The Icelandic Met Office (IMO) has updated the aviation alert for Grímsvötn from green to yellow today (30.09.2020) because the volcano’s activity is above background level, now at a level comparable to that which preceded previous eruptions. They note:
Above average seismicity for September 2020;
Deepening cauldrons in the ice-cap round the caldera from geothermal activity;
Surface deformation exceeding that which preceded the 2011 eruption;
Magmatic gases detected in the summer of 2020.
An eruption is not considered imminent.
Water levels in the sub-glacial lake are high indicating possible jökulhlaups in the coming months. Draining of the lake by a jökulhlaup depressurised the system before the 2011 eruption, so an eruption is considered possible in the event of a jökulhlaup.
Activity may decrease without an eruption in this instance; only time will tell.
Jumping the gun a bit on our next post in the volcanic risk mitigation series, the IMO’s alert is an example of using alert levels to highlight the increased risk of an eruption to those who need to know, without being unduly alarmist – a straightforward statement of the facts supporting the current status. For the exact wording of the alert, please follow the link below.
Fig 1: Eruption column 3 hours after the onset of the 2011 eruption of Grímsvötn. Source: Sigurjónsson, O. (2011 May 21). Grímsvötn: photo 10 of 14. Retrieved from http://icelandicvolcanoes.is/?volcano=GRV
Grímsvötn is located under the Vatnajökull ice-cap in an active rift zone of the Eastern Volcanic Zone, Iceland. She erupts frequently; her last in 2011 was a large VEI 4, which impacted local farmers and livestock and aviation in Europe.
Update (02/10/2020)
Googling around a bit more, I note that Iceland’s Department of Civil Protection and Emergency Management, Almannavarnir, have reported in their 25 September 2020 bulletin that an eruption is considered likely this Autumn (use Google Translate or other tool, if you need to, as it is in Icelandic).
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.
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.
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.
Krakatau’s VEI 6 eruption of 1883 is the next in our series of famous eruptions. We have also included a summary of birth of Anak Krakatau from the caldera, and the December 2018 cone collapse and resulting tsunami.
The 1883 eruption is not only famous for the catastrophic destruction of Krakatau Island, pyroclastic flows, global cooling, devastating tsunamis and a death toll of between 36,164 to 120,000 people, but it is also the first major eruption to have been reported globally by telegraph. A typo in the telegraph led to the west calling the volcano Krakatoa.
The 2018 eruption, cone collapse and tsunami are well-documented by various sources. Our go-to resource here was primarily GVP.
Fig 1: Lithograph: Parker & Coward, Britain. 1888. Public Domain
Geological Setting
Katakatau lies to the west of the Sunda Strait between Sumatra and Java. It is the site of a much larger 7 km wide caldera which may have been formed during eruptions in 416 AD or 535 AD. The edges of the caldera are marked by Verlaten and Lang Islands.
Before the 1883 eruption, Krakatau was a 9 km long verdant, wooded island formed from three volcanoes, Rakata, Danan and Perbuwatan in the caldera. Another small island in the group was Polish Hat. The islands were uninhabited but used by local fishermen, woodcutters and the Dutch and British navies.
Fig 2: Map by ChrisDHDR showing Krakatau Island before the eruption and the site of Anak Krakatau, Public Domain
Eruptive History
GVP records 56 known Holocene eruptive periods for Krakatau, of which only 10 precede the 1883 eruption. The evidence for the 10 is historical observations from 250 AD to 1684, so perhaps earlier activity has been lost under the debris from more recent events. Activity after 1883 relates to building of Anak Krakatau.
The 535 AD eruption of Krakatau may have caused the volcanic aerosol veil that dimmed the Sun (filtered out sunlight) for eighteen months, causing crop failures, cooling and hiding Canopus, a bright star used by Chinese astronomers to mark the seasons – more likely if the eruption was a large caldera forming event. The other contender (preferred by some) is the 408 AD – 536 AD Tierra Blanca Joven eruption of Ilopango, El Salvador. On the other hand, why exclude one? Both may have contributed in some way.
