Why Bali's Mount Agung's volcanic ash is a particular problem for airlines

The threat posed by a particular volcano's ash depends on what it is made of

The eruption of Mount Agung on the island of Bali in Indonesia has emitted a huge plume of volcanic ash over the region, reaching more than 9km up into the atmosphere. This has disrupted flights over Bali and nearby islands. With the Aviation Colour Code listed as red (the most dangerous), air passengers are once again being stranded, just as many were following the 2010 Eyjafjallajökull eruption in Iceland, or during the Puyehue-Cordón eruption in Chile the following year.

The volcano is still erupting. This means it could become