Big Idea 6 : Growth and movements of bubbles drives eruptions

Bubbles appear late in the magma ascent, after magma saturation due to pressure release and crystallization. They tend to change melt properties, specially viscosity and density, which induce the magma to stiffen and rise. The depressurization during the magma ascent can build an overpressure inside the bubbles and leads to explosive eruption.

Different scenarios are possible, notably depending on bubbles mobility compared to the melt. They can rise faster through a permeable foam, inducing a passive degassing or be trapped because of a high viscosity and rise at the same velocity as the melt.

Because a given volcano may have all eruptive styles with the same magma, the study of bubbles and their influence is really important to better understand how volcanoes works.

Magma contains dissolved volatiles (predominantly water, CO2, SO2). Upon ascent-driven decompression, volatile solubility decreases and magma becomes supersaturated in volatiles. Gas bubbles nucleate at the exsolution surface. Volatiles continue to exsolve and bubbles grow during magma ascent above the exsolution surface. Gas may be lost from the magma by permeable gas flow through porous and/or fractured magma. Gas may escape radially into conduit walls or vertically through the magma column to the atmosphere. During explosive eruptions, bubble walls rupture catastrophically at the fragmentation surface. The released gas expands rapidly and magma ascent changes from a viscous melt with suspended bubbles to a gas flow with suspended magma fragments.
Helge M. Gonnermann and Michael Manga

Subideas

6.1 Evidences of volatiles dissolved in the melt

6.2 The volatiles exsolve and escape from magma as volcanic gases

6.3 Bubbles nucleation appears after oversaturation of the melt

6.4 The presence of crystals within the magma may induce a different eruption style

6.5 Bubbles lead magma to ascent within the conduit

6.6 Bubbles induce an increase of the melt viscosity

6.7 The decompression rate influences bubbles number and size

6.8 Passive degassing is linked to the kind of flow during the ascension

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