Category Archives: Volcano Basics

Geological concepts relating to volcanic activity

Volcanic Risk Mitigation: Disaster Management Planning

Good Afternoon!

This is the third in our series on volcanic risk mitigation. Here, we cover the planning that needs to be done to mitigate the risk before our volcano really ramps up to an eruption or to deal with other volcanic related incidents. 

Fig 1: Sakurajima.  Photo by Tanaka Juuyou, published under CC-BY-3.0

Let’s take a moment to reflect on what we are trying to prevent.  In the first instance, this is loss of life, followed by loss of or significant damage to property, infrastructure, farming and other production in the zones at risk.  The table below shows estimated fatalities with their cause from recent eruptions.  The largest cause of death is famine and disease, followed by tsunamis, pyroclastic density currents (PDCs) and lahars.  Even relatively moderate eruptions may result in a lot of fatalities. 

Fig 2:  Analysis by the author using data from the Volcanic Fatalities Database Version 1.0a. Pink denotes eruptions with a VEI over 4; yellow denote those with VEI of 4 or less. © Copyright remains with the author; all rights reserved, 2020.

The Volcanic Fatalities Database (see Sources below) lists many more causes, including some arising from being too close to the hazard and some others from evacuation.  All of which need to be borne in mind in our disaster planning.

What Should Be in Our Disaster Management Plan?

We saw in our previous article that a lot of people are involved in risk mitigation.  They all need to be on the same page when action needs to be taken.  This requires planning, preparation and coordination.   Like all plans, our volcanic incident management plan must cover the three “W”s – What, Who & When.


We cannot stop our volcano from erupting but there are few things we can do to reduce the risk, for example:

  • Crater lakes can be drained to remove the source of lahars (e.g. Pinatubo and Kelud). 
  • Rivers can be widened and their banks raised (levées) to contain some lahars. 
  • Small lava flows can be diverted.
  • Lakes can be agitated to release carbon dioxide and reduce the accumulation (Lake Nyos).
  • Some crops may be protected by sheeting.
  • Local planning regulations can be adapted to include reinforcement of buildings for volcanic ash and to prevent building and other activities in areas of high risk. 
  • Organisations may opt to site critical facilities, such as datacentres, in less hazardous environments.
  • Some insurers may offer insurance to cover volcanic incidents; if they do, we may well have to have robust business continuity planning.

There are other hazards (e.g. pyroclastic density currents, volcanic bombs, volcanic gases, tsunamis, landslip, large lava flows) that we cannot do much about except impose exclusion zones and evacuate those in the areas at risk, when it appears likely that an eruption or other event will occur (we’ll look at “when” later).  Evacuation planning includes finding suitable accommodation, food and water, and medical supplies for evacuees.

Who: Roles & Responsibilities

The “who” is based on skills and areas of responsibility.

Civil defence authorities are responsible for coordinating the response to a volcanic incident, developing and testing emergency plans and procedures, communicating with the public and media.

The volcanologists monitor volcanic activity, participate in development of volcano coordination plans, and deliver eruption updates to the civil defence authorities.

The public (businesses, non-government organisations and individuals) need to be aware of the hazards and alerts, have their own disaster recovery and business continuity management plans, and, be prepared to evacuate, if needed.

Who: Communication Protocols

Communication protocols are necessary to avoid confused messaging to the public, and should be consistent with designated roles and responsibilities.  Countries have their own established protocols; in essence, these are:

  • Civil defence authorities advise local government / authorities, communicate with the public and the media, and issue alerts.
  • Volcanologists and other specialists advise the civil defence authorities. Where specialists disagree, the disagreement must be formally and openly communicated with reasons to the civil defence authorities, along with the consensus of opinion.
  • Volcanologists do not advise or communicate with the public, unless part of an agreed education / information program. 
  • Volcanic Ash Alert Centres issue volcanic ash advisories for aviation.
  • Governments request / accept and distribute international aid with the help of aid agencies, should it be necessary.

The protocols should include designated personnel to contact, their contact details, with deputies should the initial contact not be available.


Judging when to take action is fraught with difficulties: evacuate too soon, people will wander back into exclusion zones to look after crops, livestock or other assets left behind; do it too late, there could be serious loss of life; or, evacuate with no ensuing eruption, the political, social and economic fall-out from what may be seen to be unnecessary disruption won’t be pretty.  Timing is therefore critical.

Fortunately, our volcano is unlikely to go from inactive to a full-blown eruption overnight. Various authorities have successfully used volcano activity levels to provide a system of alerts to indicate when mitigating activity should be occurring.  The alert levels may be raised or lowered based on the level of unrest shown by our volcano.  Countries have their own systems of alerts; we have based the alerts in our sample example of a summary plan below on those used for the eruption of Pinatubo in 1991. 

The Plan

Putting all this together our plan, in summary, may look something like our theoretical sample below.  This covers both ramping up to and ramping down from an eruption, and the alert levels may be changed in accordance with the level of unrest shown by our volcano.  This is a theoretical sample only; your disaster management plans must be tailored to the risks posed by your volcanoes.

Fig 3 (1 of 2): Overview of Volcanic Incident Plan by the author showing alert levels 0 to 5 (ramping up), their criteria, hazards, interpretation, volcanologists’ responsibilities and civil defence responsibilities.  © Copyright remains with the author; all rights reserved, 2020.
Fig 3 (2 of 2): Overview of Volcanic Incident Plan by the author showing alert levels 6 to 9 (ramping down), their criteria, hazards, interpretation, volcanologists’ responsibilities and civil defence responsibilities.  © Copyright remains with the author; all rights reserved, 2020.

Note: that for each action, there will be several underlying actions.  For example, building a lahar channel will require landowners’ permissions, architects, surveyors, engineers, procurement of building materials, builders, project management, etc.. 


