Tag Archives: debris avalanche flow

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

Mount St Helens : 18 May 1980 Eruption

Good Afternoon!

The 40th anniversary of the catastrophic eruption of Mount St Helens was on 18 May 2020.  On 18 May 1980, 57 people are known to have been killed, most probably by the initial lateral blast, not taking into account anyone unwise enough to have ignored the exclusion zone.  In addition, significant damage was done to the surrounding area.

Geologists Keith and Dorothy Stoffel, on a photography expedition, witnessed the catastrophic failure of the northern flank of the volcano: the north side of the volcano slid down then disintegrated in a massive debris avalanche. This was followed by the horizontal blast and the vertical ash column which reached a height of 20 km within 10 minutes. The threat of being engulfed in the ash cloud forced them to return to base.

Fig 1: Mount St Helens and Spirit Lake before the May 1980 eruption. Image cropped from one by Jim Nieland – US Forest Service, Public Domain

Geological Setting

Mount St Helens lies at the front of the Cascades volcanic arc in Washington. She is the most active volcano in the arc. Her lavas are dacite, andesite / basaltic andesite, basalt / picro basalt, trachybasalt / tephrite basanite and trachyandesite / basaltic trachyandesite(1). The lavas are sourced from depths of 7 km – 14 km beneath the base of the edifice. Eruptions occurring after 1980 were fed from magma at a depth of less than 4 km. 

Volcanism here is driven by the subduction of the Juan de Fuca plate under the North American plate.  The arc starts in northern California and reaches up to British Columbia, including composite volcanoes and volcanic centres, calderas and back arc basalt shield volcanoes.  Lava compositions range from intermediate to high silica lava domes to low silica lave flows in the southern end of the arc.  The presence of low silica lavas at the southern end of the arc is attributed to extensional tectonics permitting the rise of more fluid magmas.

Seismicity in the Cascades

We downloaded the earthquakes between 39.470°N, 130.808°W and 50.870°N, 118.696°W from 1975 to 31 May 2020 to see what the subduction zone below the Cascades looks like.  There were 18,173 quakes over 2.5, of which 48 were between 6.0 and 7.0 and 4 had magnitudes in excess of 7.0. We found two areas where there was some evidence of subduction, one at the northern end of the region (north of latitude 45°N) and the other in the south (south of latitude 42°N), otherwise the area was relatively quiet in this period. 

Fig 2: Seismicity and some volcanoes of the Cascades plotted by the author. © Copyright remains with the author; all rights reserved, 2020.

In order to see the subduction zone clearly we have had to removed the “noise” from the Blanco Fracture Zone.

Fig 3: The subduction zone north of latitude 45N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.
Fig 4: The subduction zone south of latitude 42N from the above data set, plotted by the author.  © Copyright remains with the author, all rights reserved, 2020.

Eruptive History

Mount St Helens is a relatively young volcano, formed around 40,000 to 50,000 years ago.  The current edifice was built in the last 2,200 years.   Before the eruption Mount St Helens was conical stratovolcano made up of lava, ash, pumice and other deposits with layers of basalt and andesite.  Dacite lava domes extruded from the summit and on the northern flank at Goat Rocks. 

Mount St Helens has had 44 Holocene eruptions according to GVP(1), of which one was VEI 6, and five were VEI 5, including the 1980 eruption.  Two years prior to the1980 eruption USGS volcanologists produced a hazard assessment, predicting an eruption with the next couple of decades. 

The 18 May 1980 Eruption

A 4.2 earthquake on 20 March 1980 and increased seismic activity showed that the volcano was reawakening.  Ash eruptions followed from 27 March 1980. A harmonic tremor, indicating magma ascent, set in.  Between the initial earthquake and to the earthquake preceding the eruption on 18 May 1980 there were 228 earthquakes over 2.5 and thousands more smaller quakes.

By the end of April, a 2 km wide cryptodome (bulge) had deformed the northern flank, swelling upwards at a rate of 1 metre per day to 150 metres above the existing topography by 12 May 1980.  Ground deformation was now presenting a serious hazard.  

Fig 5   The cryptodome on the north flank 27 April 1980, cropped from an image by Peter Lipman in  – CVO Photo Archives Mount St. Helens: A General Slide Set, Public Domain

On the morning of 18 May 1980 magma had reached a level of 2 km below the edifice. The slope failure was triggered by a 5.1 earthquake on the morning of 18 May 1980 (or vice versa).  A debris avalanche of rocks, glacier ice, soil and other debris ensued, reaching Spirit Lake and the Toutle River valley and ending up east of the Camp Baker logging base, 20 km further down the valley. 

Depressurisation of the system, resulted in the violent explosive lateral blast. The blast outran the initial avalanche, felling trees in an area of 600km2 and causing the most loss of life.  The blast included heated old lava from the volcano and left a relatively thin ash layer. 

After the blast, pumice was erupted in a vertical eruption column and pyroclastic currents covered the northern side of the volcano.  Pyroclastic deposits reached a depth of 40 metres in the Upper Toutle Valley.  It is estimated that at the time of the eruption the pyroclastic flows had temperatures in the region of 700°C.  Hot ash reaching Spirit Lake caused secondary eruptions as the water flashed to stream.

During the eruption, Mount St Helens lost its pristine cone and around 400 metres height; it was left with a 1.6 km wide crater, open on the northern side.

Fig 6 The day before the eruption and a few days later.  Both images are from Johnson’s Ridge (named after the volcanologist who lost his life in the eruption) and are by Harry Glicken – USGS Cascades Volcano Observatory, Public Domain

Post Eruption

From mid-June 1980, lava emerged into the crater forming a lava dome; subsequent eruptions have been crater-based lava dome.

Could Mount St Helens produce another catastrophic eruption? 

As she has a history of VEI 5s and a VEI 6, the answer is probably, yes.  Her lava types do produce explosive eruptions, including the dacite lava domes. If any of the remaining slopes are unstable, there could be future slope failure should ascending magma cause enough deformation but it’s possible that she may have to rebuild the cone first. 

In the meantime, a risk would be to the unwary straying too near to the crater should she have a smaller e.g. phreatic eruption, which are unpredictable. Phreatic eruptions occur when ground water in the edifice heated by hot rocks or magma flashes to steam.

GVP(1) have added a commemorative feature to its website giving a lot more information, including footage of the eruption.  We recommend that you take a look; it is well worth the visit(2).

The Armchair Volcanologist

2 June 2020

References & Further Reading

  1. Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/
  2. Mount St Helens 40th Anniversary, GVP: https://volcano.si.edu/projects/sthelens40/
  3. Mount St Helens, Wikipedia: https://en.wikipedia.org/wiki/Mount_St._Helens
  4. 1980 Eruption of Mount St Helens, Wikipedia, https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens

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