Why do volcanoes eject ash?

Answer 1

It is a common misconception that there IS always ash. In some volcanoes, such as Hawaii, barely any ash is generated due to eruption.

It all depends on the type of magma. Mafic magma tends to be non-viscous (quite runny, as magma goes), and so will flow freely. Because of this, gases can escape easily and so little pressure builds up.

This is important as ash is essentially pulverised rock that is thrown up due to large-scale explosive eruptions.

In intermediate-felsic magmas (or lavas), the viscosity (I may have just made that word up), is greater, and felsic lavas in particular are very viscous, due to their relatively low temperature compared more mafic magmas. Therefore, gases find it harder to escape the magma, especially as it cools and hardens in the volcano itself, and rarely flows down the mountainside.

Therefore, pressure begins to build due to all the trapped gases. This pressure, when great enough, causes the magma above it, as well as rocks that make up the volcano itself, to blast outwards in an explosive eruption. This generates large volumes of pulverized rock (ash) and small rock fragments.

Because this ash is light enough to remain airbourne, it can stay in the atmosphere for months and even years after the initial eruption. Some eruptions from these strato-volcanoes in the past have caused widespread global cooling, leading to crop failure, and this is even one of the theories for the extinction of the dinosaurs (except dinosaurs didn't have crops, but you know what I mean).

Answer 2

Volcanic ash forms during explosive volcanic eruptions in two ways. Some magma contains a significant amounts of gas dissolved in it under high pressure. As the magma approaches the surface the pressure decreases and the gas forms bubbles. These bubbles can expand rapidly and blast the magma into tiny fragments. Ash can also form when explosions within a volcano pulverize the surrounding rock.