OK, so now we have our plans: our alert levels, our exclusion zones, our detailed evacuation plans, our communication protocols, etc.  But will they work?  Testing is needed to make sure that evacuation plans work: e.g. that people can get from the area under threat to a designated evacuation centre in the relatively short space of time available, that the evacuation centre can take them, that food, water and other supplies will be available at the evacuation centre when needed. 

We don’t need to get everyone in the area to participate in all the testing, desk-top “what if” scenario testing with representatives from local authorities, volcanologists and others involved in the actions can pick up obvious problems, e.g. a single road out for a population of 100,000 +.  Nor do we need to wait for an eruption to solve the problem, e.g. if feasible, another road could be built to provide another exit, or we may plan to evacuate earlier.

Involving the public on a voluntary basis in testing may be a good way to get the public on board and get their cooperation, if we have the resources to manage this, not least because it provides education and information.  At Sakurajima, a very active volcano in Japan, the local authorities hold an evacuation drill for the island on 12 January every year, the anniversary of a tragic eruption in 1914; this maintains awareness and means that people know what to do, should they need to evacuate for real.

Education & Information

We may have the greatest plans going but if we do not get the public on board, our plans will be less than effective.  We need to have an education programme that advises the public what they need to know in time for them to take action, if required, while minimising panic.  If our volcano has erupted violently in living memory, it will be easier to inform the public than if our volcano has been quiet for some time or considered dormant or extinct.

While our volcano is quiet, the public needs to know that our volcano is a volcano, the hazards and its eruptive history.  They also need to know that the volcano is being monitored and that there is system of alerts in place that they may need to respond to.  Organisations need to include volcanic hazards in their business continuity management plans.

When activity at our volcano ramps up, the public need to be made aware of the level of activity, its interpretation and any preparation they need to do, e.g. know the evacuation routes, have a bag packed with essentials in case they need to leave at short notice.   Organisations need to know when to invoke business continuity / disaster recovery plans.  Workshops, lectures and media campaigns can help to get the message across.  A good example of an alert is that recently issued by the Icelandic authorities in relation of increased unrest at Grímsvötn (link:   & ).

When the eruption quietens down, the public need to know whether and when it is safe to return to their properties; any aid available to cover damage to property, livestock and crops; how they will be rehoused, if they are unable to return to their property; and, ultimately confirmation when the eruption is over.

Factors to Consider

Our plans have to be flexible because our volcano may have a range of eruptions sizes, for example, with an average of VEI 3, but extending to VEI 6, or our volcano may change its behaviour. 

When activity starts to ramp up, volcanologists will monitor the volcano for any clues as to what our volcano will do next.


Significant loss of life has been prevented in large eruptions (Pinatubo, 1991, VEI 6) by following a disaster management process along the lines set out above.  However, despite the successful evacuation of 200,000 people, 359 died from the eruption, itself, and 1,200 indirectly from disease and lahars.

Since 1991, the biggest cause of volcanic fatalities has been secondary lahars: San Cristóbal, 2,547 deaths in 1999; and, Mayon, 1,266 in 2006, although there has not been a VEI 6 or more since Pinatubo’s 1991 eruption.  There have been several other fatal incidents in recent years, such as the recent eruptions of White Island, New Zealand, and Ontake-san, Japan, where visitors were caught in phreatic eruptions.

The above touches the surface of disaster management planning. Should your volcano start to ramp up, consult with the experts, such as USGS or others who have volcanoes in a similar setting to yours.

We will look at the 1991 eruption of Pinatubo and others in more detail in later articles.

The Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2020.

Sources and Further Reading

Volcanic Fatalities Database Version 1.0a:

“Volcanic fatalities database: analysis of volcanic threat with distance and victim classification”, Sarah K. Brown, Susanna F. Jenkins, R. Stephen J. Sparks , Henry Odbert and Melanie R. Auker, Journal of Applied Volcanology (2017) 6:15, Brown et al. (2017)

USGS Volcano Hazards Program:

“Volcanoes, Earthquakes and Tsunamis”, David Rothery, Teach Yourself, 2010

“Eruptions That Shook the World”, Clive Oppenheimer, Cambridge University Press, 2011

“Volcanoes”, Peter Francis, Clive Oppenheimer, Oxford University Press, Second Edition, 2004

“Volcanoes. Crucibles of Change”, Richard V. Fisher, Grant Heiken, Jeffrey B. Hulen, Princeton University Press, 1997.

Volcanic Risk Mitigation: Know Our Volcano

Our second part of our series on Volcanic Risk Mitigation covers understanding the hazards and risk posed by our own volcano.

Fig 1: Lassen Peak Hazard Map, USGS, Public Domain (for full reference see Sources below). Coloured blocks indicate lava flows (mafic blue, andesite purple, silicic red) and lahars (yellow). Circles denote the extent of ash. Black dots are vents.  Flood risk is also shown

Understanding our volcano is a vital part of volcanic risk mitigation.  If we don’t understand our volcano and what it can do, we may not take the right action at the right time – we may call for an evacuation too soon; we may not call for an evacuation when needed; or, we may evacuate to the wrong place.  If we get it wrong, the cost and disruption will be huge, and, there may be considerable preventable loss of life. We really don’t want to be at ground zero at the wrong time.

Who Needs to Know

Everyone who is likely to be impacted: the volcanologists, local authorities (government, civil defence authorities, public officials), and the general public (business, including aviation, other organisations and individuals).  The following shows our roles, based on a summary published by USGS.

The volcanologists monitor volcanic activity, issue warnings to the local authorities of impending eruptions, participate in development of volcano coordination plans, and deliver eruption updates via a formalised communication protocol.

The local authorities (civil defence) develop emergency plans, provide information about local hazards and emergency procedures, and during volcanic activity, they advise residents about closures, exclusion zones, evacuation routes, and recommendations for recovery. Meteorological agencies issue volcanic ash alerts for aviation.

 The public can help themselves to remain safe by learning about the hazards where they live or visit, following local recommendations to ensure households and businesses are prepared, including being able to evacuate at what could be relatively short notice (making an emergency plan and compiling an emergency kit (the essentials, like medications, that you can grab and go on the way out of the house)), and keeping out of exclusion zones.

