Mount Agung — Bali's highest volcano — is what geologist call a stratovolcano.
Stratovolcanoes erupt infrequently, but when they do, they are usually violent. Mount Agung's last major eruption took place in 1963. With its recent activity causing it to spew out a massive ash cloud, experts think the volcano is on the brink of a much larger eruption.
People are being evacuated from the area surrounding the volcano. The eruption's effect, however, is much more widespread. Airline passengers face huge delays through cancelled flights and a closed airport, and explosive volcanoes in the tropics — like Mount Agung — have the potential to significantly affect all around the world due to atmospheric circulation patterns.
Explosive volcanic eruptions can send ash plumes and gases into the stratosphere. One of these gases, sulphur dioxide, can react with water to produce droplets that reflect solar energy back into space before it hits the earth. If the eruption is big enough, this could cause a reduction in the global average temperature over a period of years.
Satellites show that the sulphur dioxide in Agung's ash cloud is currently only halfway to the stratosphere, though one big eruption could easily increase that height. Not all volcanoes release sulphur dioxide, but Mount Agung is known to be filled with the gas — the 1963 eruption caused global temperatures to fall by 0.1-0.2 degrees Celsius over a year.
In addition to its potential climatic impact, a large eruption of Mount Agung has the potential to change our travel plans over the Christmas holiday period, with the extremely fine volcanic ash impacting air travel.
Sometimes 10 times thinner than a human hair, volcanic ash can readily be thrown up into the jet stream, reaching similar altitudes to some airliners. Small and lightweight, the ash particles can remain up in the stratosphere for years, carried across the globe by the wind. Flying through an ash cloud is extremely dangerous.
You might think the obvious solution would be for pilots to avoid flying into them — after all, photographs from Bali show the clouds thick black plumes. High altitude ash clouds, however, don't look like this at all. They are extremely difficult to spot visually and the particles are too small to be detected by radar making it hard for pilots to know where they are to avoid them.
The tiny, sharp particles within volcanic ash are mostly made up of silicates which have a melting temperature of 1100C. When an aircraft flies through the ash, the particles are exposed to the 1400C operating temperature of the jet engines and start to melt. As the molten liquid droplets pass through to the cooler parts of the engine, they re-solidify into a hard solid glass which sticks to and coats the surfaces of the engine.
This coating changes the airflow within the engine which can cause it to stall, or — in the case of the flight BA9, which flew through Mt Galunggung's ash cloud in 1982 — total engine failure in all four engines.
Ash can also affect many of the plane's sensors, cause accelerated corrosion of the aircraft, and reduce visibility by pitting the windscreens. Luckily, in the case of flight BA9, after descending some 8000m they were able to restart three engines and make an emergency landing.
The lessons learned from that experience though mean that air travel rules around areas of volcanic activity are much more strict — necessary and far safer, but disruptive to normal operations.
If the predicted major eruption doesn't happen, the ash clouds will disperse and life will return to normal until the next time this stratovolcano decides to wake up and show its power.