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Tuesday, September 21, 2021

Decoding the triggers of volcanic eruptions

A lot depends not only on the speed at which magma accumulates in the volcanic reservoir but also on the conditions in which it does.

Written by Ritvik Chaturvedi | Bengaluru |
Updated: July 30, 2021 7:49:15 pm
Volcano eruptionA major internal eruption trigger is the ‘injection’ of magma from deeper reservoirs to shallower depths (Wikimedia Commons)

A vast majority of people continue to be vulnerable to volcanic systems due to their proximity. It is, therefore, important to understand the drivers of volcanic activity in order to better predict eruptions and develop mitigation strategies in the future. To this end, a recent article in Nature Reviews – Earth and Environment examines the various factors governing magmatic eruption.

Satellite-based remote sensing or even ground-based methods can only reveal limited information. Volcanologists often have to turn to the chemistry and texture of volcanic rocks to get an understanding of magmatic development.

A lot depends not only on the speed at which magma accumulates in the volcanic reservoir but also on the conditions in which it does. Authors note that although the likelihood of a volcanic eruption increases with the accumulation of eruptible magma, it can also decrease over time, as volatile substances are eliminated from the magma, allowing it to compress further. The continued build-up of volatile-rich magma pushes the system towards critical conditions ultimately causing an eruption.

Magma matters

A major internal eruption trigger is the ‘injection’ of magma from deeper reservoirs to shallower depths, a phenomenon also known as magma reservoir failure. An earlier 2012 study put forward the same reason for the Bronze Age (~1600 BCE) Eruption in Santorini, Greece.

Another potential eruption trigger is the rapid cooling and crystallisation of magma. This leads to the flushing out of the low-density volatile phase. This increases pressure in the magma chamber and is a veritable trigger. The eruptions in Calbuco, Chile (2015) and Kelud, Indonesia (2014) are cases in point.

Role of climate change

There are external equally important stressors that can trigger an eruption. Climate change being a notable example. During an interglacial period i.e. when glaciers melt, rocks get transported with the glacial melt. This leads to a short-term depressurisation, thereby altering magmatic and volcanic activity. Similarly, sea-level rise can also influence magmatic activity.

The authors of the study note the strong correlation between volcanic eruptions occurring after an earthquake, even if not immediately. It has been argued that earthquakes can lead to the rapid degassing of iron-rich magmas. Indeed, volcanic eruptions too can induce seismic activity.

Rain trouble

Additionally, since the host rock of any magma is porous in nature, and gas/magma transport depends a lot on the host rock, rainfall is a potent influencer of volcanic activities by changing pore pressure. Historical observations have noted a strong correlation between an increase in the frequency of eruptions and rainfall. A notable example is that of the Kilauea volcano, Hawaii, USA, where record-breaking levels of rainfall in 2018 created pathways for the magma to rise to the volcano orifice.

Rainfall also influences the hydrothermal pressurisation of domes, which leads to mineral collapse and eventually dome collapse. Rain can also help reduce pressure in the lava chamber, causing it to explode. This factor has been considered responsible for the eruptions at Soufrière Hills, Montserrat; Unzen, Japan; Merapi volcano, Indonesia and Mount St. Helens, USA.

Ultimately, all the aforementioned factors notwithstanding, a body of magma that has reached critical pressure has to ascend to the mouth of the volcano in order for an eruption to occur.

The study assesses a few major mechanisms by which this happens, other than just the properties of the magma that we discussed above.

  • * The magma fluid can fracture surrounding brittle crust, which then enables its transport to the surface. Beyond a certain stress point, magma transport can further widen the fractures surrounding the magma chamber walls.
  • * The volcanic structure: A large volcano can tend to further compress the underlying rock and tends to trap magma reservoirs at depth, preventing a further eruption.
  • * Unlike often assumed, the magma reservoir does have a neat cylindrical outlet to the top. Depending on the pressure above, the distribution of the magma vents changes fairly frequently, even after a few months.

The authors suggest that since geophysical observations often mask the evolution of a volcanic system, there is a poor understanding of magmatic behaviour. It is often impossible to study magma reservoirs. In the long term, the study hopes that better, multi-proxy, resolution underpinnings of magma build-up and transport will lead to the development of more accurate and well-constrained eruption models.

“Knowing whether or not to evacuate the population is crucial and we hope that our study will contribute to decreasing the impact of volcanic activity on our society,” lead author Luca Caricchi said in a release.

The author is a freelance science communicator. (mail@ritvikc.com)

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