What Can Go Wrong? Armero, 1985

Failure to understand hazards by decision makers has led to some of the worst volcanic disasters in recent history; for example, the loss of Armero and its 23,000 inhabitants from a lahar in the 1985 eruption of Nevado del Ruiz.  The disaster, itself, unfolded as follows on 13 November 1985: at 15:05 a phreatic eruption started at the summit; 16:00 the local geological survey recommended evacuation of Armero; 17:00 the emergency committee met for a couple of hours (the outcome was indecisive); 19:30 the Red Cross issued an evacuation order, apparently not heeded; 21:08 the main eruption started; 22:30 an alert of an avalanche approaching Armero was issued, however, the local mayor ordered residents to stay at home; 23:35 the first lahar swept through the town, destroying it and its inhabitants.

The eruption of Nevado del Ruiz was not unexpected. There were precursors: heavy steaming occurred at the summit in November 1984; a phreatic eruption in February 1985 caused ashfall on a nearby town and a lahar that reached 27km from the volcano; earthquake swarms started on 7 November 1985 and the volcanic tremor set in by 10 November – both indicating magma was ascending. 

 It was known that Armero had been destroyed by lahars in 1595 and 1845, so why would the local authorities delay evacuation or be slow to order it?  In addition to failure to comprehend the size, destructive power and possible reach of a lahar, the simple answers are cost, social and economic disruption, and no-one wants loss of credibility from crying wolf.  An evacuation is costly in terms of moving people, providing shelter and loss of incomes. 

The tragedy of Armero has led to the establishment of protocols to be followed in the event of a volcanic incident; in this instance, lessons were learned.   One of the big lessons was helping local authorities and local populations understand the hazards / risks their volcano posed.  Educational programmes, systems of alert levels and response protocols have been developed.

What Do We Need to Know?

To predict what our volcano may do we need to know what similar volcanoes and our volcano have done in the past. We then need to look at the impact of any eruptive activity on the local area in terms of how the following will be affected: the local inhabitants, property, the local infrastructure, farming and other economic activity.

We also need to know what baseline level of activity is so that any unusual activity which may herald an eruption is picked up early.

The Boring Bit – Definitions

We’ll get this over with quickly.  The important bit is that calculating risk for hazards by location helps planners work out mitigation plans later.

A volcanic hazard is defined here as a natural process that has caused loss at a particular volcano in the past for which, based on its history, a probability can be calculated.  Risk is the potential loss from a potential hazard, including loss of life, damage to property, reduction in productive capacity of the region and destruction of land, crops and habitats.  Risk is used to determine, for example, the size of exclusion zones and evacuation orders. There is a formula to calculate risk:

Risk = value x vulnerability x probability of hazard.

Where value is the number of threatened assets (lives, property, farming, other commercial activity and infrastructure).  Vulnerability is percentage value likely to be lost in an eruption. 

Note: this formula is based on the volcano’s known history.  It is a good place to start.  But it has to be borne in mind that the volcano may change its behaviour so be prepared to update the risk evaluation as we get more information.

What the Volcano is Likely to Do

  • What type of volcano is it?

Knowing the type of volcano, its geological setting and its lavas indicates how it is likely to erupt.  Our volcano is more likely to behave like others of its type in a similar geological setting than not.   A shield volcano with basaltic lavas is more likely to have effusive eruptions rather than explosive eruptions. A stratovolcano or composite volcano with more viscous silicic lavas is more likely to have more explosive eruptions. Glacier, ice-caps or high rainfall may cause lahars or jökulhlaups. Proximity to large inland bodies of water or the ocean adds the risk of tsunamis.

  • What has the volcano has done in the past?

So we know what type our volcano is, its lavas and what similar volcanoes have done, which give some indicators of what it could do.  But what has it done so far?  How big have its previous eruptions been, how far did the ash travel, what precursors have there been, etc.?

Volcanologists check out the volcano’s eruptive history by looking at both historic records and the local geology.  Historic records may indicate what the precursors for an eruption were, such as gas or steam emissions and earthquakes, as well as the eruptions themselves.  Geological records, such as tephra layers (age, depth, composition) provide evidence of previous eruptive activity, e.g. pyroclastic flows , lahars, lava flows and lava compositions, and caldera formation.  The geology can also tell us if there is ground water held in aquifers which could lead to phreatic eruptions. 

Volcanologists may also check out the stability of the edifice: how quickly was it formed; what is it made of; what is it built on; and any evidence of faults. 

  • Local Impact

Local government should have most of the data necessary to calculate the impact of a hazard at any one locations (local community, infrastructure and economic activity, etc.) from census and other activities. This should be checked out as far as is practical to make sure that it is up to date by looking at what is on the ground.

  • Hazard Mapping

With all this data, volcanologists calculate the volcanic risk for each hazard by geographic location using the formula above (a huge exercise, we are just skimming the surface here), collate their findings in reports and create hazard maps, which summarise the risk by location and by hazard (see Fig 1 for an example).  These maps are an important means to share the risks with local civil defence authorities and will be part of the decision-making process regarding exclusion zones or evacuation plans, should our volcano stir.

Baseline levels of activity

Volcanoes in repose are not quiet; they have low levels of activity from minor internal movements of magma and degassing and tectonic movement of local faults, with associated low levels of seismicity, gas emissions, minor ground deformation (inflation and deflation).

We need to know what activity is usual for the volcano while it is in repose so that any unusual activity can be detected early.  This will require seismometers, thermal imaging, tilt-meters, GPS, satellite and gas monitoring; seismometers are critical.

When Do We Need to Know It?

The short answer is the sooner the better.  The geological surveys needed can take months.  And, if we can get it, we need the base line data for the volcano in repose. On the other hand doing the hazard and risk assessments is costly so countries tend to focus on those volcanoes that pose most risk (e.g. are near large cities).

