Volcanic Activity and Hazards in Montserrat

The Soufrière Hills Volcano

The Soufrière Hills Volcano (SHV) is characterized by andesitic magmas, which typically exhibit high viscosity, akin to that of a sticky fluid. Upon reaching the surface, these magmas form lava domes, which are hemispherical to irregularly shaped mounds formed by high-viscosity lava that cannot flow. This phenomenon contrasts with the eruptions observed in Hawaii.

The eruption of lava domes, along with the production of Pyroclastic Density Currents (PDCs), commonly known as Pyroclastic Flows, represents the most typical eruptive styles of SHV. Occasionally, these eruptions can lead to powerful vulcanian eruptions.

The SHV experienced significant eruptive activity between 31,000 and 16,000 years BP (Before Present, referring to January 1, 1950). There was a reactivation 4,000 years BP, which led to the formation of the English Crater through sector collapse. Additionally, minor activity occurred approximately 350 years ago BP, characterized by small eruptions.

Lava Dome Characteristics

There are several different types of lava dome. The domes that form at the SHV are called Peléan-type lava domes that consist of a substantial blocky mound surrounded by an apron of loose debris shed from the dome as rockfalls or pyroclastic flows. Lava domes can become very large features, measuring 10s to hundreds of metres thick and hundreds of metres in diameter. The largest lava dome ever built at the Soufriére Hills volcano measured more than 400 m thick, approximately 1 km in diameter and had a volume of more than 200 million m³.

Domes grow through the accumulation of lava that is typically erupted from a single vent. Fresh lava is added to a dome either by stacking it on top of older lava (exogenous growth), or by intruding it into or beneath recently erupted lava, causing the older lava to inflate or expand (endogenous growth). Either method of growth can be quite rapid, with rates of growth reaching in excess of 40-60 m3 per second. Although rapid, the growth rate is not constant. Observations from dome growth at the Soufriere Hills volcano have shown that growth typically occurs in pulses lasting 1-2 hours separated by periods of little or no growth that last anywhere from 5 to 16 hours.

Lava domes have a wide variety of surface features and textures that are related to the rate at which the lava was extruded. Slow extrusion usually produces large features like spines or shear lobes, while faster extrusion rates will generate blocky lobes. Spines are usually formed of nearly solidified lava that doesn’t contain much gas. The spines can be short and blocky or very tall and conical, reaching tens of metres high and, depending on the size of the conduit or vent, tens of metres in diameter. Spines will always form over the vent. At the Soufriere Hills volcano, the formation of spines has coincided with dramatic decreases in extrusion rates to less than 1 m³ per second that have often been followed by explosive events. Shear lobes are the basic building-block of domes. They are large bodies of solid lava measuring tens to hundreds of metres long and tens of metres high, often with a curved, ridge-like shape to them and smooth sides. They are the result of steady extrusion of lava from a vent, with all the friction and shear concentrated around the sides of the shear lobe. At faster extrusion rates, the surface of lava lobes will be much blockier. This is because as the lava is extruded quickly, it is subjected to higher stress causing the nearly solid lava to break up, giving the lava lobes a rough appearance.

Although nearly solid when erupted, the lava in lava domes is still very hot. This is particularly so for the interior of domes and lobes, where temperatures may range from 400-600 °C. At these temperatures, freshly extruded lava and any exposed interior sections will glow incandescent reds and oranges at night. The surface of a lava dome will cool down relatively quickly and will act as an insulating outer layer, allowing the interior parts of the dome to remain hot for many years after it has stopped erupting. Thus lava domes can remain very hazardous many years after lava extrusion has stopped.

Due to the fact that they are constructed from piles of lava surrounded by an apron of unconsolidated blocky debris, lava domes can become very unstable and may eventually collapse. The partial or total collapse of a lava dome can produce rockfalls or pyroclastic flows and may lead to explosive eruptions if the dome traps a large amount of gas in the shallow conduit. Since 1995, there have been many dome collapses on Montserrat. The largest collapse occurred overnight from 12-13 July 2003 and involved more than 200 million m³ of dome rock and talus. This event is presently the largest known historical dome collapse in the world. The present lava dome, parts of which formed after the 20 May 2006 dome collapse, currently contains approximately 190 million m³ of dome rock and talus.

