Coffee in a very hot cup, please…
Like a dormant volcano, there are many people who simply cannot start the day without a coffee. Surprisingly, the physics of coffee and lava are quite similar — at least when it comes to bubbles.
A team of scientists discovered that the formation of bubbles in magma ascending follows some of the same physical principles as bubbles in a stirred coffee or a glass of champagne.
This discovery expands the traditional model of volcanic bubble formation, which has guided volcanology since the 1950s, and could improve the eruption forecast volcanic.
In their study, researchers from Université Clermont Auvergne and ETH Zurich showed that, in gas-saturated magma, the simple physical act of flowing and swirling through underground channels can trigger the formation of bubbles due to mechanical stress — even without a further decrease in pressure.
The results of the , published at the beginning of the month in Scienceshow for the first time that the shear-induced nucleation occurs when viscous liquids undergo mechanical forces resulting from the movement itself.
Essentially, It’s the same physics in action. When magma rises through volcanic conduits, the liquid is subjected to intense shear forces when flowing along rock walls and around obstacles.
These mechanical forces provide sufficient energy to trigger the formation of bubbles in a magma already loaded with dissolved gasessuch as water vapor and carbon dioxide — just like when you stir a fizzy drink and bubbles form.
When the rotation creates the eruption
The discovery arose from laboratory experiments using a heated polymeric liquid saturated with carbon dioxide, explains .
The team placed the liquid on a rotating platform inside a rheometer, a device that precisely controls and measures viscous flow. As the rotation speed gradually increased, bubbles began to appear in the outer regions, where shear stress was highest.
They then formed various types of blisters at different stress levels — some in the liquid itself, others on solid surfaces or near bubbles that have already formed. The pattern was consistent: the greater the initial carbon dioxide saturation, the lower the shear stress required to initiate nucleation.
What makes this discovery remarkable it is its deviation from the traditional focus. Since the 1950s, volcanologists have mainly explained the formation of bubbles based on the decrease in pressure: as the magma rises to the surface, the pressure reduces and the dissolved gases are released, forming bubbles.
Scientists have used this model to interpret the textures of volcanic rocks and estimate the speed of ascension of magma before the eruption. But the new experiments reveal that the mechanical energy of moving magma also contributes substantially to the formation of bubbles.
Researchers have developed a mathematical model that shows that shear stress and pressure changes contribute almost equally to overcome the energy barrier necessary for bubble nucleation in your experimental system.
The practical consequences are substantial. The formation and growth of bubbles fundamentally control the behavior of magma during eruptions. More bubbles mean lower density, which affects the rise of magma.
Bubbles too drastically alter the viscosity of magma and determine whether gases escape gradually or build up to explosive levels.
The Obsidian Enigma
This discovery may help explain an ancient enigma of volcanology. Some very viscous, gas-rich magmas somehow manage to erupt smoothly in the form of obsidian flows, instead of exploding catastrophically.
Efficient shear bubble formation at the bottom of the conduit, followed by the growth and coalescence of these bubbles, may allow these volatile-rich magmas to outgas before reaching depths of explosive fragmentation.
The investigation also questions the role of tiny crystallites called nanoliths in the formation of bubbles. Recent studies have proposed that these iron oxide crystals, less than a micrometer in size, provide nucleation sites for bubbles.
But if shear alone can trigger the widespread formation of bubbles in conduits, nanoliths may be less essential than previously thought or even formed as a result of the bubbles, instead of originating them.
Although the experiments used a polymer and carbon dioxide system rather than real magma, the researchers carefully adjusted their results to volcanic conditions. The supersaturation pressures in their tests correspond to realistic values for magmas at depths of 2 to 10 km.
Beyond volcanoes, shear-induced nucleation should influence other natural and industrial processes. The phenomenon is, for example, known to affect the formation of foam in synthetic materials.
It may also play a role in phenomena such as the formation of bubbles triggered by earthquakes, release of gases on planets and the thermal expulsion of volatiles in hydrothermal systems.