Krakatau’s lavas are typical of a subduction zone: andesite, basaltic andesite, dacite, trachyte and trachydacite; and, also basalt and picro basalt. The last mentioned indicates that there may be more rapid magma ascent through extensional faulting in the area.
The 1883 Eruption: 100 Days of Activity
The Intro
The only known precursors to the 1883 eruptions are a large earthquake on 1 September 1880 followed by a period of increasing seismicity. Unfortunately, the area is seismically very active, being near the convergent margin between the Sunda Plate and the descending Indo-Australian Plate so, without modern instrumentation, there was insufficient information to interpret escalating events.
On the 20 May 1883 eruptive activity started at the Perbuwatan crater with series of loud explosions audible 150 km away. Light ashfall covered the area and a column of steam was visible. Activity continued for a few days then calmed down enough for a party to charter a boat to the island on 27 May 1883; they were the only witnesses to the Perbuwatan crater, then about 1 km in diameter, 50 m deep, with a small pit generating a steam column and small explosions every 5 to 10 minutes. By the end of June 1883, the summit of Perbuwatan had been destroyed and a second eruption column was visible at the centre of the island.
Captain Ferzenaar, a surveyor for the Dutch government, collecting ash samples on 11 August 1883, found a thick covering of tephra, all vegetation stripped bar a few tree trunks; three active eruption columns (one at Perbuwatan and the other two near the centre of the island); and, at least eleven other sites with some activity. However, upwind of the eruption, he was unable to see more beyond the steam and ash.
The Cataclysmic Eruption
The main event occurred over 26 and 27 August 1883. There were very few survivors so it took while for Dutch investigators, led by Rogier D.M. Verbeek, a mining engineer and geologist, and British investigators from the Royal Society to reconstruct the events. Their information came from various sources, including ships caught in the Sunda Strait, pressure gauges at the Batavia gasworks on Java, Dutch officials living in Batavia and Buitenzorg, and requests for information from the Royal Society for data further afield, including one printed in The Times.
Three days before 26August, there had been a marked increase in activity on the island. By 13: 00 on 26 August explosions were loud enough to be heard 150 km away. By 14:00 a 25 km high black eruption column was visible. By 17:00 activity was audible throughout Java and pumice was raining down on vessels in the Strait. By 19:00 a Plinian eruption column with intense volcanic lightening was witnessed. Several ships were among those caught up in the eruption, one of which, the Charles Bal, trapped by poor visibility had to sail within sight of the volcano to keep its bearings amid hot ash fall, volcanic gases, lightening and St Elmo’s Fires (static electricity which lit up the mastheads).
The eruption escalated on 27 August with large explosions at 05:30, 06:44, 10:02 and 10:52 in the morning (local time). The noise from these woke people 3,224 km away in Australia; and, further away at a distance of 4,811 km, it was confused with gunfire. Atmospheric pressure changes were detected globally. These explosions generated a 40 km high Plinian eruption column that cut out the sun for up to two days in the vicinity; further away in Batavia, full loss of light lasted for just over an hour and a half. There is some debate on what caused the large explosions, including the possibility of sea water reaching either the magma chamber or ascending hot magma.
Pyroclastic density currents (PDCs) made it to southern Sumatra, killing 2,000 people. The inhabited islands of Sebesi and Sebuku between Krakatau and Sumatra were devastated, with no survivors. Hot ash from the PDCs burned people as far away as Kalimbang, Sumatra.
The tsunamis caused the most of the remaining fatalities (estimates of the total number of fatalities vary from c. 36,000 to 120,000). A series of tsunamis devasted the shores of the Sunda Strait; waves reached a height of 25m on the coast of Sumatra and 40m on Java. The town of Anjer was washed away. The largest tsunami wave at Batavia was detected at 12:36 on 27 August. The waves reached as far as Auckland, New Zealand. The tsunamis may have been caused by displacement of large amounts of seawater from the rapid deposit of ash in the caldera form discrete explosions, collapse of the eruption columns or edifice collapse. As you will see later, the edifice failure of Anak Krakatau in December 2018 caused a catastrophic tsunami.
