What Is Volcanic Ash Made Of?
But exactly what is volcanic ash made of and why is it so dangerous to air traffic? Volcanic ash and debris are known as tephra. Small, irregular shaped fragments of rocks, minerals and volcanic glasses are what volcanic ash is made of. The range of particle sizes associated with an eruption is very wide from large boulders to tiny fragments of volcanic glasses as small as 0.001 mm in diameter. Volcanic ash itself is inorganic in nature and insoluble in water. Ash particles are highly abrasive and can conduct an electric current when wet. Due to the very small particle size and the heat of an eruption, ash particles can be lifted high into the atmosphere and may travel many thousands of kilometres from their source before falling back to earth. Ash is transported by high altitude winds.
The glass fragments found in ash are due to gas bubbles being released into the magma of the eruption as it wells up from deep underground. As the magma cools and solidifies, these gas bubbles are trapped. During the eruption process, the gas inside these particles becomes heated to the point where solidified mass explodes, creating glass shards. If the magma comes into contact with water during the eruption (a phreatomagmatic eruption), a violent explosive reaction will occur. This produces ash fragments with angular shapes which are particularly abrasive (therefore more dangerous to aviation). The eruption of Eyjafjallajökul took place through a glaciated ice sheet and therefore was a phreatomagmatic eruption.
Whilst volcanic ash itself is insoluble, during its formation, ash particles can absorb soluble material onto their surfaces. This happens when the ash particles interact with gases contained within the magma (sulphur and halide compounds) and means that the ash particles become corrosive due to the formation of sulphuric and hydrochloric acid on their surfaces when they become hydrated. This explains why acid rain is encountered during an eruption.
So, why are ash clouds and aviation so incompatible? The answer to this question relates to the physico-chemical properties of volcanic ash and just what volcanic ash is made of. As we have seen, ash may be acidic and so can cause chemical corrosion to parts of an aircraft. However, the major problem is that ash particles will melt at the temperatures they encounter when sucked into a jet engine. The molten material may choke the cooling vents on the turbo fans leading to engine failure. Since ash plumes cannot be detected on an aircraft’s radar, the prudent course of action is to close airspace where there is a significant probability of an aircraft encountering a volcanic ash plume. Fortunately, ash plumes can be detected by satellites, so aviation can be protected from the dangers of flying through them.