Tag Archives: tsunami

Indonesia

Gunung Merapi, Alert Level 3, Aviation Code Orange

Good afternoon!

Today we are taking a trip to Java, Indonesia, away from the gentle effusive eruption at Fagradalsfjall, Iceland, and what looks like the growth of a shield volcano, to the more explosive activity at the subduction zone between the Indo-Australian Plate and the Sunda Plate, focussing on Gunung Merapi. 

Merapi has been erupting since the start of the year, with the growth of two lava domes, block and ash falls and pyroclastic flows generated from partial collapses of the lave domes.  The current alert level is 3 (Siaga) and the aviation code is orange.  (Siaga translates as, using Google Translate, “Stand By” (There was a significant increase in volcanic activity. Eruption is most likely to occur and the area of potential eruption hazard is in the area of ​​Disaster-Prone Areas (KRB) II. The community is prohibited from carrying out activities in the KRB II area). The exclusion zone extends 5km from the summit).

Fig 1:  Merapi 2011 with Prambanan in the foreground, cropped from an image of Prambanan by Arabsalam, published under CC BY-SA 4.0.  Source: Prambanan Java243.  Prambanan is an 8th Century Hindu temple compound located approximately 17 kilometres (11 mi) northeast of the city of Yogyakarta and designated a UNECSO World Heritage Site.

Background

Merapi is a 2,968 m high stratovolcano located 25 km north of the city of Yogyakarta on the island of Java, Indonesia.  Volcanic activity at Merapi is believed to have started around 170,000 years ago.  Since then, activity alternated between effusive and explosive, the latter with lava domes and pyroclastic flows.  While her eruptions are comparatively modest (VEI 1 to 4), her proximity to a large metropolis means she has the potential to do a lot of damage, hence her status as a Decade Volcano. Over 4 million people live within 30 km of the volcano and over 24 million people live within 100 km of the volcano.

Yogyakarta, itself, is a densely populated city in the Special Region of Yogyakarta.  373,589 people live in the city (Census 2020) and over 4 million in the metropolitan area.  It is a sophisticated centre for Javanese fine arts, culture and education.  Yogyakarta has been home to the seat of power for the Medang Kingdom between the 8th and 10th century and Mataram Sultanate between 1587 and 1755.  Explosive eruptions from Merapi, which destroyed many Buddhist and Hindu temples built between 732 AD and 900 AD (and presumably caused other significant damage), may have been a factor in the migration of the Mataram Kingdom to East Java in 928 AD or 929 AD.

Tectonic Setting

Volcanic activity on Java is driven by the northward subduction of the Australian Plate under the Sunda Plate at the Sunda Trench.   The western, southern and eastern boundaries of the Sunda Plate are tectonically very active. 

We downloaded earthquake data from EMSC for the period October 2004 to July 2021 between 14.04°S 86.16°E to 12.66°N 127.48°E to take a look. This area is larger than the area under discussion to make sure that subduction zone was captured.  The following images are from extracts of that data.

Fig 2: Subduction zone at the Java Trench section of the Sunda Trench.  Dots denote earthquakes, blue triangles, volcanoes.  The orange triangle is Merapi.  © Copyright remains with the author; all rights reserved, 2021.
Fig 3:  Tectonic setting – plot of earthquakes between October 2004 and July 2021 by the author.  Earthquakes clearly delineate the plate boundaries. Green dots denote earthquakes below 6.0 M, yellow dots, earthquakes between 6.0 M and 7.0 M, red dots, earthquakes between 7.0 M and 8.0 M, silver stars, earthquakes between 8.0 M and 9.0 M and orange star, earthquakes over 9.0 M.  Blue triangles denote volcanoes.  The orange triangle is Merapi.  © Copyright remains with the author; all rights reserved, 2021.

Merapi is located at the intersection of two volcanic lineaments:  the north – south trending Ungaran-Telomoyo-Merbabu-Merapi; and the east – west trending Lawu – Merapi – Sumbing – Sindoro – Slamet.  Ungaran-Telomoyo-Merbabu-Merapi is a double chain volcanic arc, with Ungaran as the back arc and Telomoyo-Merbabu-Merapi as the trench side volcanoes. Merapi is also bounded by two faults: the north – south Merapi – Merbabu fault and the Baribis – Semarang – Kendeng fault.  

Fig 4:  Earthquakes in Java from October 2004 to July 2021 plotted by the author.  Red and grey lines denote approximate locations of major faults, blue lines, volcanic lineaments, blue triangles, volcanoes and orange triangle, Merapi, black rectangle on the left is the area plotted.  © Copyright remains with the author; all rights reserved, 2021.

Growth of Gunung Merapi

Merapi formed in the late Pleistocene and Holocene in three stages: Proto-Merapi, Old Merapi and New Merapi.

Proto-Merapi emerged after 170 ka and includes Gunung Bibi, dated to 190 ± 60 ka, Gunung Turgo, 138 ± 3 ka, and Gunung Plawangan ,135 ± 3 ka.

Gunung Plawangan was home to a volcano observatory until the 1990s but was abandoned due to nuée ardentes from the current cone.

