Volcanic plumes and tephra

Volcanic plume from the Grímsvötn erutpion in 1998. Photo taken on the 18 of December 1998 by Oddur Sigurðsson

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.

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.