The eruption calmed down after the four large explosions, with some outbreaks of minor activity, to be quiet after 28 August 1883.
The Immediate Aftermath
Two thirds of Krakatau Island had disappeared – either blown apart by the eruption or sunk as part of the creation of a 300m deep caldera; only the southern section of Rakata remained. Pumice and a small rock were all that remained of the northern part of the island.
20 km3 of dacite pyroclastic material had been erupted as tephra, pyroclastic density currents and the rest deposited into the sea and on surrounding islands. Deposits enlarged Verlaten and Lang Islands and two new islands were formed: Steers and Calmeyer. Polish Hat, however, had disappeared. Steers and Calmeyer were later eroded by sea water.
The 40 km high eruption column had reached into the stratosphere, where ash was spread round the globe, initially in the tropics but then migrating northwards and southwards, lingering for a couple of weeks. Aerosols filtered sunlight resulting in vivid sunsets; the sun is reported as appearing as green or blue, depending on its angle in the sky. Filtering of the sunlight caused global cooling probably in the order of 0.34°C in 1884.
The Birth, Collapse and Regrowth of Anak Krakatau
Krakatau has remained active with over 40 eruptive episodes since the 1883 eruption. Anak Krakatau (Child of Krakatau) emerged in 1927 from the caldera and had reached a height of 338m by 2018, only to lose a large part of the new cone in December 2018 when a relatively small eruptive episode (VEI 3), which started in June 2018, caused edifice failure. The edifice collapse was preceded by an eruption at 21:03. Øystein Lund Andersen, a photographer, recorded that by 21:05 a dark plume obscured the volcano and earlier incandescence. At 21:27 the first tsunami wave hit the shore, travelling 15m inland; at 21:31 a second much larger wave followed.
Two thirds of Anak Krakatau had been destroyed. The tsunamis killed 437 people, injured 31,943, displaced a further 16,198 and damaged 186 miles of the shore line in Sumatra and Java. Ash and gases cleared Kecil Island and a large part of Anak Krakatau, itself, of vegetation.
This edifice collapse had been predicted. Volcanologists from the University of Oregon had noted in January 2012 that the cone, formed on a steep slope of the 1883 caldera, was vulnerable to edifice collapse, especially on the western side.
The eruptive activity has continued, initially underwater, producing Surtseyan activity. Cone rebuilding is continuing with both submarine and subaerial activity. Recently, there was a small magmatic eruption in April 2020, producing two ash columns that reached 14 km and 11 km height and lava fountains.
Recent Seismicity
Volcanism in the area is driven by the subduction of the Indo-Australian Plate under the Sunda Plate. The Sunda Strait is seismically very active, possibly because it is accommodating the change in direction between the northern and eastern arms of the plate boundary. Krakatau, itself, lies in the bend of the Arc above the Wadati-Benioff zone.
We looked at the earthquakes in the region 8.67°S 101.00°E to 3.94°S 110.09°E for the period 1971 to 14 July 2020; this area includes the southern end of Sumatra, the Sunda Strait and the western end of Java. We downloaded the earthquake data from IRIS’s earthquake browser. The download comprised mostly earthquakes with magnitude over 4.0; smaller volcanic / tectonic earthquakes were not included in the data set.
Our plots show the subduction zone in the curve of the Sunda Volcanic Arc, with more intense seismic activity in the northern arm of the arc. The intense areas of activity in the northern arm starts in 2000, preceding the 2004 Banda Aceh earthquake, which is north of the area in our plot, and continuing for a few years afterwards.
