What Are The Most Important Volcanic Hazards?

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Volcanic eruptions produce lots of materials that have sometimes cause hazards. Volcanic eruptions are divided into two groups for their size and intensity features; effusive and explosive respectively. These type of eruptions lead to many volcanic hazards specially explosive ones. Some volcanic hazards have the biggest impact on society , agriculture areas , air transportation ways etc… according to their flow and dispersion speed and bulk density of volcanic material. The most important ones such as tephra (ash) fall out, lahar and floods, pyroclastic flows with pyroclastic density currents ).

Hazards means results of volcanic activity and it is estimated by their footprint (area affected) but if these hazards threat to humanity then it must be considered as a risk. Since according to risk definition; Risk = hazard * vulnerability * exposure. The aim of hazard assessment is to take possible approach to volcanic events and try to assess their nature. Probabilistic hazards assessment mainly based on Bayer’s theory and expert elicitation. Since hazards have great potential to be risk, exact timing and concise hazard assessment may be life saver.

Scientists work in co-operative and must warn the authorities and army if it is necessary to evacuate. To diminish hazard risk, volcanologists have to interpret previous case studies carefully and use the tools like hazard maps, event trees, exposure and vulnerability relationship, population exposure index (PEI) tables, volcanic risk distribution diagrams accurately. In general when volcanic emergency shows up, scientists work with their colleagues (international collaboration) from other countries to determine the alert level. Risk management is a body part of hazard assessment and it requires lots of experiments, planning and consult for many number of different stances for the specific issue.

Lahar Hazards

Lahars are very fast mudflows (they contain water, volcanic rock particles and debris) and in history they’ve caused so many fatalities like over than 35 thousand people (Witham 2005). The biggest problem of lahars is to predict the exact occurrence time. Lahars vary with velocity of flow and they always follow topography of slopes. Thickness of slope is the key parameter that is used in lahar –slope speed tables (Pierson et al, 2014).

When lahars travel tens of kilometres away from volcanoes, they may trigger floods in some cases. Also they can destroy paths, railways and bridges when they get turbulent flow regime. They occur mostly during or after eruptions and estimation of these kind of hazards is based on distance-travel time from source to reach point area (Pierson et al, 2014).

Example events for Lahar Hazards (USGS photos from Janda 1985(a), TCP 1991(b)):

  • 1985 Colombia, lahar event – nearly 21 thousand people death
  • 1991 Philippines, repeated high-concentrated lahar flows lead to erosion

In lahar hazard assessment there are four groups that bound to risk mitigation (Pierson et al. 2014).

Four ways to diminish risk about lahar hazards. (Pierson et al. 2014):

  • Hazard avoidance; construction planning for homes
  • Hazard modification; drainage channels and berms with the help of design techniques
  • Hazard warning; enough time for evacuation
  • Hazard response and recovery; reduce the residue effects after lahar flows.

JökulhlaupsJökulhlaups means flood in Icelandic language. According to some scientists, this hazard is occurred as a result of melting of huge amount of glaciers such cases in Iceland mountains. They do not have to be connected with lahars, volcanic eruptions might lead only for jökulhlaups (glacier deterioration). In spite of the fact that lahars and jökulhlaups are still be confused to differentiate in between scientists due to their identical characteristics like total debris volume content, discharge rate, travel distances, observable situations, size and thickness (Blong, 1984).

Tephra (Ash) Fallouts

Explosive eruptions generate volcanic ash into air which is also called tephra. Volcanic ash is about 20 µm in diameter (Prata and Rose, 2015). After volcanic material rising through volcanic conduit then erupt, tephra is separated and entrained into air thus forms tephra cloud. Tepra (ash) cloud is controlled by atmospheric force like wind. After that tephra cloud disperses along with the wind direction. Tephra travel distance is proportional to wind speed and plume height (Blong, 1984). Tephra deposits can threat living creatures’ respiration systems. Also they might easily destroy the environment and structures. Besides they may deteriorate path ways and when they are mixed with water (mud) could lead to roof collapse (Brown et al. 2015).

Because ash is too small and thin to visible, remote sensing is used to determine tephra (ash) dispersion from volcano and dispersion models give prior insights to volcanologists to predict the ways and to estimate the stop points (fallouts) for them. These models indicate the emissions which happen after a long period from eruption. SO2 is a basic key parameter for satellite instruments because it is the easiest measurable volatile (Sigurdsson et al. 2015).

Pyroclastic Flows and Pyroclastic Density Currents (surge)

Pyroclastic flows are hot and very speed hazards which is not mainly followed topography (because of thickness) and generated from explosive eruption column collision. Because they are less viscous than lava flows they can travel further and they can be extremely dangerous to humanity. They are comprised of volcanic rock particles, tephra and volcanic gases. Lateral volcanic blasts also sometimes create this flows. These flows can make damage on vegetation, fertilizer areas and infrastructures (Brown et al. 2015).

On one hand, pyroclastic density currents (PDC) have much higher temperature and much faster characteristics and they are also more dangerous surge component of pyroclastic flows. PDCs can be occurred from so many different sources and their natural behaviours are so problematic that it is so hard to predict and estimate when will they happen. These surges form more dilute and denser cloud than pyroclastic flows (Baxter et al. 2005). In general, whilst doing hazard assessment, earth scientists are benefited from their deposits. Due to their higher speed, they can go over hundreds of thousands kilometres and across continents or oceans longitudinally.

