An effusive eruption began on La Soufriere volcano on St Vincent (St Vincent and the Grenadines) island in the eastern Caribbean Sea in Dec. Note that this is currently an ongoing dome building eruption.
La Soufriere (Soufriere Saint Vincent) volcano is the currently active volcano on the island of Saint Vincent in the Windward Islands in the Caribbean. The volcano’s name in French is “Sulfur Mine.” This particular volcano at 1,234 m is highest point on the island and the highest point in the nation of Saint Vincent and the Grenadines. It periodically hosts a crater lake. Many volcanoes in the island chain on the eastern end of the Caribbean Plate are named Soufriere including Soufriere Hills on Montserrat and La Grande Soufriere on Guadeloupe.
The two government organizations monitoring activity at La Soufriere are The University of the West Indies Seismic Research Centre and the National Emergency Management Organization (NEMO) of The Government of Saint Vincent and the Grenadines. NEMO has regular bulletins up to La Soufriere Bulletin #12, Jan 11, 2021 as of this writing.
Current status is Alert level at orange. No reconnaissance flights were done Jan 11 due to weather. They have been testing drone surveillance of the volcano and have so far been successful. NEMO continues to appeal to the public to stay off the volcano until they are advised it is safe to do so.
There is a 300 m platform south of the volcano called Belmont Lookout / Belmont Observatory on the western side of the island. Link to a Jan 9 time lapse video of the volcano posted on FaceBook can be found here.
The volcano is well monitored with five seismic stations, at least eight GPS stations, and five dry tilt stations reporting activity as of Jan 2005.
The island itself measures 29 x 18 km, located 160 km W of Barbados. It is mountainous and heavily forested. Climate is tropical and humid. Most of the beaches of the island are black volcanic sand. The island has a population of around 100,000, most of whom are in the largest and capital city of Kingstown, just over 25,000. These islands achieved their independence from Great Britain in 1969. The economy of St Vincent and the Grenadines is heavily dependent on agriculture, with bananas and root crops. In recent years, Tourism has become an important part of the economy.
Volcano Discovery has extensive coverage of Soufriere St Vincent, though most of the live webcams were not available at this writing.
La Soufriere is the youngest volcanic center on St Vincent. It occupies the northern third of the island and is considered to be the only remaining active vent on the island. Apparently, activity has migrated northward over time. There is no detailed geologic map of the volcano, though primary formation have been identified by researchers 1978 – 1996. The volcanic edifice consists of an older stratocone somma, 2.5 km in diameter. This forms a steep ridge to the N. There is a younger pyroclastic cone what has been the source of all eruptions since 1700. The older cone is thought to have been active some 700 ka.
The main crater of the active pyroclastic cone is 1.6 km in diameter, 300 – 600 m deep. There is a neighboring crater immediately to the NW, some 450 m in diameter, 60 m deep, which was formed during the 1812 eruption sequence. The currently active crater is the largest of three summit craters. The other two, the Somma and the 1812 crater are both currently dormant.
There are four rock formations in the crater. The lowest is the debris flow formation, a massive deposit with angular basaltic blocks in a poorly sorted matrix. It is overlain with the brown tuff, a 20 m thick well-bedded succession of ash and scoria deposits and minor surge layers. This has angular basaltic fragments. The crater lavas are a series of thick basaltic and andesitic lava flows forming the lower half of the vertical eastern and northern crater flows. The top layer exposed in the crater is a thick layer of pyroclastic and airfall deposits called the pyroclastic formation.
The oldest formations on the flanks are basaltic lavas forming the Somma crater. These are overlain with pumice rich yellow tephra (yellow tephra formation) which are yellow pyroclastic fall deposits covering the island. These deposits contain black scoria and yellow lapilli sized pumiceous tuffs ranging from basalts to andesites. The yellow tephra is covered mainly in river valleys with a combination of alluvial, basaltic andesitic pyroclastic flow deposits, and mudflow deposits. The mudflow deposits are massive, up to 25 m thick. On the lower flanks the mudflow deposits are interbedded with pyroclastic flow deposits, thin tephra fall deposits, and minor alluvial deposits.
