Volcanic eruption gases
Volcanic eruptions can emit large quantities of rock fragments and fine particles (ash) into the atmosphere, as well as several gases, such as:
- Water vapour
Gas rises high and over very large distances
The rock fragments usually fall back to Earth quite quickly. The ash and the gases, however, can rise high up into the troposphere and even reach the lower stratosphere, up to 15 or even 20 km. The elevation reached by the material depends on the strength of the volcanic eruption, which in turn depends on the kind of volcano that erupts.
Air traffic dangers
The ash emitted by volcanic eruptions is a major hazard to aviation.
The ash can:
- severely damage the material of the aircraft
- clog its sensors
- limit the view of its pilots
- severely scratch ("sandblast") the windows of the aircraft
And when it enters the aircraft's engines, the ash can melt (it has a melting point of about 1100°C), as a result of which the engine may fail.
Ash clouds in the lower stratosphere can be transported over very large distances, and thus pose hazards many days after an eruption. The ash emitted during the eruption of the Pinatubo volcano (1991) has damaged aircraft as far away from the volcano as 1000 km.
Dangerous volcanic eruption gases
Of the gases emitted during a volcano eruption, sulphur dioxide (SO2) is in itself also a hazard to aircraft, as SO2 reacts with water vapour to form sulphuric acid (H2SO4), which is corrosive and can therefore
- scratch the paint and the windows of the aircraft
- create sulphate deposits in the engines
Depending on the kind of eruption, the SO2 may be inside the ash cloud, but it may also be above or below the ash cloud. In general the ash will drop due to gravity effects faster than the SO2, so that some distance away from the volcano the ash and SO2 clouds may be separated.
From all these considerations it is clear that the safest procedure for aircraft is to stay clear of volcanic clouds. Pilots cannot always see an ash cloud (e.g. at night) and the ash does not show up on radar. And SO2 and H2SO4 are colourless gases, and therefore invisible.
If it penetrates into the aircraft; sulphuric acid is noticed easily because of its strong smell, but then the aircraft is already inside the cloud. Hence, it is of major importance to know in advance where volcanic clouds are and what elevation they reach.
Complete article: Hazard to aviation of volcanic eruptions