If the first inkling we have that our volcano is active is a seismic swarm and steam jets, the volcano is stirring so the hazard and risk assessments are critical. As time may not be on our side, we may need to call in experts from other countries to help out.  

What is in Place Now?

It’s been a while since Armero, where are we now?  Most countries with active volcanoes have the basics in place to monitor their known active volcanoes.  The level of monitoring depends on the wealth of each country and the perceived risk those volcanoes pose.  Some organisations, such as USGS and various universities, offer assistance in volcanic risk mitigation, which includes hazard identification, evaluation and mapping.

Following the Armero disaster, the International Association of Volcanology and Chemistry of the Earth’s Interior, IAVCEI, successfully created an educational video, “Understanding Volcanic Hazards”, to help decision makers and local people who had little awareness of what a volcano can do.  Since then, with the advent of the internet, there is a wealth of educational material available to draw on.  Many volcano monitoring organisations make some data publicly available.

There are various programs in place to study volcanic risk, including the Decade Volcano Program, set up by IAVCEI, the Yokohama strategy, the biennial Global Assessment of Risk (GAR) report published by the United Nations Office for Disaster Risk Reduction (UNISDR), the EU has also started major research programs dealing with risk assessment.

Data bases have been set up by various bodies describing volcanoes, their eruptive history and known hazards, such as the Global Volcano Model, the Smithsonian Institute’s Global Volcanism Program and Icelandic Volcanoes – the latter two have been referred to in this blog on many occasions.

Hopefully, we will know our volcano before she wakes up fully and be prepared for the crisis management mode if an eruption ensues.

The Armchair Volcanologist,

7 September 2020

© Copyright remains with the author; all rights reserved, 2020.

Sources and Further Reading

Fig 1: Lassen Peak Hazard Map, from Clynne, M.A., Robinson, J.E., Nathenson, M., and Muffler, L.J.P., 2012, “Volcano hazards assessment for the Lassen region, northern California”, U.S. Geological Survey Scientific Investigations Report 2012–5176–A, 47.

USGS Volcano Hazards Program:

“Volcanoes, Earthquakes and Tsunamis”, David Rothery, Teach Yourself, 2010

“Eruptions That Shook the World”, Clive Oppenheimer, Cambridge University Press, 2011

“Volcanoes”, Peter Francis, Clive Oppenheimer, Oxford University Press, Second Edition, 2004

“Volcanoes. Crucibles of Change”, Richard V. Fisher, Grant Heiken, Jeffrey B. Hulen, Princeton University Press, 1997.

Volcanic Risk Mitigation: Volcanic Hazards

Good Afternoon!

We are looking again at the volcano basics, this time the hazards posed by volcanoes.  This is not intended to put you off visiting a volcano; understanding the hazards is the first step towards mitigating volcanic risk. 

This image has an empty alt attribute; its file name is fig-1-pyrclastic-flow-mayan-1984.png
Fig 1: Pyroclastic flows at Mayan, 1984, Philippines by C. G. Newhall, Public Domain

Visiting a volcano

A quick word about visiting a volcano before we get into volcanic hazards.

It is usually possible to view an erupting volcano from a safe distance if you follow local official advice.  Where there is an exclusion zone in place, the local authorities consider it too unsafe for the general public to go into that zone.  Your insurance company and local rescue teams would probably take a very dim view of any accidents arising should you stray into the zone and need rescuing.

If you want to visit a volcano, do your research, take advice and have an enjoyable trip.

Volcanic risk mitigation

The purpose of volcanic risk mitigation is to reduce the losses from a volcanic eruption or other volcanic hazard.  This involves understanding the hazards posed by the volcano, the losses that would be incurred should an event occur and putting appropriate measures in place, where possible.

Volcanic hazards

This is a list and brief description of the major volcanic hazards, both when the volcano is in repose and when it is erupting.

Volcano not erupting

When a volcano is not erupting the hazards are pretty much the same as for any mountain: altitude, weather, avalanche, rock fall, getting lost in unfamiliar terrain, not being properly prepared, and/or not looking where you are going while taking that selfie / photo.

There may be the additional hazards from gas emissions, hot springs or fumaroles:  for example, carbon dioxide is an invisible odourless gas that can accumulate in depressions in the ground or caves, replacing the oxygen in the air leading to asphyxiation; hot springs can cause chemical and heat burns; fumaroles emit gases.

Volcano erupting

The main additional hazards from an erupting volcano are: blast wave (directed blast), eruption clouds, tephra, pyroclastic flows, lava flows, lava domes, debris avalanche flows, lahars, jökulhlaups, gas emissions, earthquakes, caldera formation and tsunamis.

Blast wave:

Explosive eruptions triggered by a sudden release in pressure can generate supersonic blast waves (directed blasts).  These blasts can flatten trees, destroy property and kill (directly or via debris hurled by the wave).  If the gases and ash released in the explosion are hot, they sear everything in their path.  An example is the lateral blast wave produced by Mount St Helens at the start of the May 1980 eruption.

Eruption clouds:

These are the clouds of ash, gases and rocks propelled by the volcano into the atmosphere; the cloud rises through kinetic energy from the eruption and heat.  Before the advent of aviation only the larger ash clouds posed a serious threat.  Ash clouds may reach heights from hundreds of metres to tens of kilometres.  If the ash cloud reaches the stratosphere it can have regional and global impacts on climate; ash filters out sunlight, cooling the air.  Examples of climate impacting eruption clouds are Krakatau, 1883, and Tambora, 1815.

