Tag Archives: Lahar

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. https://pubs.usgs.gov/sir/2012/5176/a/sir2012-5176-a_plate1.pdf

USGS Volcano Hazards Program: https://www.usgs.gov/natural-hazards/volcano-hazards/be-ready-next-volcanic-event

“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