A large earthquake swarm started on the morning of 19 July at around 1:30 am at Fagradalsfjall on the Reykjanes Peninsula. The largest earthquake had a magnitude of 5.1M. At the time of writing, there had been 1,635 earthquakes in the last 48 hours recorded on IMO’s website (note that not all of these have been confirmed). IMO’s map and breakdown of the swarm are shown below:
Fig 1: Map of earthquakes in Iceland over the past 48 hours. Source: IMO
Close up of the Reykjanes Peninsula:
Fig 2: Map of earthquakes in the Reykjanes Peninsula over the past 48 hours. Source: IMO
Magnitude
Number
< 1.0
720
1.0-2.0
723
2.0 -3.0
162
>3.0
30
Fig 3: Breakdown of earthquakes by magnitude
This swarm is occurring on the east side of the swarms on the Reykjanes Peninsula which started late last year. IMO have reported that these swarms (still ongoing) are associated with multiple magma intrusions. The aviation code for the area is still green (IMO). IMO are in the process of evaluating the Fagradalsfjall swarm.
The swarm at the Tjörnes Fracture Zone is still ongoing.
The eagle-eyed amongst you will note that there is some seismic activity at Katla. Whether this will result in anything is anyone’s guess at the moment.
We have not yet updated our earthquake data-set for the current swarm. We will wait until IMO has had a chance to confirm more earthquakes
Update 24 July 2020
The swarm at Reykjanes is now less intense. In the meantime, Katla produced a shallow 3.0 M. IMO have remarked that earthquakes in the summer at Katla are not uncommon.
From memory, Katla was seismically active before the intense swarms started in August 2014 at Barðarbunga in the run up to the eruption at Holuhraun. This may have been a coincidence.
Fig 4: Map of earthquakes in Iceland over the past 48 hours. Source: IMO
Fig 1: Barðarbunga: cropped image from photo 3 of 11 by Erik Sturkell, retrieved from Icelandic Volcanoes
In this post we continue our journey round Iceland’s many volcanoes. We have reached the mighty Barðarbunga at the northwest corner of the Vatnajökull icecap.
Barðarbunga volcanic system lies in the Eastern Volcanic Zone, Iceland, near where the head of the mantle plume is thought to be. The system comprises a 2,000 m high central stratovolcano with a 65 km2, 700 m deep caldera, the Veiðivötn fissure swarm running NE to SW, and the Tröllagigar and the Dyngjuháls – Holuhraun fissure swarms running NE; the entire system is c. 190 km long and 25 km wide.
There is second central volcano in the system, Hamarinn, 20 km south west of the Barðarbunga central volcano. Hamarinn may be younger, indicated by the absence of both an intrusive complex and a caldera.
There are geothermal areas near the caldera rim of Barðarbunga and the east of Hamarinn, the latter is the source of jökulhlaups.
Fig 2: Map of the Barðarbunga volcanic system:central volcanoes, fissure systems and lava flows. Green barred squares indicate the locations of various volcanoes; BAR is Barðarbunga. Retrieved from Icelandic Volcanoes. See Sources for full accreditation.
The area is tectonically very active: the Eastern Volcanic Belt accommodates much of the separation between the North American and Eurasian Plates. The area is close to the junction with the Northern Volcanic Zone, where Barðarbunga’s neighbours, Askja and Herðubreið, can be found.
Eruptive History
According to GVP, there are 55 identifiable Holocene eruptive periods for the Barðarbunga system. Some of the eruptive history has been hidden by the ice-cap. However, lavas and tephra deposits on the ice-free sections of the fissures are more accessible.
Barðarbunga Central Volcano
The central volcano has had around 22 eruptions in the last 1000 years, most occurring between 1200 -1500 and in the 18th century. The last known subaerial eruption was in 1910.
Barðarbunga’s lavas are mainly basalt/ picro basalt. Her eruption types are explosive, phreato-magmatic with jökulhlaups, reflecting the impact of the ice-cap. The central volcano produces eruptions in the order of VEI 3 to 4, producing tephra – both airborne and waterborne. There is a silicic tephra layer in the ice-cap dating to the early 18th century but it is not clear that this came from Barðarbunga. If it did, any rhyolite would have come from partial melting of the basaltic crust.
