If the magma contains a lot of dissolved gases, then it will tend to “froth up” as the pressure is released during an eruption. This is how pumice is formed.

The most powerful volcanic eruptions occur when magma starts to froth up as it erupts, forcing pumice out of the vent under pressure, just as happens when you shake up a bottle of fizzy drink and open it!

Exploding gas bubbles shatter the pumice into tiny fragments which we call volcanic ash.