Two Eruptions, Both Alike in Dignity

Physics

By Allison Kubo Hutchison

Left: Glowing basaltic eruption in Iceland taken at night (image: Áslaug Arna Sigurbjörnsdóttir / twitter). Right: Grey ash clouds rise into the atmosphere over St. Vincent (image: University of West Indies Seismic Research Center / twitter).

In Iceland, where we lay our scene, lava spills orange and black tendrils from three fissures in Geldingadalir. Meanwhile across the globe in St. Vincent ash rises into the sky in a large plume from the La Soufrière volcano. Both eruptions were preceded by numerous earthquakes which warned volcanologists that magma was coming toward the surface. However, these two eruptions have very different behaviors and thus very different hazards (though both do have hazards).

On St. Vincent 16,000 people have been evacuated from their homes meanwhile hundreds gather to watch the fissures in Iceland. The eruption at St. Vincent is what volcanologists call explosive behavior. The ash plume rising 20,000 feet high is produced by energetic fragmentation of the magma due to gas pressures. As the magma rises, the gas that was dissolved in the magma — like CO2 in soda — forms bubbles, and eventually as the pressure increases cause the magma to fragment into shards of glass. However, in Iceland, this lava at the surface spatters out of the vent forming a cone and flows out in lava flows and cools into a dense, black rock called basalt. This style of eruption is called effusive. This lava was able to reach the surface without fragmenting partially due to lower gas content but also due to the fact that the lava is significantly less viscous than the magma currently erupting at St. Vincent. Viscosity is a measure of how easily fluids can deform, for example, honey is more viscous than water. The lava flowing in the Geldingadalir valley is significantly more viscous than water, about 10,000 to 100,0000 times more.

The magma that was fragmented in the eruption at St. Vincent is even more viscous, likely 10x 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. The viscosity of magma is affected by its composition, water content, and temperature. Generally higher silica, higher dissolved water, and lower temperatures lead to more viscous magmas. Explosive eruptions produce volcanic ash which is extremely hazardous to breathe in and can clog plane engines. This behavior is also associated with pyroclastic flows, the most dangerous volcanic phenomena. Pyroclastic flows from La Soufrière killed 1,565 people in 1902 most of which were indigenous Carib. As of the writing of this article, there have been no instances of pyroclastic flows but these hazards have led to thousands being displaced from their homes.

However, the eruption at St. Vincent began with an effusive phase building up a lava dome which is as it sounds a mound of degassed lava slowly extruding from a vent. The lava is much more viscous so it does not form ropey lava flows or spread far from the event. The eruption transitioned from effusive to explosive the morning of Friday, April 9. The cause of this transition is still not well known and is a focus in the field of volcanology but scientists theorize conditions in the conduit bringing magma to the surface change the way it is expressed. Fast versus slow ascent rates through the crust, the amount of gas in the magma, and even the geometry of the volcano play a role. Understanding how and more importantly when volcanoes erupt effusively or explosively is a grand challenge for volcanologists.

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