Why Some Volcanoes Don't Explode

The explosiveness of a volcanic eruption depends on how many gas bubbles form in the magma - and when. Until now, it was thought that gas bubbles were formed primarily when the ambient pressure dropped while the magma was rising. Gases that were dissolved in the magma in lower strata - due to the higher pressure - escape when the pressure drops and form bubbles. The more bubbles there are in the magma, the lighter it becomes and the faster it rises. This can cause the magma to tear apart, leading to an explosive eruption.

This process can be likened to a bottle of champagne: while the bottle is closed and therefore pressurised, the carbon dioxide remains in solution. When the cork is removed from the bottle, the pressure drops and the carbon dioxide forms bubbles. These bubbles draw the liquid upwards with them and cause it to spray out of the bottle explosively.

However, this explanation is incomplete - because the lava from some volcanoes, such as Mount St. Helens in the state of Washington, USA, or the Chilean volcano Quizapu, has sometimes flowed out gently despite the presence of highly explosive magma with a high gas content. Now, an international research team including a scientist from ETH Zurich has provided a new explanation for this riddle, which has puzzled volcanologists for a long time.

Shear as a new factor

In a recent article in the journal external page Science , the researchers show that gas bubbles can form in the rising magma not only due to a drop in pressure but also due to shear forces. If these gas bubbles grow deep in the volcanic conduit, they can combine with one another and therefore form degassing channels. Gas can then escape at an early stage, and the magma flows out calmly.

We can imagine the shear forces in the magma as being like stirring a jar of honey: the honey moves faster where it is being stirred with the spoon. At the edge of the jar, where the friction is higher, it moves slower. A similar process is taking place in volcanic conduits: the magma moves more slowly at the edge of the conduit, where the friction is greatest, than it does in the interior. This essentially "kneads" the molten rock, producing bubbles of gas.

"Our experiments showed that the movement in the magma due to shear forces is sufficient to form gas bubbles - even without a drop in pressure," explains Olivier Bachmann, Professor of Volcanology and Magmatic Petrology at ETH Zurich and one of the co-authors. The researchers' experiments show that bubbles are formed primarily near the edges of teh conduit, where the shear forces are strongest. Existing bubbles further strengthen this effect. "The more gas the magma contains, the less shear is needed for bubble formation and bubble growth," says Bachmann.

Why explosive volcanoes sometimes don't explode

According to the new findings, magma with a low gas content that seems not to be explosive could nevertheless lead to a powerful explosion if a large number of bubbles form due to pronounced shear and the magma therefore shoots upwards quickly.

Conversely, shear forces can also cause bubbles to develop and combine at an early stage in gas-rich and potentially explosive magma, leading to the formation of degassing channels in the magma that bring the gas pressure down. "We can therefore explain why some viscous magmas flow out gently instead of exploding, despite their high gas content - a riddle that's been puzzling us for a long time," says Bachmann.

One example is the eruption of Mount St. Helens in 1980. Although the magma was gas-rich and therefore potentially explosive, the eruption began with the emplacement of a very slow lava flow inside the volcanic cone. The strong shear forces acting on the magma produced additional gas bubbles that initially allowed a release of gas. It was only when a landslide opened the volcanic vent further and there was a rapid drop in pressure that the volcano exploded. The study's results suggest that many volcanoes with viscous magma allow gases to escape more efficiently than previously thought.

Special laboratory experiment

In order to visualise the processes inside a volcano, the researchers developed a special experiment: they took a viscous liquid resembling molten rock and saturated it with carbon dioxide gas.

Then they observed what happened if the lava-like liquid was set in motion by shear forces. As soon as the shear forces exceeded a certain threshold, gas bubbles suddenly formed in the liquid. The higher the initial gas supersaturation, the less shear was needed to form further gas bubbles. The researchers also found that the presence of existing bubbles favoured the formation of further bubbles in their immediate environment.

The researchers combined these observations with computer simulations of volcanic eruptions. By doing so, they showed that the effect is particularly likely to occur in areas where viscous magma flows along the walls of a conduit and therefore experiences strong shear forces.

With their work, the researchers provide a vital new piece of the puzzle when it comes to better understanding processes taking place inside active volcanoes and more precisely assessing how volcanoes will erupt. "In order to better predict the hazard potential of volcanoes, we need to update our volcano models and take shear forces in conduits into account," says study co-author Bachmann.