Influence of Non-Newtonian rheology on magma degassing

Many volcanoes exhibit temporal changes in their degassing process, from rapid gas puffing to lava fountaining and long-lasting quiescent passive degassing periods. This range of behaviors has been explained in terms of changes in gas flux and/or magma input rate. We report here a simple laboratory experiment which shows that the non- Newtonian rheology of magma can be responsible, alone, for such intriguing behavior, even in a stationary gas flux regime. We inject a constant gas flow-rate Q at the bottom of a non-Newtonian fluid column, and demonstrate the existence of a critical flow rate Q* above which the system spontaneously alternates between a bubbling and a channeling regime, where a gas channel crosses the entire fluid column. The threshold Q* depends on the fluid rheological properties which are controlled, in particular, by the gas volume fraction (or void fraction) {\phi}. When {\phi} increases, Q* decreases and the degassing regime changes. Non-Newtonian properties of magma might therefore play a crucial role in volcanic eruption dynamics.

💡 Research Summary

The authors present a laboratory study that demonstrates how the non‑Newtonian rheology of a magma analog can alone generate the intermittent degassing behavior observed at many active volcanoes. A vertical column filled with a shear‑thinning, yield‑stress fluid (a diluted hair‑gel solution) is supplied with a constant air flow rate Q at its base. Pressure inside the gas reservoir is recorded continuously, allowing the identification of two distinct degassing regimes.

When the imposed flow rate is below a critical value Q* the system remains in a “bubbling” regime: individual bubbles nucleate at the nozzle, rise through the column, and burst at the free surface. The recorded over‑pressure shows a series of sharp rises and drops, each corresponding to the formation and release of a single bubble. In this regime the fluid’s yield strength traps small satellite bubbles generated by surface bursting, leading to a gradual increase in the void fraction φ within the column.

If Q exceeds Q*, the column spontaneously alternates between the bubbling regime and a “channeling” regime. In the latter a continuous gas channel, resembling a Saffman‑Taylor finger, grows upward from the nozzle to the free surface, allowing a quasi‑steady gas flux. The pressure signal in this state is low and only slowly varying, reflecting the pressure drop across the channel. The channel is not permanent; it collapses after a finite time, after which bubbling resumes, and the cycle repeats.

A key finding is that Q* is not a fixed property of the apparatus but depends sensitively on the rheology of the fluid, which in turn is controlled by the void fraction φ. As bubbles accumulate, both the apparent viscosity and the yield stress decrease, causing Q* to shift to lower values. Consequently, a system that initially operates below Q* can, simply by the progressive enrichment of bubbles, cross the threshold and enter the intermittent regime without any change in the imposed gas flux.

The experiments also reveal that the gas channel forms by a finger‑like instability rather than by bulk coalescence of bubbles, a mechanism that is promoted by shear‑thinning and reduced yield strength. The authors quantify the evolution of φ by measuring the column height increase and demonstrate that φ reaches a maximum near Q* during the first flow‑rate cycle; subsequent cycles show a saturated φ and a lower, stable Q* (denoted Q*₂).

By analogy, the authors argue that volcanic conduits filled with crystal‑laden, bubble‑rich magma could experience similar transitions. A volcano supplied with a constant gas flux Q that is initially below the magma’s critical flux would exhibit regular Strombolian bursts (bubbling regime). As the magma’s bubble content grows—through bubble nucleation, coalescence, or exsolution—the effective rheology softens, lowering the critical flux. Once Q exceeds the new Q*, the conduit could spontaneously switch to an intermittent state where a gas channel intermittently opens, allowing continuous gas escape (analogous to annular flow or sustained lava fountains) interspersed with discrete explosive bursts.

The study therefore provides experimental evidence that non‑Newtonian magma rheology alone can drive the observed alternation between explosive and quiescent degassing without invoking variations in magma supply or gas input. This insight complements previous theoretical work emphasizing the role of crystal content, temperature, and gas exsolution, and suggests that monitoring changes in magma rheology (e.g., via seismic velocity, tilt, or gas composition) could improve forecasts of transitions between eruptive styles.