PDCs carry more weight of hotter and denser volcanic material than pyroclastic flows so they have a capacity of much higher fatality percentages. Despite the fact that high advanced software programmes have been used for volcanic hazard assessment for years, there are still many concerning unknowns and limited knowledge for PDC behaviour. Most common tangible and beneficial event for assessing and observing PDC hazards is Soufriere Hill volcano eruption case (Sigurdsson et al. 2015). These sort of hazards occur more often than the others (PDCs, Tephra(ash) fallouts, lahar flows and floods associated with them etc…) but with low intensity and impacts. Only just for a volcanic aerosol SO2 might be more remarkable and concerning because of volcanic winter – cool warming periods (Brown et al. 2015).

Risk Determination For Volcanic Hazard Assessment

Volcanic hazards are natural facts that human beings have confronted them for ages. To determine and mitigate risk requires lots of joint works with geologist (volcanologists), authorities, military services and societies. Volcanoes has no limit and they can affect both border neighbour countries simultaneously. Good example for that, Chilean volcanoes and their tephra fallout in Argentina (Viramonte et al. 2001). In assessment; primary and secondary volcanic events must be considered and examined links between cause – reason for them. Some useful methods to determine risk from volcanic hazards can be ordered like;

Position of volcanologists

Because of scrutinizing how Earth works, it is expected that Earth scientists have more information than any other disciplines. When volcanic emergency shows up, scientist make right decisions to evacuate (timing) and inform authorized offices (Brown et al. 2015).

Alert levels

Alert levels are illustrated in three sometimes four colours as a traffic lights. They are kind a early warning systems to hazards for before – after eruptions. 4 range of different level scales ( I – IV) indicate the emergency situation for each hazards. They are also used in hazard maps to symbolize the zones of footprints (Brown et al. 2015).

Kinships and communication

Volcanic hazards have complex characteristics and behaviours so people who are supposed to work together for a volcanic crisis, must use similar scientific language to make easier whole assessment process. In addition, now social media has more significant value by sharing a great number of data quickly within people while compared to past times. (lack of technology)

Long distance co-operation

The primary and secondary effects of volcanic hazards are known as globally. Volcanic eruptions do not always impact the country they situated in. Because tephra (ash) and PDCs travel fast far away from vent, agreements for memorandum of understanding can make strong relationship between nations about taking precautions for the next possible hazard (Brown et al. 2015).

Risk education

Universities and Institutes are able to open courses, seminars, conferences and symposiums to educate and inform people specially who live in risky places. These activities can help to increase awareness levels for hazards and alter people’s stances to assess the importance of hazards nature. Moreover, risk education programmes may teach people how to train and cope with sudden disaster (Pierson et al. 2014).


Management under risk conditions is highly depend on government policies. During crisis, scientists have to do right moves to save people, money and time. They are supposed to be well educated and civil offices are expected to be well equipped. Only in this way, (working together) the effective methods can be found and used to study hazard cases (Brown et al. 2015).

Plan and preparation

Volcanic hazard assessment and risk management process must be worked with several step lines and aim to prevent deaths and other values for societies. These are such as aware the emergency level of risk, recognition the characteristics of risk by aid of early warning systems, consult experts opinions and previous events, building framework and determine factors and take a precautions according to hazards risk levels. (Brown et al. 2015)

Wrap up

Determining risk is made up with periodic working of different programmes. All programmes must be associated with risk reduction. They ought to be taken into account either volcanic unrest phase or alarming levels from observatory and forecasting systems (Molist, 2017).

In conclusion, hazard assessment is the major part of the determining and reducing volcanic risk. The main goal of assessment is combined with scrutinizing historical hazards and try to understand the occurrence time of feasible ones in the future. It helps making decision stage, interpreting scenarios, informing and rising public awareness, identify possible economic loss and damage costs (Molist, 2017)


  1. Baxter PJ, Boyle R, Cole P, Neri A, Spence R, Zuccaro G, The impacts of pyroclastic surges on buildings at the eruption of Soufriere Hills volcano, Montserrat, Bulletin of volcanology, 2005
  2. Blong, Russell J. Volcanic hazards: a sourcebook on the effects of eruptions. Elsevier, 2013.
  3. Jenkins, Susanna F., et al. “Developing building damage scales for lahars: application Merapi volcano, Indonesia. “Bulletin of Volcanology 77.9 (2015): 1-17.
  4. Loughlin, S. C. et al. Global volcanic hazards and risk: Summary background paper for the UN-ISDR Global Assessment Report on Disaster Risk Reduction 2015. Global Volcanism Model, 2014
  5. Molist J. M. , Assessing Volcanic Hazard : A Review, Physical Sciences, Natural Hazard Science, Geology, Geophysics, 2017
  6. Pierson T. C., Wood N. J., and Driedger C. L. “Reducing risk from lahar hazards: concepts, case studies, and roles for scientists.” Journal of Applied Volcanology 3.1 (2014): 1 – 25
  7. Sigurdsson H. ed., The Encyclopaedia of Volcanoes, Ch. 54, 943-9

Cite this paper

What Are The Most Important Volcanic Hazards?. (2021, Nov 19). Retrieved from https://samploon.com/what-are-the-most-important-volcanic-hazards/

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