There is a thick series of well bedded pyroclastic fall deposits on much of the island. These are correlated with coarse tephra beds exposed in the crater and on the flanks of Soufriere. These range from black scoria to yellow lapilli and pumiceous tuffs, the Yellow Tuff Formation, with a volume around 48 km3. These date 4.5 – 3.6 ka. This formation appears to have been deposited quickly likely from the central Soufriere vent.
There are two structures on La Soufriere identified as possible collapse scars. Both are horseshoe shaped structures, though there are no debris avalanche deposits identified on land as yet.
The first and oldest scar is the Baleine scarp, which cuts a pre-existing primitive volcanic edifice. It is open to the sea to the west. Only its northern and eastern rims are well preserved, extending from the coastline to the summit, forming a cliff up to 300 m bounded by the Baleine River. Valleys N of the rim generally point toward the N. Those inside the structure generally point W. The southern rim is masked by construction of a new cone named Somma inside or on its border. The southern rim of this structure may extend into the water to the W of the island. The structure is estimated at 5 x 3.5 km with missing volume over 10 km3. Age of this structure is estimated at less than 50 ka. Deposits of this cone probably cover debris avalanche deposits on land.
The Somma cone in the Baleine structure is cut by a second structure that is a possible partly preserved structure, the Larikai structure. This is 2.5 x 3.5 km open to the SW. Its NW rim is well preserved with cliffs up to 250 m. The SE rim is covered by products of the current active cone of Soufriere. This structure is not less than several thousands of years. The missing volume from this is 3 – 5 km3. Construction of the Somma cone was very fast following the Baleine collapse, with increased magma production possibly due to unloading following the collapse.
There is a final horseshoe shaped structure on the neighboring Morne Garu volcano, open to the NW. It is poorly constrained due to erosion. There are no associated debris avalanche deposits described in the area.
There are at least three-layered deposits discovered via sonar to the W of St Vincent. These are interpreted as debris avalanche deposits with lobes as far as 53 km offshore. These do not have large blocks. They cover around 600 km2 with an estimated volume of 9 km3.
The current eruption began Dec 27 when the new dome broke through the crater floor. It continues to grow, expanding in a westerly direction. Seismicity began to increase in early Nov. Changes in the water lake and fumarole area were noted on Dec 16. There was a persistent thermal anomaly identified in satellite data which prompted a NEMO field trip to the crater.
Residents of St Vincent and the Grenadines were put on alert Dec 30 as activity at Soufriere volcano on St Vincent increased to the point where local government and the Caribbean Disaster Emergency Management Agency warned the volcano could erupt with less than 24 hours. The Prime Minister noted that some communities may receive evacuation orders on short notice. This was the first of its kind since the 1979 eruption.
The island has not had an eruption since 1979. Officials on neighboring Martinique are also closely watching Mount Pelee volcano as activity has ramped up over the last month. Pelee and Soufriere both erupted in 1902, killing thousands. The simultaneous increase in activity at both volcanoes is not linked.
Current activity at Soufriere is dome building next to the existing dome in the crater. So far, the eruption is effusive.
In 2005, sulfur smells from the volcano were reported as far south as Kingstown. This was not accompanied by any other volcanic indicators and explained by a change in prevailing wind direction from E-W to N-S. The reports prompted a two-day field trip to the crater rim taking measurements. No increase in activity was observed. The alert level was not increased for this event. These reports are common for St Vincent and other volcanoes in the region.
Activity in 1979 took place Apr – Nov. It began with a series of powerful explosions producing ash clouds and pyroclastic avalanches. 17,000 were evacuated. The eruptions were preceded by a strong local earthquake on Apr 12. Activity increased over the day, eventually with continuous harmonic tremor that saturated the seismometers.
The first explosive event took place at dawn on the next day. It was followed by multiple explosions through Apr 25. The largest of these put a plume to 18 km, up to 140 km wide. Most explosions took place near the time of diurnal tide maximum. One took place at the tidal minimum. Pyroclastic flows followed three of the explosions. The largest of these was on Apr 14. These mainly moved down river valleys several kilometers to the sea and on out to sea. The deposit on the Apr 14 flow was 1.5 m thick at the beach. The Apr 17 explosion produced multiple hot avalanches down several valleys on the volcano flanks.
