Volcanic eruptions are one of the many splendid wonders Earth has to offer. Yes, they are devastating; yes they are, undeniably, natural disasters. That does not stop them from being truly magnificent to behold. Toxic plumes rising in the sky, competing with the densest of cumulonimbus clouds; molten fire erupting in infernal glory, destroying everything in its wake; ash covering land and sky alike, suffocating all who breathe it – these are all catastrophic, yet spectacular, events which show the true, inexorable power of Mother Nature.
Perhaps unbeknownst to the human population in general, the nineteenth century saw the introduction of a volcano eruption classification system based on three distinctions: explosive, intermediate and quiet. In 1908, Professor A. Lacroix designed a new system comprising four divisions; a system which has since then seen the introduction of a further two divisions. The Vulcanian, Peléan and Plinian (originally grouped with Vulcanian) divisions are the main ones dealing with explosive eruptions, with the latter being the strongest of all.
According to the volcanologist G. Paulette Scrope, an explosive eruption takes place when lava plugs a volcano’s crater. Due to the plug, gas and magma build up, resulting in a drastic increase in pressure. Once the pressure increase becomes too large to contain, the gas and magma explode into the air, obliterating the crater and devastating the surrounding area in the process. While this is a fairly accurate explanation, it is a rather simplistic version of events leading to such an eruption. In actuality, two causes have been identified as being the main contributors leading to an explosive volcanic eruption: high magma viscosity and high dissolved gas content.
High magma viscosity
Relatively low temperatures (not exceeding 800°C), high silica content (SiO2) and a high level of dissolved gas are the three main features characterizing the most viscous of magmas – Rhyolitic. Apart from being highly resistant to flow, Rhyolitic magma has an adverse affect on the dissolved gas within it and thus initiates the process which ultimately culminates in a devastatingly explosive volcanic eruption. Although less viscous than Rhyolitic, Andesitic magma may from time to time cause an explosive eruption, especially if it contains a high level of dissolved gas.
High dissolved gas content
Magma is made of three components: molten rock, crystals and dissolved gas. It is this latter constituent which causes the explosive eruption. As magma rises from the vast depths of the Earth, pressure starts to decrease. Consequently, the dissolved gas will start separating itself from the magma by forming bubbles. As the magma continues to rise, the bubbles will increase both in quantity and in size due to the incessant release of dissolved gas.
In the case of Basaltic magma, gas bubbles find no difficulty in expanding due to the magma’s characteristically liquid nature. Due to their expansion, the bubbles will benefit from a decrease in pressure, making them more akin to atmospheric pressure as they rise towards the surface. Once the surface is reached, the gas bubbles convert in an apparent seamless way into atmospheric pressure. This enables the magma to simply flow out of the volcano’s vent.
Contrastingly, the high viscosity levels of Rhyolitic magma tend to greatly hinder the expansion of gas bubbles, causing an inevitable increase in pressure within the bubbles. Upon reaching the surface, the disparity between the bubbles’ internal pressure and atmospheric pressure will cause the gas bubbles to explode. Thus, an explosive volcanic eruption is created; an eruption marking the first phase of a pyroclastic flow.
As the gas and magma erupt from the volcano’s vent in explosive glory, debris and lava are launched across great distances, eradicating anything they come into contact with. Toxic gases released during the eruption have the tendency to poison anyone who inhales them and cause acid rain. Ash clouds will make it near impossible for aircraft to utilize the affected air space due to their thickness and properties.
Such an eruption has the potential of obliterating entire cities. A case in point is the infamous Roman city of Pompeii, which in AD 79 was completely destroyed and buried by the explosive eruption of Mount Vesuvius.
While causing incontrovertible devastation and regrettable loss of life, an explosive volcanic eruption is still a dazzling spectacle; catastrophic, yes, but a visual splendour none the less. An attestation to this is the occasional eruption of the infamous Mount Etna, especially if the devastating event takes place during the nightly hours. The amber magma erupting against the dark canvas that is the sky is a sight which, given the opportunity, none should miss; as long as a safe distance from the toxic gases preceding it and the devastation following it is maintained.