 Ash, especially ash with a high silica content, is also bad for aircraft.  Flying through an eruption cloud at speed is akin to flying through a large sand blaster.  Ash abrades the outer surface of the aircraft, including the windshields and lacerates vulnerable parts.  Engine heat also melts the ash, coating fuel nozzles and turbine blade with glass, which may stop the engines in seconds. An eruption as small as a VEI 3, is enough to put an aircraft at risk.  Examples of aircraft in trouble are the VEI 4 eruption of Galungung, 1982 and VEI 3 of Redoubt, 1989. A BA flight encountered the eruption cloud from Galungung 150 km away from the volcano; all the engines cut out, were successfully restarted 2,000 m from the sea, after falling 9,000 m, only for the experience to be repeated when the plane flew back into the cloud (the plane later landed successfully in Java, despite having an abraded opaque windscreen); a Singapore Airlines flight was only able to recover two engines from the loss of all four after flying through the same cloud two weeks later.  A KLM flight lost all four engines in the Redoubt ash cloud and got uncomfortably close to the Alaskan mountains before being able to recover.


What goes up, must come down.  Tephra is the ash and rock that falls out of the eruption cloud due to gravity.  It is densest nearest to the volcanic vent; it can be dense enough to cut out all light by filtering out starlight, moonlight and sunlight; it may also cut the electricity supply, block roads and hamper rescue attempts. 

Fig 2: Light-coloured tephra deposit from one of Hekla’s largest explosive eruptions, H-3. The total thickness is of the order of several meters. Photographer: Guðrún Sverrisdóttir, Photo 5 of 5. Retrieved from Icelandic Volcanoes

The weight of accumulated tephra on buildings may cause them to collapse. If the tephra is combined with water, it may turn to concrete and be very difficult to remove.

Accumulation of tephra on the ground may cover vegetation, cutting off air and water to the plants:  a layer as thin as 2.5 cm is enough to kill some plants, vegetation recovers within a year; a covering of 15 cm or more is enough to kill vegetation and sterilise the soil from which it takes decades to recover. 

Tephra may also be rich in chemicals that are toxic to plants and the livestock that feed on them and contaminate the water supply.  Fluorosis from ash is a common cause of death in livestock and people.  Some minerals in tephra are also carcinogenic.

There is no shortage of examples of eruptions where tephra has caused loss of life; here are three:  Pinatubo, 1991, Vesuvius, 79 AD, and Tambora, 1815.

Pyroclastic flows:

These are big killers.  You cannot outrun a pyroclastic flow so unless you are right on the edge of a cold flow, or in an air-tight robust building, your chance of survival is zero.  Pyroclastic flows are fast-moving surges of gases, ash and rocks propelled by gravity that flow down the sides of the volcano at speeds of up to 200 km per hour; heavy ash and rocks form a basal surge, whereas lighter ash may form phoenix clouds, propelled upwards by heat and turbulence.  Small flows may travel up to 20 km and larger ones more than 100 km from the vent. Pyroclastic flows are generated by collapse of the eruption column or by lava domes (collapse of the dome or detonation by pressurised gas). 

Pyroclastic flows deposit large volumes of ash and rocks; lumps of lava may be the size of a vehicle.  If the flow is hot, the ash and rocks may be welded together as ignimbrite.  Hot flows will ignite anything that is flammable.  The area covered by the debris depends on the size of the eruption, a very large one may cover an area the size of Switzerland.

Again, there is no shortage of examples of pyroclastic flows, in addition to the three eruptions noted above, there is the VEI 4 1902 eruption of Mount Pelée, Martinique, in which two pyroclastic surges caused by dome collapse wiped out the capital St Pierre killing 30,000 and leaving 50,000 homeless; a third pyroclastic flow killed 2,000 in Morne Rouge. 


Lava flows are the lava which flows down a volcano or across open ground.  Most lava is viscous and can be outrun; hazards tend to be damage to property, farm land, power supplies and communication lines.  There are rare instances where lava flows have killed, such as Nyiragongo in 1977, when a fissure emptied the lava lake, producing basaltic lava flows with speeds of 30km per hour; where observers getting too close were casualties chunks of lava  breaking off the flow or from the explosions where hot lava met water or ice; or where lava flows have cut off escape routes. 

Lava domes are formed when more viscous magma builds up as a dome rather than flows.  They tend to have a solid surface and may have spines of solid lava pushed up by magma from the centre of the dome.  Lava domes may collapse under gravity causing block and ashflow eruptions or there may be a directed blast where a crack or collapse releases pressure. Examples where lava domes have claimed lives are Mount Pelée, 1902, Mount Unzen, 1991, Merapi, 1930, Mount St Helens, 1980 (cryptodome).

Debris Avalanche Flows & Lahars:

Volcanoes are made up of layers of loose ash and lava so are not very stable.  They may suffer partial edifice collapse as a result of gravity, erosion or from the pressures generated during an eruption.  The collapse causes debris avalanche flows – a fast moving, gravity driven currents of rocks, water and other materials. 

Mount St Helens is an example of the debris avalanche flow triggered by magma: a dacite cryptodome destabilised the northern slope; a small earthquake caused the bulge to fail, its collapsing released superheated steam, resulting in the catastrophic eruption.  Bandai volcano in Japan suffered failure of the northern flank set off by a small phreatic eruption in 1888; the avalanche had a volume of 1.5km3 and killed 461 people. Unzen in 1792 suffered sector collapse with no eruptive activity; the edifice failed under gravity alone.

A lahar is a water saturated debris avalanche flow (mudflow).  Lahars may be triggered by heavy rainfall typical of the tropics or by the rain storms generated by an eruption.  Rain washes loose ash and rocks into river valleys or gullies where they mix to torrential mud flows. Lahars can move at speeds of 90 km per hour.  Mudflows, plus debris picked up, form torrents which destroy property and kill those caught in their path.   A famous example of a catastrophic lahar is the destruction of the town of Armero in 1985 from a series of lahars resulting from pyroclastic flows 50 km away at the summit melting snow in a VEI 3 eruption of Nevado del Ruiz; most of the population of 28,700 were buried in several metres of mud. 


These are glacial outburst floods caused by the melting of glacier ice by heat from magma near the surface or during an eruption.  The water may accumulate under the ice in lakes, if its exit is blocked by ice, to be released as a flood when the ice dam breaks or floats.  Jökulhlaups carry debris from ash, boulders and ice bergs from the glacier.

