1b), as it can damage engines and, in the worst case, cause a loss of power to the aircraft.
Ash fall (Figure 1) is the most common and hazardous fall process. Depending on the volume and duration of the eruption, ash can cover vast areas and affect regions far from the volcano. When it falls, it can cause road and communication route closures, respiratory problems, damage to plants, crops, and livestock, and even a small accumulation on building rooftops can easily cause them to collapse—ash is very dense!
Figure 1. a. Eruptive column and cloud produced during the eruption of Mount Bromo (Indonesia) in 2011; https://unsplash.com/. b. Ash accumulated on an aircraft during the eruption of Pinatubo (Philippines) in 1991. The photograph was taken at the Cubi Point Naval Air Station, about 40 km from the eruption site, where ash accumulation ranged from 10 to 15 cm; photograph by R.L. Rieger, 1991 (U.S. Navy), Global Volcanism Program, Smithsonian Institution https://volcano.si.edu.
The high-velocity ejection and fall of volcanic bombs during explosive eruptions is another common, but more localised, fall hazard (Figure 2a). Their impact areas are restricted to the area closest to the volcano (Figure 2b), generally within about 5 km around it.
Figure 2. a. Strombolian eruption of the Acatenango volcano (Guatemala), showing the ejection of volcanic bombs; https://unsplash.com/. b. Damage to infrastructure caused by the impact of volcanic bombs emitted by the Bromo volcano (Indonesia); http://www.photovolcanica.com/.
FLOW PROCESSES are the most deadly direct volcanic hazards, and compared to fall processes, their impacts are much more restricted to the areas surrounding the volcano. These processes mainly occur as lava flows and pyroclastic density currents.
Lava flows (Figure 3a) are the most common volcanic hazard in non-explosive eruptions and, due to their slow movement, rarely threaten human life. However, they can be highly destructive, causing road and communication route closures, building destruction, and wildfires (Figure 3b).
Figure 3. a. Lava flow emitted by the Fagradalsfjall volcano (Iceland) in 2021; https://unsplash.com/. b. Road closure caused by the eruption of Kīlauea (Hawaii) in 2003; https://www.usgs.gov/.
Pyroclastic density currents (Figure 4a), although more localised than ash fall, are the most destructive and deadly direct volcanic hazard. They are generated by a gravitational dome collapse, by a detachment of part of the eruptive column, or by a combination of both. Unlike lava flows, they travel at extremely high velocities for tens of kilometres and, in their path, cause asphyxiation, burial, and incineration of all human and animal life, making escape impossible! (Figure 4b).
Figure 4. a. Pyroclastic density currents generated during the eruption of Pinatubo (Philippines) in 1991; photograph by Alberto García. b. Plaster cast of one of the thousands of victims of the eruption of Vesuvius (Italy) in 79 A.D.; https://unsplash.com/.
In addition to the fall and flowage processes, there are other processes such as GAS EMISSION, which are very common before, during, and after an eruption. Volcanic gases can be released in low amounts over large areas of land, or in higher amounts at very specific points, forming fumaroles (Figure 5a). The most abundant are water vapor (H2O), carbon dioxide (CO2), and sulphur dioxide (SO2), although in smaller amounts, there are other types as well, such as hydrogen sulphide (H2S) and carbon monoxide (CO). Some, such as water vapor, are harmless, but others, like sulphur dioxide, are very toxic! (Figure 5b).
Figure 5. a. Gas emission and sulphur mineralisation (yellow tones) in a fumarole of the Kawah Ijen volcano (Indonesia); https://unsplash.com/. b. ‘Hazard area’ signs due to volcanic gas emission in Puerto Naos (La Palma); photograph by Arcadio Suárez, https://canarias7.es/.
INDIRECT HAZARDS
Indirect volcanic hazards (Figure 6) are secondary consequences of volcanic activity and include: wildfires, earthquakes, landslides, tsunamis, lahars, acid rain, climate change, famine, and epidemics.
The wildfires that occur during an eruption are primarily caused by the flow of lava. Large areas of vegetation can be destroyed (Figure 6a), and houses and infrastructure can be reduced to rubble.
