Explosivity of volcanic eruptions: current assessment methods

Explosivity of volcanic eruptions: current assessment methods

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Assessing the level of explosiveness of a volcanic eruption is a real challenge. But what are the parameters that are currently used in this regard?

Incandescent fireballs that are violently fired into the air, gigantic clouds of ash rising into the sky, unstoppable fiery avalanches that descend from the flanks of a volcano, this set of words leads to imagine an explosive volcanic eruption, such as could Vesuvius. If you turn your gaze to Stromboli, however, things change, you can always notice an explosive type of activity, but "how explosive"? Why are some volcanoes characterized by very violent explosions while others by small explosions and still others by lava flows alone? Currently some parameters are being used to clarify this question and, only recently, in addition to high improvements in the prediction and evaluation of the explosiveness of volcanic eruptions, new methods are starting to be considered.

Volcanoes are not all the same, some have explosive and others effusive activity, the latter characterized only by the emission of lava flows and / or degassing. In general, in explosive volcanism we distinguish: volcanoes with Strombolian activity, characterized by small explosions and lava fountains; Vulcan activity, given by violent explosions with launches of pyroclasts of modest size even at a great distance from the crater; plinian activity, large explosive eruptions with a column of smoke and ash that can extend up to the stratosphere and generate violent pyroclastic flows; ultraplinian activity, linked to catastrophic eruptions and immense proportions of lava material. Non-explosive eruptions are instead defined as Hawaiian. An example of a volcano with Plinian activity is given by Vesuvius, while an example of Hawaiian-type volcanism can be observed at Kīlauea (Hawaii). There is, however, an index to calculate the "power" of a volcanic eruption, the Volcanic Explosivity Index (VEI), which ranges from 0 (for effusive eruptions) to, theoretically, infinite depending on how explosive and large a volcanic apparatus.

What, however, could immediately leave you perplexed is the fact that the same volcano behaves in a completely different way, even within a few days. This can be explained by important physico-chemical parameters that are directly related to the degree of explosiveness that a volcano can present. The main parameters are: volatile content and viscosity. Lately, the chemical interaction between magma and water and the phenomenon of magma mixing are also starting to be considered, as two potential triggers of explosive eruptions.

Volatiles consist mainly of H2O and CO2 (water and carbon dioxide) and, to a lesser extent, CO, SO2, H2S, H2, S and O, dissolved in molecular solution in magma. However, the volatiles represent only one of the three components of a magma, the other two are given by a liquid part, with a temperature between 650-1200 ° C (essentially consisting of mobile ions), and a solid part, including the crystals already formed. from the same liquid part. In general, the higher the volatile content, the more the magma is capable of generating explosive eruptions.

The viscosity of a magma represents its resistance to flow, in other words, the less viscous a magma is, the more it is "fluid and free to move". To better understand how this important parameter affects the degree of explosiveness, it is good to clarify how magma behaves at the atomic level. A magma is mainly composed of a fusion of silicates in the form of tetrahedra [SiO4] 4-, these are linked together by bridging oxygen and have a silica particle (network-forming ion) in the center, this process is called polymerization. If, to modify this atomic structure, other atoms (such as Ca and Mg) intervene, these will turn out to be bond modifiers (network-modifying ion), breaking the bridge oxygen and the entire structure, making it no longer polymerized. Therefore, in the polymerized case the viscosity is high because the magma is less inclined to flow, being well bound by the bridging oxygen (the single units are subject to a considerable internal friction), in the second case the viscosity is low due to the other atoms upon entering the system, they destroy the previous atomic structure, making everything much more "mobile" (image 1). The low-fluid magmas, with high viscosity, are those capable of generating the largest explosive eruptions. It follows that the more a magma is rich in silica (acid magma), the more the viscosity, and therefore the explosiveness, increases. A large amount of crystals also contributes to making the magmatic melt more viscous. The viscosity also depends on the temperature and the dissolved H2O, the higher these two values ​​are, the less viscous the magma will be.

Image - 1 - Diagram showing the difference between the atomic structure of a high viscosity magma (left) and a low viscosity one (right). In the first case, the eruption will be more explosive
(Credit: Alessandro Da Mommio, note 1)

Eruptions in which there is a chemical interaction between magma and H2O (both in liquid and solid state) are called phreatomagmatic. Generally, this eruptive style presents itself as immediate and highly explosive. A relatively recent example is the 2010 eruption of the Eyjafjallajökull volcano (Iceland), in which the high degree of explosiveness and dispersion of the ashes was caused by the contact between the magma and the ice that covered the top of the volcano (image 2 ). From some laboratory experiments it has been observed that, depending on the water-magma ratio, the explosiveness of the eruption varies regularly: for low ratios the activity could appear as Strombolian (non-violent), while it reaches the maximum point at the water-magma ratio of 0.3, increasing the ratio even more, however, the efficiency decreases as the abundance of water will tend to completely cool the magma (as can happen in a submarine eruption).

Picture - 2 - Pheatomagmatic eruption of Eyjafjallajökull volcano in 2010 (Iceland). It is possible to note how the magma-ice interaction plays an important role in the level of explosiveness of an eruption
(Credit: Patrick Mylund Nielsen, note 2)

Magma mixing phenomena consist in the mixing of two or more magmas of different chemical composition and thermodynamic state such as to form a hybrid magma, having intermediate properties between the previous ones. The entry into the magmatic system of a new magma, not in thermodynamic equilibrium with the one already present in the magma chamber of the volcano, creates a general state of imbalance in the crystals already present, which must therefore "reprogram" and continue the process of crystallization under new conditions. Only recently it is being considered how magma mixing processes can make an eruption more explosive than normal, in particular for the concept according to which this contact between two different thermodynamic states increases the heat movements (convective motions) in the magma chamber. and this, in turn, increases the volatile content which, as explained above, increases explosiveness.

The parameters that play a fundamental role in the explosiveness and eruptive style of a volcano can therefore be summarized in a high volatile content and high viscosity of the magma, to which, according to certain situations, contact between magma-H2O and magma phenomena can be added. mixing. The same volcano can exhibit different activity depending on how these physicochemical and geochemical parameters change in the magmatic system. If, with future studies and research on these important parameters and methods, it will one day be possible to establish how much it takes, at a temporal level, for a volcano to erupt in a highly explosive way, then we will make great progress in forecasting volcanic hazard and risk.

dr. Nicola Mari

Newhall, C. G. and Self, S. The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism, J. Geophys. Res, 87, 1231-1238, 1982.

Sheridan, M.F. and Wohletz, K ... Hydrovolcanism, basic considerations and review. Journal of Volcanology and Geothermal Research, 17: 1—29, 1983.

1. Alex Strekeisen
2. Eyjafjallajokull eruption photo

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