The term is Icelandic and here we can find many examples.   One is the 1996 jökulhlaup from Grímsvötn with a volume of 3.5 km3, which reached a peak discharge of 45,000 cubic metres per second.  Most of the damage was to roads, bridges and power supplies; not many people lived in the vicinity. Flood deposits covered 750 km2 and extended the coastline out by 800m.

Volcanic Gases:

Volcanoes emit gases (volatiles) both when erupting and not.   The volatiles include sulphur dioxide, steam, carbon dioxide, hydrogen fluoride, hydrogen chloride, hydrogen sulphide and carbon monoxide; they may also include toxic compounds of arsenic, bismuth, cadmium, copper, lead, mercury, thallium and zinc.  Volatiles can tell us much about the lavas: unevolved magmas (mid ocean ridge basalts) tend to contain less water than arc basalts and rhyolites.  However, here we are concerned with the hazards posed by volcanic volatiles. 

Volcanic emissions damage vegetation, mostly by the acidic effects on soil and foliage, reducing crop yields or stripping vegetation.  Acidic gases may damage skin and irritate eyes and lungs.  Hydrogen sulphide, like carbon dioxide, can accumulate in volcanic or geothermal areas, causing fatalities from neural, respiratory and cardiovascular damage or by asphyxiation. One such tragedy occurred at Lake Nyos on 21 August 1986 in Cameroon when a cloud of carbon dioxide released from the lake flowed down the volcano, asphyxiating the 1,700 people plus pets, livestock and wild animals in its path.


Volcanic activity rarely produces large earthquakes (over 6.0 M); the ground motion required to accommodate magma movement is relatively small.  However, magma movement generates swarms of earthquakes which can run into the thousands.  The accumulated shaking may damage property, weakening it to be more vulnerable to the effects of the eruption which may ensue.

Caldera formation:

I have not seen this mentioned as a specific hazard in its own right in my text books, probably because much of the hazard is covered by other factors, such as: the eruption cloud, tephra, lava flows, tsunamis and earthquakes, which may be considerable in a caldera-forming eruption.

A caldera is formed when the roof of the magma chamber sinks while its contents are being evacuated.  In addition to the eruptive products, this may result in loss of land as we saw with Krakatau.  The depression may fill rapidly with water (sea water or fresh water).  Land loss is likely to be permanent resulting in loss of property, businesses, flora and fauna and anything else left on that land.


These are big killers.  Volcanic tsunamis are generated when a large volume of sea water is displaced by ash deposited by a collapsing eruption columns, pyroclastic flows and edifice failure. Tsunamis may travel a long way, devastating shore-lines.  Examples are Krakatau, 1883 and Tambora, 1815.

So why live near a volcano?

If volcanoes are such a threat, why do people live near them, or even on their slopes (assuming that they know that the beautiful wooded mountain is a volcano)?  We’ll look at the reasons in more detail later, but in summary they are that large eruptions are not that frequent  (the volcano may be in repose for decades / centuries); volcanic ash contains a lot of nutrients which lead to fertile soils, good for crops and livestock; the scenery is often unusual or attractive leading to tourism; and, if you were born or raised in the area, you may accept the risk.

We will look at volcanic risk mitigation over the coming weeks.

The Armchair Volcanologist

24 August 2020

© Copyright remains with the author; all rights reserved, 2020.

Sources and Further Research

“Volcanoes, Earthquakes and Tsunamis”, David Rothery, Teach Yourself, 2010

“Eruptions That Shook the World”, Clive Oppenheimer, Cambridge University Press, 2011

“Volcanoes”, Peter Francis, Clive Oppenheimer, Oxford University Press, Second Edition, 2004

The Magic of Magma

Good Afternoon!

Magma is the molten source of many of the rocks at the Earth’s surface; the others come from meteorites or other terrestrial processes.  Here, we look at how magma is generated and how it affects volcanic eruptions.   This is a long one; you may want to dip into it as a reference rather than read it all in one go, but bear with me.

Fig 1: Holuhraun on 4 September 2014 by peterhartree.  Published under CC BY-SA 2.0. Note the fire fountains, effusive lava flows and gas emissions.

There are several different types of volcanic eruption, varying from effusive to explosive.  The type is determined by the magma composition- both its silica content and its volatile (gas) content.

How is magma generated?

From our earlier post, we know that the heat required to generate magma comes from nuclear reactions at the Earth’s core.  This heat makes its way to the surface via conduction, convection and radiation.  The surface rocks are cold enough to be solid.  However, we saw that the heat from the core drives plate tectonics; it is the variations in temperature and pressure caused by plate motion, combined with changes in rock composition, that create the conditions for rocks to melt and create magma.

Effusive Continental Rifts / Ocean Ridges

Effusive eruptions (think Hawaiian eruptions such as Kilauea) tend to occur at rifts or fissures, cracks in the Earth’s crust that let magma reach the surface relatively quickly and allow gases to escape gently.  Such fissures are common at constructive plate boundaries (continental rifts or mid ocean ridges), caused by plate separation.  Magma is generated in a process called decompression melting; as the plates move apart the overlying pressure is reduced, which in turn reduces the melting temperature of the mantle.  Because magma ascends relatively rapidly there is less time for it to mix with other magma or the crust.  This magma tends to be basic basalt. 

Magma ascent at ocean ridge
Fig 2:  Magma ascending at a Mid Ocean Ridge by the author after the many examples available.  Arrows denote plate motion.  Not to scale. © All rights reserved, 2020.

