By Phil Shane

Why do volcanoes erupt and can we forecast future activity? Phil Shane explains the science behind volcanoes.

One of the grand challenges of volcano science is understanding the behaviour of a volcano, and whether can we forecast the likelihood of future eruptions. Despite decades of high technology monitoring, and even longer historic records for some volcanoes, there is currently no single model that explains the behaviour of a particular volcano. Indeed, volcanoes that are in close proximity may exhibit a completely different history of lava and ash eruptions, such as Ngāuruhoe and Red Crater at Tongariro, New Zealand, which are just a few kilometres apart.

For most volcanoes, magma (molten rock) originates in Earth’s mantle. However, the composition and character of what gets erupted is strongly controlled by the torturous pathway (the conduit) to the surface. This is a clue to why volcanic activity is so hard to predict. Magmas generated in the mantle may raise due to buoyancy. Thus, ascent will be controlled by their temperature and volatile (gas) content. However, during ascent, the magma may start to crystallise, and will stall at depths of 20-25 km in the lower and middle crust, or at various depths. This is due to changes in viscosity that prevents flow and because the magma may reach neutral buoyancy with its surroundings. Magma is a mixture of hot liquid, gas and solid. The gases segregate during ascent, and can travel to different parts of the system, leaving behind a magma with changed rheological properties (ability to flow). Most volcanoes have been active for thousands of years. Thus, magmas have incrementally ascended and stalled in their subterranean systems. This phenomenon alters the thermal and tensile strength of the surrounding crust which channels the magma flow. As a result new magmas encounter older cooling magma bodies in the crust. Magma mixing can occur, and older magmas can be thermally re-activated. All of these factors make it difficult to identify precursors to pending unrest or to model ascent from geophysical data collected during unrest.

Seismic, gravity and electrical field surveys of the deep and shallow plumbing beneath current active and dormant volcanoes around the world have failed to identify sizeable bodies of molten magma stored in the crust. This is an enigma considering many volcanoes erupt vast quantities of material (0.01 to 1000 cubic kilometres of rock) sometimes in the space of days to a few years. Instead, geophysical data collected from beneath large volcanoes such as Yellowstone (Western US) and Toba (Sumatra) reveal bodies of hot rock that contain only a few percent to perhaps ten percent melt. Beneath Toba, these small melt bodies occur as a series of stacked lenses through the crust. This, along with geochemical and mineralogical evidence, suggests to most geologists that magma bodies are in fact crystal mush zones – bodies that are mostly crystal and the melt occurs as a thin film around crystal surfaces. These mush zones are narrow, but vertically extensive. Being mixtures of solids, liquids and gases that change due to segregate with time, there are critical points in space and time when a magma will be more prone to eruption. The challenge is for geologists to recognise the state of the system that is most likely to tip toward a new eruption.

Even when a magma body is prone to erupt, what is the final triggering/tipping point or cause? There have been many historic eruptions that are apparently triggered by new magma from the mantle entering a crustal reservoir of older magma, and thermally triggering it back to life. Examples include Pinatubo (Philippines) and Montserrat (Caribbean). In such cases, the erupted rock contains microscopic geochemical evidence of two magmas mixing, and real-time seismic data tracks the rise of new magma entering the system. However, sometimes there is a lack of good evidence for magma mixing or it is uncertain (e.g. Mt St Helens, USA). Recent studies by researchers at the University of Auckland suggest that some eruptions lag behind the entry and mixing of new magma to the shallow reservoir. It is likely that crystallisation of the magma expels volatiles into the remaining melt, explosively triggering it to erupt. If correct, such phenomena could be difficult to detect by seismic and gas monitoring because it may not cause a significant signal. Furthermore, other eruptions are not connected to the arrival of new magma. Gases that easily segregate from the magma will collect in sealed conduits beneath the vent. Ultimately, there will be an explosion once the strength of the surrounding rocks is overcome. It is possible that the December 2019 event at White Island was this type of eruption.

A compounding factor in forecasting eruptions is that for most volcanoes long periods of dormancy can last decades to tens of thousands of years while unrest can represent a relatively short event lasting days/months to perhaps a decade. In many cases last century, the biggest eruptions on Earth have occurred at volcanoes that had no record of prior historic activity (e.g., Pinatubo), and indeed local people were unaware that they were even volcanoes!  Hence, years to a few decades of monitoring with advanced technology may not be a representative guide to how a volcano behaves.

Despite the lack of long-term monitoring databases, and the complexity of how magma behaves as it rises to the surface, increased technological surveillance, including of remote regions via satellite imaging, is leading to better mitigation of potential hazards associated with volcanoes. It is unlikely that risk will ever be completely eliminated because some volcanic phenomena have such rapid onset times (seconds to minutes). Also, the planet’s increasing human population is pushing settlements into areas unsuitable for long-term habitation close to volcanoes.

Phil Shane is an Associate Professor in the Environment at the University of Auckland. He is an expert in volcanos and volcano hazards. 

Disclaimer: The ideas expressed in this article reflect the author’s views and not necessarily the views of The Big Q. 

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