Magma is sourced from a depth of 10 km or more below the caldera; above this source is an intrusive complex and a lower density region, probably of caldera in-fill. Magma may also be sourced direct from the mantle in the fissure swarms.
Fissure Swarms
Fissure swarm eruptions are basaltic in the order of VEI 1 to 2, with a maximum of VEI 6 on the Veiðivötn fissure.
The last three eruptions on the south west fissure swarm were the VEI 4 at Vatnaöldur in 877, the VEI 6 at Veiðivötn in 1477 and the VEI 2 at Tröllagigar in 1862-1864. The first two of these were explosive tephra eruptions, producing 5 km3 to 10 km3 of tephra and small lava flows. Both the Vatnaöldur and Veiðivötn fissures cut into the Torfajökull volcano, causing it to erupt with silicic tephra and lava. The largest known effusive eruption on the SW fissure swarm is the Great Þjórsá lava which covers 900 km2 and reached the south coast via the Tungnaá and Þjórsá river valleys.
The Gjálp eruption in 1996 occurred on a subglacial fissure that links the Barðarbunga and Grímsvötn volcanic systems. While it is thought that the magma was sourced from beneath Barðarbunga, based on seismic and geodetic data, the magma erupted subaerially was characteristic of Grímsvötn’s lavas.
The frequency of eruptions on the northern fissure swarm is not known; the last eruption was at Holuhraun which started on 29 August 2014 and lasted until February 2015. Precursors to this eruption were a build up of seismic activity at Barðarbunga over seven years, which stopped immediately after the Grímsvötn 2011 eruption but resumed soon afterwards. The largest known effusive fissure eruption north of Vatnajökull is the mid Holocene 15 km3 Trölladyngja lava shield.
Holuhraun Eruption 2014 – 2015
Fig 3: Holuhraun eruption. Cropped image from Barðarbunga: photo 8 of 11 by Alessandro La Spina, 4 September 2014. Retrieved from Icelandic Volcanoes. Note the fire fountains, spatter cones and volcanic gases.
The subaerial eruption started on 29 August 2014 at the Holuhruan vent 45 km NE of the Barðarbunga caldera; the eruption ended on 28 February 2015, having left an 85 km2 lava field and a 65 m deep depression in the Barðarbunga caldera’s ice cover. The eruption was a large SO2 and other volcanic gas producer, however there was little ash or tephra.
The central volcano, Barðarbunga had inflated prior to the eruption then deflated during the eruption as evidenced by subsidence in the ice covering. The volume of the subsidence was consistent with the dyke intrusion and the lava erupted at Holuraun, although there is seismic and geochemical evidence that some of the lava erupted at Holuhraun was fed direct from the mantle. It is estimated that 1.6 km3 lava was erupted.
Since September 2015, seismic and GPS data show that the volcano has started to refill at a depth of 10 to 15 km.
Recent Seismic Activity
We looked at earthquakes in the Barðarbunga, Askja, Herðubreið and Holuhraun area (64.56°N, 17.65°W to 65.3°N, 16.1°W) for the period 1 January 2009 to 28 June 2020. Not much activity had been noted in the area to the west and south west of Barðarbunga in our earlier plots; however, we had noted that heightened activity at Askja and Herðubreið had preceded the 2014 eruption at Holuhraun which lies between the three volcanoes, hence we included them in our plots. The link between the centres is rifting in the crust to accommodate the separation of the North American and Eurasian Plates.
There were 70,128 earthquakes in the period, of which 16,573 occurred before the 2014 -2015 eruption, 19,061 during the eruption and 34,494 post eruption; the average per calendar month was 247 pre eruption, 2,723 during the eruption and 539 post eruption; the maximum magnitude earthquake pre eruption was 3.9 M, during the eruption 5.5 M and 4.9 M post eruption; and, the deepest quakes had respective depths of 33.9 km, 31.0 km and 33.9 km. These numbers include activity at Barðarbunga, itself, the Holuhraun fissure, Askja and Herðubreið. The larger magnitude earthquakes occurred near the north and south caldera rim during the eruption. Since the eruption all four centres have had elevated seismic activity.