More Explosive Subduction Zones

Magma may also be generated at what is called subduction zones.  Subduction zones occur where plates meet: a denser oceanic lithosphere descends beneath either a less dense continental lithosphere or other oceanic lithosphere. These zones were discovered by two scientists independently researching earthquakes, Hugo Benioff and Kiyoo Wadati. Earthquake foci delineate the descending slab; this zone may be referred to as the Wadati-Benioff zone.   Magma is created from the descending slab by a process called hydration melting: as the slab descends water is squeezed out which mixes with the overlying asthenosphere, lowering its melting temperature.  Hydration melting occurs between 50 km to 200 km, with volcanoes accumulating at around 70 km above the descending slab.  

Magma mixes with the asthenosphere and the crust which increases its silica content. Partial melting of the mantle results in basalts (<52% silica); partial melting of the descending ocean crust provides andesites (52% to 65% silica); and, partial melting of the continental crust gives rhyolites (> 65% silica).  Increasing the silica content of the magma increases its viscosity and the explosivity of eruptions.

These plate boundaries are called destructive boundaries; it is believed that the descending plate is destroyed in the process. 

Fig 3:  Magma generation at a subduction zone by the author, after the many sources available.  Large arrow denotes plate motion; stars, the site of hydration melting; and, the smaller arrow, magma ascent.  The accretionary wedge is rock scraped off as the plates meet. Earthquake foci, not shown, would indicate the path of the descending plate. Not to scale. © All rights reserved, 2020.


There is another process where plates meet called obduction; one plate rides over the other.  It is mentioned because obduction revealed the origins of oceanic crust.  Ophiolites, the remnants of old oceanic crust, show that this crust is made up of peridotites from the mantle overlain by old magma chambers, sheet dyke complexes, pillow lavas and sediments.  The magma was sourced from partial melting of the peridotite.  Sheet dyke complexes are complexes of vertical magma intrusions where magma has filled gaps caused by extension from plate separation.  Pillow lavas occur when basalt is erupted underwater.

Magma Evolution

We know that not all magma is basic basalt: there are others, including andesite and rhyolite.  These are formed from basalt by a process called magma evolution.

Magma rises slowly under its natural buoyancy, being around 10% less dense than the surrounding rock.  It tends to accumulate in magma chambers / reservoirs at depths of around 5 km to 20 km below the volcano.  Magma evolves by one or more of the following:

  • Magma mixing: one batch of magma mixes with another in the magma reservoir;
  • Assimilation: the rising magma collects surrounding rock during its ascent;
  • Fractional crystallisation: as magma rises, it cools, which causes the components with higher melting points to crystallise out.

Evolution tends to increase the silicate content of the magma and increase its viscosity; the higher the viscosity, the slower magma moves.

The table below shows a comparison of rock and magma compositions for some its key elements.  The convention for magma composition is based on the oxide equivalents of the elements, presumably on the basis that oxygen does not occur on its own in rock. 

Fig 4:  Comparison of key elements in the composition of different magmas by the author after various sources.  © All rights reserved, 2020.

So how does magma get to the surface?

The short answer is plate tectonics. Distortions in the crust from plate motion either squeeze magma out via existing or new fractures, or provide a wide enough pathway for the magma to ascend.  This is why we see so many volcanoes around plate boundaries.

Eruption Style

Magma contains dissolved gases (e.g. water, carbon dioxide, sulphur, and halogens: chlorine and fluorine).  When the constraining pressure decreases, the gases are released from solution: e.g. carbon dioxide exsolves at depths of several kilometres and sulphur at around 1 km.   This is a process called degassing (think of carbon dioxide released from a fizzy drink when the cap is loosened.) These gases may be held in the magma as bubbles or it may escape via fractures in the crust to the surface.  Measuring gas emissions around volcanoes indicates whether or not new magma is close to the surface.  We have seen with Grímsvötn that Icelandic scientists have reported the presence of sulphur dioxide near the caldera rim, indicating that Grímsvötn may be heading for a new eruption.

Effusive eruptions tend to occur with the less viscous basaltic magmas from which gases can escape gently.  More explosive eruptions are caused when the gases are trapped in more viscous magma and accumulate either as foam or as bubbles; it is their rapid expansion causes an explosive eruption. If a degassing event is large enough, less viscous magmas may also produce explosive eruptions.

When gas bubbles coalesce and expand, large bubbles form which rise with the magma.  If the magma reaches the vent, the bubbles explode with the reduction in constraining pressure, throwing out chunks of lava and ash; some of the chunks can be large boulders.  If gas bubbles get trapped in the magma chamber, when the accumulated pressure is high enough, the gas is forced out as a jet strong enough to cause fire fountains from less viscous basaltic magmas or ash / shards from fractured more viscous magma.  Fire fountains are / were visible in some of Etna’s eruption, in the 2014 eruption at Holuhraun or in the 1783 eruption of Laki. Famous explosive eruptions include Pinatubo, 1991, Krakatau, 1883, and Vesuvius, 79AD,

Impact of ground water or ice

We have seen explosive eruptions from some of Iceland’s basaltic volcanoes.  Another source of volatiles is ground water (aquifers) or glaciers.  When magma gets close enough to these, the heat from the magma flashes the water to steam; the rapid expansion of the water / steam results in an explosive eruption.  If only old rock is involved, the eruption is referred to as phreatic; if new magma is involved, the eruption is referred to as phreatomagmatic.  Phreatic eruptions may be a vehicle to clear the vent / provide a pathway for new magma to reach the surface. Recent phreatic eruptions include White Island, 2019, Mount Ontake, 2014; both caused loss of life because of the unpredictable nature of this eruption type.

Future Topics

There is another important process for magma that we have not touched here: mantle plumes.  We will look at this later.

We will also look in more detail the various locations for volcanoes, including the earthquake foci plots that indicate activity (usually tectonic).  Our Icelandic posts show the influence of the Mid Atlantic Ridge; others will look at subduction zones.

We may get diverted / distracted on the way to look at current events of interest.  (Probably, no “may” about it 😉 ).

