When was the last eruption in Grímsvötn?

The last eruption at Grímsvötn volcano took place in May 2011, the recent eruptions before that occurred in 2004 and 1998. The eruption in 2011 was significantly larger than other recent eruptions at Grímsvötn volcano. To find an eruption of similar magnitude one needs to go back to 1873. The 2011 eruption started in the evening of 21st of May and lasted for almost seven days. The ash cloud reached maximum 20 km a.s.l. and was seen in the beginning of the eruption from Egilsstaðir in East-Iceland to Selfoss in South-Iceland. The intensity of the eruption was highest the first couple of days but declined after that. Tephra fall was observed outside Vatnajökull ice cap but after first two days the fallout was mainly confined to the ice cap.

what kind of eruption are usually in Grímsvötn?

Eruptions at Grímsvötn central volcano are most commonly basaltic explosive with small to medium size and last for 1-2 weeks. They occur inside the caldera and the most recent eruptions have taken place along the southern caldera rim. The initial pahse of such eruptions is sub-glacial but the magma often melts it way through the glacier, and when it happens we refer to phreatomagmatic eruptions. it depends on the ice thickness the magma encounters when traveling to the surface, how long time that takes but it can take from minutes to hours. Because of interaction between magma and water the eruption become explosive and produces a volcanic plume that reaches commonly 5-12 km a.s.l. while the volcanic plume/cloud is transported by the wind, it spreads tephra over Vatnajökull ice cap and only a minor fraction outside the ice cap. In phreatomagmatic eruptions several hazards might be generated like a glacier flood, tephra fallout, lightning, pyroclastic flows and ejection of bombs.Typically, the eruptions are most intense for the first couple of days but after that the instensity declines rapidly.

What kind of natural hazard can occur when Grímsvötn erupt?

Glacial outburst floods:

Glacial outburst floods (jökulhlaups) are water floods released from ice dammed marginal or subglacial lakes. Subglacial volcanic eruption and other glacier related events can also cause jökulhlaups. These floods are transient events and can last from a few hours up to a few weeks. Whereas glaciers cover about 10% of Iceland, including many of the most active volcanoes, jökulhlaups are frequent. See also Catalogue of Icelandic volcanoes. 

Jökulhlaups can be of various sizes, where the maximum discharge and volume of floodwater spans orders of magnitudes. The largest jökulhlaups in historic times, (e.g. outbursts from Mýrdalsjökull caused by Katla eruptions), have had a maximum discharge on the order of 100.000 m3s-1, comparable to the discharge of the Amazon river. The rate of discharge increase during the rising phase of a jökulhlaup is also variable, causing some floods to reach maximum discharge in hours while other take days. Hydrographs of different events, therefore, span a spectrum from rapidly rising jökulhlaups to slowly rising jökulhlaups.

Jökulhlaups can carry various materials other than water, such as eroded ice or sediments, or eruptive material in the case of subglacial volcanic eruptions. Usually solid particles are only a small portion of the flood, but solid material can be a high portion or even the majority of the volume, in special cases. Subglacial volcanic eruptions where solid material accumulates and is then transported with the flow, and floods down steep slopes where lose soil, dust, rocks or other material is incorporated into the flow, are examples of such cases. Floods where the ratio of water and solid material is similar are also called lahars. This name originates from Indonesia, where such floods occur on the slopes of active volcanoes.

Jökulhlaups can be categorized to different groups depending on their origin but these are connected to volacanic eruptions:

  • Jökulhlaups from subglacial lakes. Such lakes are generally formed due to geothermal activity at the base of a glacier. These settings are common in Iceland and they are the cause of a number of subglacial lakes, i.e. the eastern and western Skaftá ice cauldrons and Grímsvötn.
  • Jökulhlaups caused by volcanic eruptions underneath a glacier. Meltwater formed in subglacial eruptions often flows immediately away from the eruption site, but it can also accumulate around the site. Increased accumulation leads to the reservoir becoming unstable and the release of a jökulhlaup. Jökulhlaups from Eyjafjallajökull in 2010 are an example of this type of flood. In large eruptions, such as Katla eruptions underneath Mýrdalsjökull, ice melting can be very rapid and substantial in volume, resulting in large floods.
  • Jökulhlaups cause by melting due to hot pyroclastic flow onto snow and ice in explosive eruptions in stratovolcanoes. This type of floods usually starts as an avalanche of hot eruptive material that transforms into a rock flood that converts into a mud flood, as more water is entrained in the flow. This has happened during 20th century eruptions at Hekla.

Most jökulhlaups in Iceland are water floods, but it is believed that some flood events caused by Katla eruptions and Öræfajökull eruptions can be classified as mud flows.

The concept “jökulhlaup” is known worldwide among geoscientist and reflects the importance of the theory, which evolved from observations of floods in Iceland.

A Volcanic emissions:

A volcanic plume is a mixture of hot volcanic particles, water vapor, other magmatic gases and air injected in the atmosphere during an explosive eruption. As function of their size, some of the particles are initially moving upwards coupled with the gas stream, whereas the largest particles detach immediately from the main flow and follow a ballistic trajectory. The altitude that the plume can reach depends on several factors, primarily on the mass flux of the eruption. Other environmental factors, like wind field and atmospheric stratification, act on the plume dynamics affecting the final top altitude. Very strong wind can bend the rising mixture causing a lower top altitude and the maximum plume height to be located downwind the eruptive vent. The type and style of the eruption also affect the plume altitude as well as the size of the erupted material. Pyroclastic material in the plume is subjected to two main forces: the force of gravity and the drag of the rising gas stream. Small pyroclastic material will then be transported to the top of the plume, whereas larger particles will lose momentum quicker and will abandon the plume during its ascent. Only the finer fraction of the erupted material (what is called volcanic ash) will be transported by the wind within a volcanic ash cloud.