Seismicity during the 2014 to 2015 Holuhraun eruption
Three months prior to the eruption, there was an earthquake swarm at Herðubreið, noted here because the rifting event that preceded the Holuhraun eruption occurred on the same plate boundary. Seismic activity at Herðubreið or Askja may be precursors to activity at Vatnajökull, if they, themselves, are not the main event or brewing something. Something to watch out for bearing in mind the recent large swarms in the Tjörnes Fracture Zone and on the Reykjanes Peninsula.
The earthquake plots for August 2014 and November 2014 show the intense swarms from caldera collapse and also the rifting event.
We will look at Askja and Herðubreið in future posts.
“Barðarbunga”, Guðrún Larsen and Magnús T. Guðmundsson (Institute of Earth Sciences – Nordvulk, University of Iceland). In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=BAR
Fig 2: Map. After Björnsson (1988), Gudmundsson and Högnadöttir (2007), Jóhanneson and Saemundsson (1998a & b), Sigurgeirsson et al (2015). Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO. In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=BAR
Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
All good books on volcanoes describe well-known eruptions, including Vesuvius 79 AD, Tambora 1815, Krakatau 1883, Mount Peleé 1902, Katla 1918, Mount St. Helens 1980, Pinatubo 1991. Who are we to be any different?
There is a good reason for this. Most famous eruptions are large and explosive, causing loss of life and considerable property damage – in other words, headline-grabbing. Later eruptions have impacted aviation – volcanic ash and jet engines are not a good mix; airports near erupting volcanoes are closed and flights re-routed to avoid the ash clouds.
Catastrophic events are the reason volcanology is such an important subject. Understanding these and other eruptions is important to find out how to minimise the risk; i.e. reduce loss of life.
Researchers have shown that volcanoes usually very kindly give us some warning in the lead up to an eruption. Magma is viscous, rarely moves fast and, when moving, causes earthquakes, usually small, but some are felt without equipment. As magma ascends, degassing starts to occur, some of which is detectable at the surface. Rising magma also causes ground deformation, e.g. it causes the volcano to inflate, measurable by tilt-meters and GPS, and some of which may even be visible to the naked eye (Mount St Helens).
I read somewhere that the first piece of equipment required to monitor a volcano is a seismometer, the second is another seismometer, as is the third; earthquakes are often the first sign of impending trouble (if I can remember / find the source, I will accredit it properly). Scientists observing volcanoes will have a raft of equipment in place: seismometers, GPS stations, tilt-meters and gas monitoring; satellite monitoring may also detect ground deformation. Drones may be used to inspect craters which are not readily accessible or where it would be unsafe to visit. Our maps of Iceland include some GPS stations as markers to indicate where the earthquakes are occurring but still evidence that Iceland has several in place at its active volcanoes.
Given that volcanoes give some warning, why are people killed? Unfortunately, magma ascent and the build up to an eruption is a slow process with many stops and starts. A major eruption may be preceded by a few small throat clearing events before the volcano unleashes the main eruption. An evacuation may take place, but if it is quiet between the precursors and the main event, people may think that the volcano has finished and return home to be at ground zero at the wrong time, irrespective of whether or not any alert is in place. People living close to volcanoes tend to be farmers with strong bonds to both their land and livestock. On the other hand, the volcano may not behave as expected – e.g. a phreatic eruption or edifice failure. Volcano hazard assessment is an interesting topic for future posts.
We will also look at some of the more famous eruptions over the coming weeks.
Tambora produced one of the largest known eruptions in recorded history with a climate impacting VEI 7 and possibly the largest Holocene eruption (other contenders for a VEI 7 being Kurile Lake, 6440 BC, Mazama, 5700 BC, Kikai Caldera, 4300 BC, Cerro Blanco, 2300 BC, Thera (Santorini), 1620 BC, Taupo, 180 AD, Baekdu, 946 AD, and Samalas (Rinjani), 1257 ).