Thank you for staying with me,

The Armchair Volcanologist

1 July 2020

© Copyright remains with the author; all rights reserved, 2020

Sources and Further Reading

“Volcanoes, Earthquakes and Tsunamis”, David A. Rothery, “Teach Yourself”, 2010

“Volcanoes”, Peter Francis, Clive Oppenheimer, Oxford University Press, 2004

“Volcanoes Global Perspectives”, John P. Lockwood and Richard W. Hazlett, Wiley-Blackwell, 2010

“Wadati-Benioff zone”, Wikipedia:

“Igneous rock”, Wikipedia:

“Magma”, Wikipedia:

What is a Volcano?

Good Morning!

As this blog is about volcanic and seismic activity, a word or two on what a volcano is might be helpful.

A volcano is defined by the Oxford English Dictionary as mountain or hill with a crater or vent through which rocks, rock fragments, lava, hot vapour and / or gases are or have been erupted through the Earth’s crust.  However, said mountain or hill may be quite small, even just a depression or a rupture in the Earth’s surface.

The island of Vulcano is the source of the term, volcano, itself.  Vulcano Island is located in the Tyrrhenian Sea, north of Sicily, made up of several active volcanoes, including calderas. 

Fig 1: Vulcano Island by Brisk g. in the public domain. Source

So what causes the lava and other matter to be erupted?  What is the Earth’s crust?  These are questions some of the questions we will look at in this blog. 

Heat generated at the Earth’s core drives the geological processes which result in volcanic activity. For starters, we will look at the basics of the earth’s composition, magma and the source of the energy to enable eruptions to occur.

Basics of the Earth’s Composition

This is a very basic description of the Earth’s composition.  The Earth is made up of three main parts: the core, the mantle and the crust. The core is very hot and temperatures decrease towards the Earth’s surface.  Most of the Earth is solid, only the outer core is liquid.  Evidence for this structure has been gleaned from seismic studies, notably how the different wave types generated by an earthquake pass through the Earth, geophysics and the study of rocks.

Fig 2:  Earth’s structure and composition, not to scale, by the author. © Copyright remains with the author; all rights reserved. 2020.

The Core

The radius of the Earth is around 6,378 km, in other words the centre of the Earth’s core can be found 6,738 km down.  The core makes up around one third of the Earth’s mass.  It is made up of an outer core which starts at around 2,900 km down and an inner core which starts at around 5,100 km down.

Material in the core is too dense to make its way to the surface, so there is some uncertainty over its composition.  What we do know is inferred from geophysical studies of the Earth and the chemical analysis of meteorites.  During the Earth’s formation, as rocks and fragments combined to form the planet, denser matter sunk towards the core under gravitational and other forces.  Iron is the chief component of the core, with nickel at the inner core and a lighter element in the outer core (possibly, oxygen, sulphur, carbon, hydrogen or potassium).  The iron in the core and the electrical currents in the molten outer core are the source of the Earth’s magnetic field. 

Seismic studies have shown that the outer core is impermeable to earthquake shear waves (S waves) so acts like a liquid.  Whether or not a layer is liquid or solid is down to the balance between temperature, pressure and chemical composition: while the inner core is around 4,700°C, immense pressure keeps the rock solid.

The Mantle

The mantle is composed of solid rocky materials that are less dense that the outer core; it makes up two thirds of the Earth’s mass.  Density differences mean that the mantle is a distinct layer from the outer core.   The most abundant elements in the mantle are silicon and oxygen, that form silicates.  The mantle is made up of around 45% silica. Magnesium and iron are the third and fourth most abundant elements.  Many other elements are to be found in the mantle, but these tend to be depleted near the boundary with the crust.

The composition of the mantle is inferred from xenoliths (small fragments of rock) contained in some basalt magmas and kimberlites.  Whether or not these are representative of the mantle as a whole or just the fragments that have been erupted is open for debate.

The upper mantle is joined to the crust; the combined layer is referred to as the lithosphere. Below the lithosphere, also in the upper mantle, is the asthenosphere.  The asthenosphere, being weaker than the lithosphere, enables lithospheric slabs to move around (plate tectonics).  The asthenosphere moves at the rate of a few centimetres a year from a process called solid-state convection; hot mantle rises, transfers heat to the lithosphere and the resulting cooled mantle sinks.  The heat in the lithosphere is dissipated through conduction or via rising magma.

The lithosphere is around 120 km thick.  It’s boundary with the asthenosphere is defined by the temperature at which rocks become ductile, around 1,350°C.

The Crust

The crust is a silicate rich brittle layer covering the mantle; it comprises less than 0.5% of the Earth’s mass.  There are two types of crust: oceanic crust, c. 6 km to 11 km thick, mostly basalt, which makes up ocean floors; and, continental crust, c. 25 km to 90 km thick, composed of igneous rocks (granite and andesite), sedimentary rocks and metamorphic rocks, which, as the name suggests, make up the continents and the continental shelves.  Igneous rocks are those resulting from volcanic processes. Sedimentary rocks are those made up of fragments produced by erosion or decay of rocks on the surface.  Metamorphic rocks are sedimentary or igneous rocks altered by changes in temperature and / or pressure. 


Magma is the molten rock from either the mantle or the crust, itself, that makes its way through the crust to where it may be erupted as lava at a volcano or volcanic fissure.  Magma and lava are the same rock: it is magma until it is erupted; and, lava is the erupted matter.

The composition of the magma and how it is generated determine the eruptive style of the volcano: e.g. effusive or explosive.


The energy required for matter to be erupted is heat from the Earth’s core.  The Earth’s core is made up of radioactive materials; their radioactive decay generates heat.  Most heat today is generated from four long-lived radioactive isotopes: two uranium isotopes,235U and 238U; one thorium, 232Th; and, one potassium, 40K.  Additional heat came from the decay of the shorter-lived aluminium isotope, 26Al earlier in the planet’s formation.  Asteroid bombardment has also added kinetic energy. 

So we know have the Earth’s crust, magma and heat.  What happens next?  Watch this space. 

The Armchair Volcanologist

24 June 2020

© Copyright remains with the author; all rights reserved. 2020.