Earthquakes are another indirect hazard associated with volcanic activity. The release of internal energy during an eruption, caused by the movement of magma beneath the surface, can generate large tremors that, among other things, weaken the structure of buildings and cause cracks in the ground. These ground shakes, torrential rains, or explosions can lead to large and destructive landslides (Figure 6b). These usually occur on volcanic slopes that are very steep and unstable. If the large landslides reach the sea, they can form enormous waves or tsunamis, which are very dangerous for coastal areas (Figure 6c). Tsunamis can also be generated by submarine eruptions.
Lahars are the second leading cause of death among indirect hazards. They can form when storms occur, during or after an eruption, or when snow or ice melts upon contact with lava or volcanic material at high temperatures. Composed of a mixture of pyroclastic material, rocks, and water, lahars have a strong capacity for carrying debris… they are very energetic and destructive! (Figure 6d).
Acid rain is also an indirect hazard in active volcanic areas. When volcanic gases, such as sulphur dioxide, react with water molecules in the atmosphere, they produce acids which, when precipitated as acid or snow rain, mainly cause damage to vegetation and water contamination (Figure 6e).
Figure 6. a. Wildfire caused by the passage of lava flows emitted during the eruption of Kīlauea volcano (Hawaii) in 2003; https://www.usgs.gov/. b. Landslide on the SE flank of Ontakesan volcano (Japan). The resulting avalanche travelled about 13 km to the bottom of the nearby valleys; photograph by Lee Siebert, 1988, Global Volcanism Program, Smithsonian Institution https://volcano.si.edu. c. Devastation caused by the 2018 tsunami in the Sunda Strait (Indonesia), triggered by the eruption of Anak Krakatau volcano; photograph by Daryono, https://tribunnews.com/. d. Blocks and vegetation mobilised by the passage of a lahar after the eruption of Mount St. Helens (The United Stated) in 1980; https://www.usgs.gov/. e. Devastating effects on the vegetation of Yellowstone National Park (The United Stated) due to acid rain; https://unsplash.com/. f. Loss of crops due to ash fall, causing shortages in the affected communities in northern Sumatra (Indonesia) after the eruption of Mount Sinabung in 2014; https://usgs.gov/.
Large eruptions can contribute to climate change if substantial amounts of particles (such as ash) and volcanic gases (such as sulphur dioxide) are injected into the atmosphere, which reflect sunlight and cause temporary global cooling. This extreme phenomenon, known as “volcanic winter”, can alter climate patterns and affect agriculture and ecosystems.
The immediate loss of livestock and crops, as well as the long-term loss (from years to decades) of agricultural productivity in lands buried by eruptive materials, can cause famines in affected communities (Figure 6f). If we add to this poor hygienic conditions and water contamination, we have the perfect environment for the spread of epidemics.
The most common causes of death during a volcanic eruption are: famine and epidemics with 30.3 %, pyroclastic density currents with 26.8 %, lahars with 17.1 %, tsunamis with 16.9 %, and other factors with 8.9 %. Despite the direct and indirect hazards that an eruption entails, volcanoes also bring great benefits to our society. This is why, since ancient times, humans have settled in volcanic areas. Over time, these early populations have grown and, today, millions of people live in active volcanic zones and make use of the resources provided by volcanoes.
References
Bolós, X., Barde-Cabusson, S., Geyer, A., Sánchez, N., Galindo, I., Fraile-Nuez, E., Vázquez, J.T., 2024. El riesgo volcánico. Editorial CSIC, Ciencias para las Políticas Públicas. http://doi.org/10.20350/DIGITALCSIC/16384
Myers, B., Brantley, S.R., Stauffer, P., Hendley II, J.W., 1997. What are volcano hazards? USGS Fact Sheet 002-97, 2. https://doi.org/10.3133/fs00297
Tilling, R.I., 2005. Volcano hazards. In: Martí, J., Ernst, G.G.J. (Eds.), Volcanoes and the Environment. Cambridge University Press, Cambridge, 55–89.
If you want to cite this entry from the volkiblog:
Prieto-Torrell, C., Geyer, A., & Schamuells, N. (2025). What volcanic hazard do we face?. Zenodo. https://doi.org/10.5281/zenodo.14775740
Textos: Claudia Prieto-Torrell (GEO3BCN-CSIC), Adelina Geyer (GEO3BCN-CSIC)
Ilustraciones: Noah Schamuells (GEO3BCN-CSIC)
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