This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Wednesday, April 14, 2021

La Soufriere St. Vincent: Background


La Soufriere St. Vincent last erupted in 1979 with a volcanic explosivity index (VEI) of 3, causing evacuation of 20,000 people. In 1902-1903 it had a VEI4 eruption that caused ~1500 deaths. Ironically, this eruption was somewhat overshadowed because it was one day before the catastrophic eruption of Mt. Pelee in Martinique, an eruption that destroyed St. Pierre.** There were also eruptions in 1718 and 1812, giving a rough cyclicity of explosive eruptions every 77-94 years.

(left figure) Soufriere St. Vincent is shown at the north end of the island in brown shading. It rises to a height of 1204 m. 

The island developed as a result of subduction of the Atlantic/North American plate beneath the Caribbean plate. The oldest dated rocks on the island are 2.74 Ma and Soufriere's activity began around 0.69 Ma. 

    At present no one knows or is predicting how this eruption cycle will turn out: what kind of eruptions? what kind of magma? how long will the eruptions persist.  But let's look at one possibility, the well documented 1902-1903 eruptions. There were three main phases in that eruption: May 6-7th; September-October 1902; and March 1903.      

    The first eruption began on May 6th, 1902, after 13 months of felt seismicity. Eruptions involved a pre-existing crater lake in a series of steamy explosions, some reaching 1 km in height.  Incandescence (=lava) was observed on the evening of May 6th. Tephra fallout began around 11:00 a.m. on May 7th, with fine ash changing to larger particles ("lapilli") around noon. The paroxysmal stage occurred around 2:00 p.m. with the formation of the "Great Black Cloud," caused by pyroclastic density currents (PDC's) travelling down all flanks of the volcano killing >1500 people. In places the PDC's reached the ocean and traveled across the sea for several kilometers.  Further explosive activity occurred in October 1902 and March 1903. Short summary of eruption history is available here.

    The 1979 eruption began with an explosive phase of 11 Vulcanian type explosions over 13 days, and the eruption columns from these explosions reached 18 km above sea level.  The April 17th eruption generated a PDC that reached out 2 km from the crater.

    In December 2020, lava began oozing quiescently from the summit, posing no serious threat to the 110,000 residents of the island of St. Vincent. However at the end of March, seismicity increased and at 8:41 a.m. on April 9, several major explosions occurred. 20,000-30,000 people living north of the volcano were endangered and required evacuation either by ship or by roads. The north end of the island is connected to the south end by only a single road. Evacuation by ship has been complicated because of the pandemic and evacuees must prove that they have been vaccinated against COVID-19 before they can get on a ship.

    The magma is a basaltic andesite. In contrast to the basalts that erupt in Hawaii or Iceland, for example, this magma is more viscous and thus the magma doesn't degas easily. The dissolved gas is then available to power explosive, rather than gently effusive, eruptions. Volcanologists couldn't predict exactly when an eruption would occur and were thus apparently relieved when the Prime Minister ordered people in the northern sectors to immediately evacuate on April 8.  Early on the morning of April 9, a huge summit explosion sent ash 10 km into the atmosphere; a second blast happened a few hours later. As of today, April 14, water shortages have been developing because ash is contaminating all surface sources. Imagery from the eruption can be seen here.

    As of Wednesday, April 14, pyroclastic flows have reached stretches of coastline and strong explosions are continuing.

**This material is taken from P.D. Cole, R.E.A. Robertson, L. Fedele, and C. Scarpati, Explosive activity of the last 1000 years at La Soufriere, St. Vincent, Lesser Antilles.