The summit of Gunung Turgo is home to sacred graves including that of Sheikh Jumadil Qubro, a direct descendant of the Islamic prophet, Muhammad.

Old Merapi started to grow around 30 ka, reaching a height slightly more than the current cone; growth ended with flank failures 4.8 ± 1.5 ka.  Collapse of the caldera with debris avalanche flows to the south and west left a Somma rim on the eastern slope. 

Gunung Batulawang and Gunung Kendil are part of Somma – Merapi. Gunung Batulawang is the highest peak of Old Merapi.  Lake Borobudur formed c 3,400 14C years BP. 

The latest Somma collapse was around 1,900 14C years BP. 

New Merapi, the current cone, grew soon after the growth of Old Merapi ended.  Partial edifice collapse of New Merapi may have occurred 1,130 ± 50 14C years BP. 

Gunung Selokopo Ngisor, Gunung Pusunglondon, Gunung Patukalapalap, Gunung Dengkeng, Gungung Selokopo Duwur and Gunung Gadjah Mungkur are hills which are thought to belong to New Merapi.

Volcanic Activity

Merapi’s lavas are typical for a subduction zone, being andesite / basaltic andesite, trachyandesite, basaltic trachyandesite with some basalt / picro basalt and trachybasalt / tephrite basanite.  Her lavas have evolved over time. Her early lavas were effusive basaltic.  The K2O content of the lava has increased over time; Old Merapi had lower K2O lavas than New Merapi and nearby Gunung Telomoyo and Gunung Merbabu. 

According to GVP, there have been 111 Holocene eruptions, ranging from VEI 1 to 4.  The most recent eruption, currently a VEI 1, started on 31 December 2020, with new lava domes extruded in January and February 2021, and, at the time of writing is ongoing.  The most recent VEI 4 was the devastating October 2010 to July 2012 eruption in which partial dome collapse caused pyroclastic flows that destroyed villages, led to the evacuation of more than 300,000 people and caused 386 fatalities; the ash plume reached 18km between 4-6 November; and, the largest pyroclastic flows occurred on 26 October 2010 and 5 November 2010 – the latter produced the widest pyroclastic flow seen in Indonesia for 100 years.  By mid-November, eruptive activity subsided, to be followed by lahars as the main hazard.

The 26 October eruption started 19 hours after tsunamis caused by earthquakes on the Sunda Trench swept away villages on the Mentawai Islands, killing 428 people and displacing thousands. The earthquakes were a 7.7 M, preceded by a 5.8 M and followed by aftershocks that included a 6.1 M and 6.2 M.  Whether or not the earthquakes triggered the eruption, itself, is open to debate; the volcano was ready to erupt. Since 2007 swarms of volcanic earthquakes had been occurring; deformation and gas emissions increased in September 2010; and, seismicity increased between 15 – 26 October, ramping up during 20 – 26 October.  The alert levels were raised to level 2 on 20 September 2010, level 3 on 21 October 2020 and level 4 on 25 October 2020.

A phreatic eruption on 11 May 2018, heralded a new phase of lava dome growth. In November 2020, evacuations were ordered due to decreasing stability of the lava dome. Eruptions commenced on 4 January 2021, leading to further evacuations in the Yogyakarta region.

Here’s hoping the current eruption remains a VEI 1.

The Armchair Volcanologist

©Copyright remains with the author; all rights reserved, 2021.

Sources and Further Reading:

Raw earthquake data, EMSC: https://www.emsc-csem.org/Earthquake/info.php

The Smithsonian Institution’s Global Volcanism Program (GVP): Merapi

Ralf Gertisser, Silvain J. Charbonnier, Jörg Keller, Xavier Quidelleur, “The geological evolution of Merapi volcano, Central Java, Indonesia”, Bulletin of Volcanology (2012) 74: 1213=1233. DOI: 10.1007/s00445-012-0591-3

Newhall, C. G., Bronto, S., Alloway, B., Banks, N. G., Bahar, I., del Marmol, M. A., Hadisantono, R. D., Holcomb, R. T., McGeehin, J., Miksic, J. N., Rubin, M., Sayudi, S. D., Sukhyar, R., Andreastuti, S., Tilling, R. I., Torley, R., Trimble, D., Wirakusumah, A. D., “10,000 Years of explosive eruptions of Merapi Volcano, Central Java: archaeological and modern implications”, Journal of Volcanology and Geothermal Research 100 (2000) 9 – 50. DOI:10.1016/S0377-0273(00)00132-3

Wikipedia:

Mount Merapi: Mount Merapi – Wikipedia

Yogyakarta: Yogyakarta – Wikipedia

Volcano Basics, Volcano Risk Mitigation

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.

What

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.

When

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.. 

Testing

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: https://en.vedur.is/   &  https://en.vedur.is/about-imo/news/the-aviation-color-code-for-grimsvotn-changed-from-green-to-yellow ).

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.

Successes

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: http://globalvolcanomodel.org/volcanic-fatalities/

“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: 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.

Volcano Basics, Volcano Risk Mitigation

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.

Tephra:

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:

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. 

Jökulhlaups:

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.

Earthquakes:

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.

Tsunamis:

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