spaceSpace and Physics

Pressure Drop Unleashes Eruptions


Stephen Luntz


Stephen Luntz

Freelance Writer

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

28 Pressure Drop Unleashes Eruptions
Erosion can reveal the complex of magma dykes and sills that lead to volcanic eruptions / Dr Janine Kavanagh

A laboratory model of a volcano made of jello has given vulcanologists a new clue as to what causes eruptions, potentially leading to better warning systems for those in danger zones.

Professor Sandy Cruden of Monash University, Australia, points out that volcanoes can be hard to study. They are often in inaccessible locations and obscured by volcanic ash. There's also the whole thing about possibly getting killed.


Cruden decided on a safer option. “We studied the plumbing systems of volcanoes by modeling how magma ascends from great depths to the surface through a series of connected fractures (called dykes and sills)," he says. "To do this, we used a tank filled with gelatine (jelly) into which we injected colored water to mimic ascending magma.”

In case you think this sounds altogether too much like something you could do at high school, Cruden and colleagues used a high-speed camera and synchronized laser to give them an exceptionally detailed view of what happens as the magma/jelly rises.

The water was mixed with fluorescent particles, which, when lit up by the laser, could be tracked with unprecedented precision.

Water and magma may not seem alike, but Cruden tells IFLS that given the small scale, water's lower viscosity, and the softness of gelatine compared to rocks, it is appropriate. The gelatine was constructed with two layers, modeling the common circumstances of a stiffer layer of surface rocks sitting above more elastic materials.


The model shows a dyke of rising magma cutting through the rocks until it hits the boundary between the two layers. It then runs roughly horizontally along the boundary, forming a sill. Cruden says this is a well established behavior observed in many volcanic hotspots, and visible in the geology of formerly volcanic provinces.

A vertical dyke of simulated magma rises until it reaches the point where rock's elasticity changes / J. Kavanagh et al.

Upon reaching the boundary, the magma runs sideways, creating a sill, which leads to a loss of pressure in the dyke, potentially causing an eruption. Credit: J. Kavanagh et al.

What is new in this study, reported in Earth and Planetary Science Letters, is the observation that as the sill forms, pressure in the dyke drops suddenly. This, Cruden says, explains much of what we have been witnessing.


“A pressure drop can drive the release of dissolved gases, potentially causing the magma to explode and erupt," he says. "It's similar to removing a cap from a bottle of shaken soda – the pressure drop causes bubbles to form and the associated increase in volume results in a fountain of foam erupting from the bottle."

Cruden describes the discovery as, “completely unexpected, but we think we now know why it happens. As the magma starts moving along the horizontal sill it sucks fluid out of the vertical dyke.”

“In our paper we speculate that the pressure drop can be responsible for triggering eruptions,” Cruden says. “The next challenge is to test this idea to look at data for evidence of sill forming being connected to eruptions in the field.” If the theory turns out to be correct, better volcano alerts will follow, with eruptions less likely to take us by surprise.


spaceSpace and Physics
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