Tambora’s once proud 4,300 m stratovolcano lost around a third of its height and acquired a 1 km deep, 6 km wide caldera over the space of a few days in April 1815. Sumbawa and the surrounding islands were devastated. Climate abnormalities (cooling and severe storms) were noted round the northern hemisphere along with crop failure and famine. Tambora is accredited as the cause of the 1816 “year without a summer”. The eruption released 50 km3 of magma, 150 km3 of tephra, 80 million tonnes of sulphur dioxide and 18 mega tonnes of fluorine, along with water vapour and other aerosols.
The eruption was chronicled by local eye witnesses. However, the then lieutenant governor of Java, Sir Stamford Raffles, keen to develop trade in the area, was not so keen to broadcast it further afield to potential investors; outside the region, the eruption went largely unnoticed by the West, already distracted by the Napoleonic wars.
Geological Setting
Before the eruption, Sumbawa Island was a pleasant prosperous island, trading mung beans, corn, rice, coffee, pepper, cotton, wood and horses. Despite a wealth of natural resources, the people to farm them were in short supply so there was also a large slave trade and piracy.
Tambora occupies the entire Sanggar Peninsula on Sumbawa Island in the Sunda Arc of the Indonesian Archipelago. Here the Australian Plate subducts beneath the Sunda Plate at a convergence rate of 7.8 cm per year. Plotting the earthquakes in the region for 1972 to date clearly shows the Wadati-Benioff zone, with volcanoes sitting around 70km above the descending plate.
Tambora is a 60 km wide stratovolcano with trachybasalt and trachyandesite lavas. She formed a caldera c.43,000 years ago which was in-filled by Pleistocene lava flows. During the Holocene her eruptions have been explosive: three eruptions in the Holocene occurred before the 1815 event, identified by radiocarbon dating, 740 AD, 3050 BC, and 3910 BC; three further smaller eruptions have been observed since the 1815 event, 1819 VEI 2, 1880 VEI 2 and 1967 VEI 0, which extruded lava domes and small lava flows on the caldera floor.
1815 eruption
Tambora had been dormant for over a thousand years; magma cooling and fractional crystallisation had been occurring along with the exsolving of high-pressure magma; over pressurisation was happening. Tambora awoke in 1812 with minor activity during the period 1812 to 1815 while magma ascended from the reservoir c.4 km below the edifice.
The 1815 eruption proper started with a short Plinian eruption on 5 April 1815 of trachyandesite, lasting two hours, producing a 33 km eruption column; the explosions were heard as far away as Sumatra and were confused with gun fire. Following this, between 5 April and 10 April, there was a relatively low level of activity.
On the evening of 10 April, a second short Plinian eruption occurred, lasting three hours, producing a 44 km eruption column. Pumice rained down on local villages for 2 hours. Volcanic winds destroyed trees and property. The eruption column collapsed as the vent was eroded. Pyroclastic density currents (PDCs) raged over the next three to four days, creating phoenix ash clouds, covering the area in ash and destroying villages, along with inhabitants. During this phase the volcano, no longer supported by magma, subsided, creating the current caldera.
Tsunamis were generated when the PDCs reached the sea. Sanggar was engulfed in a four-metre high tsunami at around 10 pm on 10 April; this tsunami reached Java a couple of hours later with a height of two metres.
Explosions were heard during the night of 10 to 11 April up to 2,600 km away. Locations within a 600km radius suffered darkness for a couple of days and a chilling of the atmosphere while sunlight was blocked.
Seven or more ignimbrite layers were deposited during the second phase of the eruption. Only 2.6 km3 of deposits remain on dry land; the heavier ejecta ended up in the sea and lighter aerosols were scattered round the globe. Pumice rafts up to 5 km wide and trees trunks littered the Flores Sea, providing a hazard for shipping for several months.
Eruptive activity continued intermittently up to August 1819.