Seismic activity ended almost entirely by Apr 29. A new dome was observed growing on May 3. This was located at the center of the now-destroyed 1971 dome. As the dome grew, so did the new crater lake. Most of the evacuees were allowed to return by May 14, though restrictions remained in place for 4,000 in the 1902 eruption zone. Extrusion continued through the summer, tailing off in Aug – Sept. It had stopped by Oct 25. This sequence is classed as a VEI 3.
The 1971 – 1972 eruption sequence began in Nov with sulfurous smells from the crater lake in Nov. An island in the lake was observed Nov 23, the top of a new lava dome. No clear volcanic earthquakes were recorded, though there were a series of explosions heard on Nov 20 in the crater. These were obscured by steam. Multiple islands in the lake coalesced into a single dome by Dec 1. Growth at varying rates continued through March, stopping by Mar 20. Water temperature cooled down by May. Erupted lavas were more basaltic than andesitic and gas poor.
The typical explosive Soufriere eruption is a highly explosive magmatic eruption preceded by frequent, strong earthquakes. They typically produce large volumes of new ashfall from eruptive columns, pyroclastic flows and surges. The 1902 – 1903 and 1979 eruptions are typical of these. Effusive eruptions are generally not associated with earthquakes. The 1971 – 1972 and current eruptions are examples of these. Eruptions over the last 250 years have generally alternated explosive and effusive eruptions, with an explosive eruption taking place once a century.
The 1902 VEI 4 eruption killed 1,680 mostly Carib natives, destroying the last large remnant of Carib culture in the Caribbean. The previous eruption were in 1814 (VEI 1) and 1812 (VEI 4). The 1718 eruption is classed as a VEI 3. There were multiple eruptions recorded over the last 4,000 years.
The Lesser Antilles is an 850 km convex island chain stretching from Sombrero in the N to Grenada in the S. The southern portion of the inner arc extends from Grenada to Saba and is the currently active volcanic arc. The dominant tectonic process is active subduction as the Caribbean Plate is overriding the Atlantic Ocean crust, the North American Plate to the north and the South American Plate to the south. Dividing lines between the two plates this far east is not well defined. Subduction rates are estimated at 2.2 – 3.8 cm/yr.
The Caribbean Plate is moving generally eastward relative to the North American Plate. While the subduction rate has varied over the last many millions of years, the rate of convergence in the Lesser Antilles estimated at 1.3 – 4.0 cm/yr is lower than most arc systems.
The Benioff zone has an average dip angle of 45 degrees and a depth 100 – 120 km beneath the active volcanic arc. The dip angle varies slightly along the arc and is vertical at and south of Grenada. Crust thickness beneath the arc is around 30 km. This area is seismically active.
Arc volcanism has been going on in the Lesser Antilles for 40 Ma. There is an outer arc north of Dominica where carbonate rocks cover volcanic basement of the islands. The inner arc are volcanic rocks less than 20 Ma, including all active volcanoes. South of Dominica, there is only a single string of islands. The back arc Grenada Basin behind the arc is 2,900 m deep.
Composition of volcanic rocks vary along the active arc, allowing the islands to be grouped according to three magma series, tholeiitic north of Montserrat, calc-alkaline in the central islands, and alkaline in the southernmost islands, Grenada and the southern Grenadines. This means that magmas at St Vincent are transitional between tholeiitic and calc-alkaline.
St Vincent itself is a series of N-S trending stratovolcanic centers as old as 3 Ma near the south of the island to 600 ka at the Soufriere volcano in the northern part of the island. The Southeast Volcanics, dated 2.74 Ma, the Grand Bonhomme, dated 1.33 – 1.16 Ma, and Morne Garu volcanic centers are pre-Soufriere centers of the island. The Morne Garu crater is eroded and estimated at 3 km in diameter. Volcanism here is dated as recently as 20 – 11 ka and may have overlapped activity with Soufriere.
La Soufriere St Vincent is an active volcano currently undergoing an effusive dome-building eruption. Historically, these sorts of eruptions are non-explosive from this particular system. But this does not mean that behavior cannot change very quickly. The system appears to have a prolific magma supply and builds cones quickly. It also has undergone multiple flank collapses in a relatively short time span.