How far a volcanic ash cloud can be transported depends on the height of the eruption column, the size of the ash, the wind circulation and the efficacy of the removal processes. When the ash plume is within the troposphere, the first 9-12 km of the atmosphere at Icelandic latitudes, the ash tends to fall back to the ground relatively quickly (within hours up to days). At these heights the ash cloud can have some effects on the local weather without affecting the climate. If the plume reaches up to the stratosphere, which resumes after the troposphere and up to 50 km altitude, the ash will fall slowly back to the troposphere and can therefore be distributed over a large area, even the whole globe. In those instances, ash and volcanic aerosols can cause a temporary cooling effect by reflecting the incoming solar radiation, i.e. the volcanic cloud can have an impact on the climate. As an example, the eruption in Pinatubo in the Philippines occurred in 1991. In that eruption about 20 million tons of sulfur dioxide (SO2) was released into the stratosphere. The gas cloud was distributed around the world, and decreased sun radiation on the surface and because of that, caused a temporary cooling of 0.5°C around the world in 1991-1993.

Tephra impact on the ground

With the term tephra we mean all the pyroclastic material released during an explosive eruption that is injected into the atmosphere. Tephra include ballistic (everything > 64 mm in diameter), lapilli (2-64 mm) and ash (<2 mm) (Table 1). During an explosive eruption pyroclastic material of various sizes impact the ground. Large clasts up to few meters-size (bombs and ballistics) can land up to few km from the vent and can represent a serious hazard in the proximity of a volcano. Those particles that decouple quickly from the volcanic plume due to their size and weight fall close to the volcano and constitute the proximal deposit. Smaller particles can reach higher altitudes and persist in the atmosphere for days and weeks and be advected far away by the wind. This fraction of the pyroclastic material can possibly generate a distal deposit very thin and covering wide areas.

Term Size 
Ballistic d > 64 mm 
Lapilli 2 mm < d < 64 mm 
Ash d < 2 mm 
Fine ash  d < 0.063 mm 

Table 1 Terminology of pyroclast material and its size.

At ground level volcanic tephra can cause: 

  • Health issues; 

  • Roofs/building collapse; 

  • Poor visibility conditions; 

  • Dangerous road conditions; 

  • Contamination of water reservoirs and vegetation; 

  • Damages to electrical infrastructures; 

  • Transportation system disruptions; 

  • Impact on telecommunication networks.

Composition of the tephra, its grain-size distribution and presence of precipitation might enhance some of these hazards, e.g. roof collapse conditions, damages to electrical infrastructure and contamination of water and vegetation. Wet ash can reach higher load due to contribution of trapped rain in the ash deposit indicating different impact of tephra fall on buildings if it rains during the eruption or immediately after. Similarly, wet conditions might affect the conduction properties of ash enhancing its effect in flashover events. Finally, silicic ashes can have a toxic impact on water and grazing animals.

Instructions for preparedness before, after and during ashfall can be found on the webpage of the Icelandic Civil Protections.

Volcanic ash impact on the aviation

A volcanic cloud can reach the altitudes of air traffic routes and threaten the safety of aircrafts. If an airplane flies through a volcanic cloud, the volcanic ash can reduce significantly the visibility and damage airframes, especially aero-engines. This might imply loss of operability, reduction of engine performance and reduced component lifetime. The entity of the damage is function of several factors, including the exposure time to the ash, ash chemical properties and flight condition (climb, cruise and descent). Possible problems on the aircraft could be: 

  • Malfunction or failure in one or more engines leading to reduction or complete loss of thrust and failures of electrical, pneumatic and hydraulic systems. 

  • Disruption of pitot and static sensors resulting in unreliable airspeed indications and erroneous warning.  

  • The windscreen of the plane is rendered partially or completely opaque.  

  • Smoke, dust and/or toxic chemical contamination in the aircrafts cabin air requiring crew to use oxygen masks and therefore impacting communications. In this situation, electronic systems may be affected. 

  • Erosion of external and internal aircraft components. 

  • Effects of electronic cooling efficiency is reduced leading to a wide range of aircraft system failures.  

  • Volcanic ash deposit on a runway will degrade braking performance, most significantly if the ash is wet. This can lead to runway closure in extreme cases. 

This list is not exhaustive and other unusual occurrences can also develop.

Hazards posed by volcanic ash cloud to aviation is known since the 1950's and since then more than 120 encounters have been documented and reported. During the Eyjafjallajökull eruption in 2010 there were not dramatic reports of volcanic ash encounters, but it caused a great disruption of the air traffic all around the world because of ash dispersal over the Atlantic sea and Europe. In that occasion more than 100,000 flights were canceled with € 1.3 billion estimated loss of revenue for the airlines.

In the 1990's, to address the problem of hazard due to volcanic ash encounter the International Civil Aviation Organization (ICAO) established consultant centers around the world for advice regarding ash dispersal. These are the Volcanic Ash Advisory Centers (VAAC) and they are currently nine worldwide: Anchorage, Buenos Aires, Darwin, London, Montreal, Tokyo, Toulouse, Washington and Wellington. Each of these centers have the responsibility to advice about ash dispersal from volcanoes in fixed defined areas around the world. These centers monitor and produce information about the activity of volcanoes in their area of responsibility forecasting those areas possibly interested by ash contamination. 


Lightning are common during explosive volcanic eruptions. Processes in the ash plume can lead to electric charge separation similar to thunderstorms. Most volcanic lightning strike close to the eruption vent, but can also strike down tens of km from the eruption site. In the 1755 Katla eruption, two people were killed when hit by a volcanic lightning in Svínadalur in Skaftártunga, about 30-35 km from Katla.

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