Local and global impact of the eruption
Sumbawa was stripped of its vegetation. The death toll in Sumbawa and neighbouring Lombok was in the order of 60,000, 10,000 from the eruption itself with the remainder from disease and famine from polluted water and loss of crops and livestock. Children were sold into slavery in order for them to survive or apparently killed to avoid a slow death from starvation or worse.
The effects were also global. The volcano had lobbed 50 km3 of matter plus gases into the stratosphere. Larger particles fell back to Earth but smaller aerosols hung around for three years causing adverse weather phenomena (e.g. storms), global cooling, failed harvests, famine and disease in much of the northern hemisphere.
The global effects were exacerbated, according to ice-core sampling, by another large catastrophic eruption in 1809. The source for the 1809 has yet to be identified; the volcano and eye witnesses may not have survived the eruption. However, the 1809 and 1815 eruptions together caused a little ice age.
Can this happen again?
VEI 7 eruptions are rare. As mentioned earlier, there are currently only eight other contenders in the Holocene. That is not to say they won’t happen again in the future.
My guess is that to get another large event, not only do you need a large magma source and a build-up of pressure but also an element of edifice failure. The largest part of the Tambora eruption in terms of magma output was while the PDCs were in full swing; these occurred as or after the vent eroded. We will see in the accounts of other eruptions, there was some edifice failure.
In the meantime, Mount Tambora and her Indonesian sisters are well-monitored.
The Armchair Volcanologist
6 July 2020 (updated 23/08/2020 & 25/08/2020 to include more potential VEI 7s (the list is getting longer!)
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.
EffusiveContinental 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 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.
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.
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 😉 ).
A large earthquake swarm started in Iceland in the Tjörnes Fracture Zone (TFZ) on 19 June 2020, still ongoing at the time of writing. The Civil Protection Authority, Iceland, has declared a state of uncertainty; the TFZ is capable of producing large destructive earthquakes.
The Icelandic Meteorological Office (IMO) has reported that over 9,000 earthquakes have been detected by their SIL earthquake monitoring system. The swarm includes three earthquakes over 5.0: on 20/06/2020 a 5.6 and a 5.4, both 20 km north east of Siglufjörður; and, on 21/06/2020, a 5.8 30 km north, north east of Siglufjörður. The day before the swarm started there was a small earthquake with a depth of 92.3 km, which is unusually deep for the area.
IMO is in the process of manually confirming the earthquakes – a mammoth task! We have updated our earthquake data set up to 28 June 2020 based on the earthquakes confirmed so far in order to take a look at the swarm. In the area, 65.25°N, 21.5°W to 67.25°N, 15.0°W, 2,010 earthquakes from 1 June 2020 to 28 June 2020 were available to download.
Our plot for month 54 (1 June 2020 to 28 June 2020) shows that the swarm is occurring on the western end of the TFZ, near the junction of the Eyjafjarðaráll Rift and the Húsavík-Flatey Fault.
Prior to this swarm, most activity in our data set from 1 January 2016 had been on the Grímsey Oblique Rift and Húsavík-Flatey Fault, with a large swarm occurring to the north west of Grímsey in February 2018 (see our earlier post). The current swarm appears to be the western end of the system catching up.
This swarm is, however, relatively deep for oceanic crust. Whether or not there is any associated volcanic or geothermal activity remains to be seen.
The swarm on the Reykjanes Peninsular that started in late 2019 is still ongoing, albeit at a reduced rate. Are the two swarms linked? The obvious answer is yes – they are both on transform fault systems which accommodate the spreading from the Mid Atlantic Ridge as it crosses Iceland. But is there more to it? Possibly, the crust could be fracturing to accommodate other activity in Iceland – e.g. uplift from the mantle plume. If yes, we may see some more activity e.g. in the vicinity of the Vatnajökull Icecap in the next few months. But we’ll have to wait until the real experts opine.
For updates, please consult IMO.
The Armchair Volcanologist
29 June 2020.
Source for earthquake data: Icelandic Meteorological Office (IMO): https://en.vedur.is
An Armchair Volcanologist 
