By Allison Kubo Hutchison
|Left: Glowing basalt eruption in Iceland at night (Image: Áslaug Arna Sigurbjörnsdóttir / twitter). Right: Gray ash clouds rise into the atmosphere over St. Vincent (Image: University of West Indies Seismic Research Center / Twitter).|
In Iceland, where we place our scene, lava spills orange and black tendrils from three cracks in Wallachadalir. Meanwhile, in St. Vincent, worldwide ashes rise into the sky in a large cloud from the volcano La Soufrière. There were numerous earthquakes ahead of both eruptions, which volcanologists warned that magma would come to the surface. However, these two eruptions have very different behavior and therefore very different dangers (although both have dangers).
On St. Vincent, 16,000 people have been evacuated from their homes while hundreds gather to watch the rifts in Iceland. The eruption in St. Vincent is described by volcanologists as explosive behavior. The ash plume, which rises 20,000 feet, is created by energetic fragmentation of the magma due to gas pressures. As the magma rises, the gas dissolved in the magma forms bubbles – like CO2 in soda – and when the pressure rises, the magma eventually splinters into pieces of glass. In Iceland, however, this lava splashes out of the vent on the surface and forms a cone. It flows out in streams of lava and cools down to a dense black rock called basalt. This type of eruption is known as exuberant. This lava was able to reach the surface without fragmenting partly due to the lower gas content, but also due to the fact that the lava is significantly less viscous than the magma that is currently erupting in St. Vincent. Viscosity is a measure of how easily liquids can deform. For example, honey is more viscous than water. The lava flowing in the Wallachadalir Valley is significantly more viscous than water and around 10,000 to 100,000 times higher.
The magma that was shattered in the St. Vincent eruption is even more viscous, probably ten times more sticky and immobile than the basalt. The higher the viscosity of the magma, the more gas pressure can build up before it erupts, and the more explosive the eruption is. The viscosity of magma is influenced by its composition, water content and temperature. In general, higher silica, higher dissolved water, and lower temperatures result in more viscous magmas. Explosive eruptions create volcanic ash, which is extremely dangerous to breathe and can clog aircraft engines. This behavior is also associated with pyroclastic currents, the most dangerous volcanic phenomena. Pyroclastic currents from La Soufrière killed 1,565 people in 1902, most of whom were native to the Caribbean. At the time of this writing, there have been no cases of pyroclastic currents, but these hazards have resulted in thousands being driven from their homes.
However, the St. Vincent eruption began with an exuberant phase of building a lava dome, which is a mound of degassed lava slowly emerging from a vent. The lava is much more viscous so it does not form roped lava flows or spread far from the event. The outbreak went from effusive to explosive on the morning of April 9th. The cause of this transition is not yet known and lies in the field of volcanology. However, scientists theorize that the conditions in the conduit that bring magma to the surface change the way in which it is expressed as changed magma. Fast and slow rates of ascent through the crust, the amount of gas in the magma, and even the geometry of the volcano play a role. Understanding how and what is more important when volcanoes erupt profusely or explosively is a major challenge for volcanologists.