The bubbles cause the magma to froth violently and burst out as a volcanic eruption. When the boiling fragments of liquid magma hit the cold air they freeze into individual dust particles, driven upwards towards the high atmosphere by the power and heat of the eruption.
As the turbulent torrent rushes out it rips and pulverises rocks from the volcano, adding to the growing cloud of ash.
“Think about when you cough. Not all of the material that comes out is from lower down, some is dragged from further up as well,” says Colin Macpherson, a volcano expert at Durham University.
Even if it does spread, the high-altitude ash cloud is unlikely to settle on the ground or even be visible from it. The curious are advised to wait until sunset, when they could be in for a treat. The retreating light could be scattered as it bounces from the shiny dust particles, which can enhance the red colour and bring stunning displays. The gigantic eruption of Mt Krakatoa in Indonesia in 1883, the loudest recorded bang in history, threw out so much ash that spectacular sunsets were recorded for years afterwards.
The dust can also help reduce global warming. The effect of the Iceland ash cloud will be small but larger eruptions help to cool the planet as they reflect sunlight back into space. The 1815 eruption of Mt Tambora in Indonesia produced so much volcanic ash that it triggered the “year without a summer” that brought widespread failure of harvests across Europe, famine and economic collapse.