6.9

6.9 The violence of eruption is due to an accumulation of potential energy within the bubbles

Bubble development can cause explosive eruptions for viscous and volatile-rich magmas. A viscous magma resists both the growth and the movement of bubbles. If bubbles rise with the magma in the conduit, there is a decompression, but because the bubbles can’t grow because of viscous resistance, an over-pressure develops inside bubbles. When this over-pressure strengthens, bubbles may burst, forming a gas-pyroclast mixture which is ejected to the atmosphere. This is magma fragmentation.

The Mount St. Helens eruption in 1980 is a good example of very violent explosion. The material ejected to the atmosphere reached 20 km height.

One of the most important thing in this big idea is to understand that all processes and magma properties inside the magma chamber and the conduit interact and are coupled : nucleation, growth, coalescence, fragmentation, crystallization, volatiles content, viscosity, decompression rate, rheology (volume fraction and size of bubbles, volume fraction and shape of crystals).