Infographic Soil & geology

Unwin, Holly E. // Can tuffisites lower a volcano’s stress levels?


This infographic is aimed at geologists, who will understand technical terms, but do not necessarily have a background in volcanology. It shows how tuffisites are formed, their potential role in moderating the explosivity of an eruption, and what they might be able to tell us about how fluid pressure evolves inside a volcano in the run up to an eruption.


As the global population and demand for natural resources increase, volcanoes are threatening more lives than ever before. An explosive volcanic eruption may produce a large plume of gas and ash that can spread around the globe, while the impacts of effusive eruptions are more focussed in the area directly surrounding a volcano. These different eruption styles carry different hazards, and so it is important to be able to predict how explosive an eruption will be.

Volcanic eruptions are driven by gas pressure, and how high the gas pressure is determines whether an eruption will be explosive or effusive. Volcanoes may have “escape valves” that can relieve this gas pressure, causing an eruption to be less explosive. These “escape valves” are fractures that act as pathways through which the gas can escape. If filled with ash and clasts of the surrounding rock, the “escape valves” can be preserved as tuffisites, found in the remains of old eroded volcanoes. By visiting old volcanoes and studying tuffisites, we can learn more about the processes occurring inside volcanoes, refining eruption models and improving volcanic hazard prediction.

How do tuffisites form?

Tuffisites form when the fluid pressure is great enough to break open the surrounding rock, producing a fracture. This acts as a degassing pathway, allowing volcanic gases to escape and diffusing the pressure inside a volcano. The gas carries ash and other rock clasts into the fracture where it deposits them, propping the fracture open and allowing gas to continue to escape. The hot particles inside the fracture can weld together, like grains of sand forming glass, gradually sealing the fracture and preventing further gas flow.

The big question is: Do tuffisites allow gas to escape for long enough to relieve enough pressure to moderate eruption style?

What are we working on?

By examining tuffisites preserved in Iceland and the UK we can start to unravel some of the processes occurring inside volcanoes. The ash and rock clasts inside tuffisites are deposited in a variety of different units and structures that record how the fluid pressure varied during tuffisite formation. The characteristics of different particles inside the tuffisite help us to determine how efficiently gas might have been able to escape, and for how long.

Holly E. Unwin

Holly E. Unwin

Holly is a 2nd year PhD student based at Lancaster University and the British Geological Survey researching tuffisites. Her research combines fieldwork in Iceland and the UK with modelling and experiments to characterise different fractures and unravel the processes that might have formed them. Holly loves exploring the outdoors and planning her next big adventure.

Holly’s Envision profile


Twitter: @hollyeunwin

Organisation: Lancaster University and the British Geological Survey