Vulcanian Explosions

Explosive eruptions are powerful events capable of generating widespread ashfall, pumice fall, ballistic projectiles, and pyroclastic flows. These eruptions encompass various types, ranging from small Strombolian explosions to massive Plinian eruptions.

Vulcanian explosions are characterized by their short duration and violent nature. They typically involve the rapid fragmentation of magma within a conduit or collapse of a lava dome, resulting in the formation of a large eruption column that can ascend more than 15 km into the atmosphere. This leads to extensive ashfall in the surrounding areas.

The ash columns produced by Vulcanian explosions can be exceptionally dense, particularly if the destruction of a lava dome is involved. Such dense columns lack buoyancy and do not ascend rapidly or far into the atmosphere. Consequently, these columns collapse back to the ground, generating pyroclastic flows known as column-collapse flows. These flows may contain significant proportions of both dense and partially vesiculated lava fragments.

Vulcanian explosions have been a prevalent aspect of the activity at the Soufrière Hills Volcano, with over 100 documented explosions since 1995. In 1997, a series of 88 explosions occurred between August and October, averaging one explosion every 10 hours. These explosions occurred subsequent to two lava dome collapses. Some of the ash columns generated by these explosions reached heights exceeding 10 km.

Additionally, Vulcanian explosions have occurred during periods of lava dome growth. For instance, in January 2010, three explosions took place between 8th and 10th January. The first explosion, the largest of the three, generated an ash column reaching 8.3 km into the atmosphere, resulting in widespread ashfall and pyroclastic flows. Although not associated with a dome collapse, this explosion removed over 6 million cubic meters of material from the dome and upper conduit. Other Vulcanian explosions at SHV have occurred before the initiation of lava dome growth. An example of this is the explosion on 28th July 2008, which led to widespread pyroclastic flows and ashfall, followed by the gradual growth of the lava dome.

Video of vulcanian eruption at Soufrière Hills Volcano. See also ballistics in the right side of the image.

Main Volcanic Hazards – Pyroclastic Flows

Pyroclastic Density Current (PDC, commonly known as Pyroclastic Flows)

“Pyroclastic Density Current” is a broader term that encompasses phenomena such as pyroclastic flows. However, for public materials, we prefer to use “Pyroclastic Flow” as it is widely understood by the local community.

Pyroclastic flows are the most hazardous phenomena associated with activity at the Soufrière Hills volcano. They consist of a mixture of hot ash, lava blocks, pumice, and volcanic gases, and can travel at extremely high speeds over the slopes of a volcano, reaching velocities exceeding 50 meters per second (about 110 mph or 180 km/h). Typically, pyroclastic flows comprise two main components: a dense basal flow containing higher concentrations of particles, some of which may exceed 1 meter in diameter, and a more diffuse, billowing cloud of gas and fine ash particles.

We use the categories “Pyroclastic Flow” and “Surge,” respectively, to refer to them in the Pyroclastic Flow hazard map (see Hazard Map Series). While pyroclastic flows usually descend along valleys, the accompanying cloud of gas and ash can detach and traverse areas between valleys, and even ascend over ridges and small hills.

The left image shows a pyroclastic flow descending through a river valley. Although it is a different location, in the right image, the riverbed is clearly affected by the dense basal flow, while the lateral walls of the valley are mostly affected by the more diffuse billowing cloud of gas.

Video of a Pyroclastic Flow in Gingoes Ghath, west Montserrat. Note the dense basal flow following the riverbed and the billowing cloud of gas passing over the topography.

Video of a Pyroclastic Flow downflow Tyres River in 2010, northwest of Soufrière Hills Volcano. Taken by Gianni Draghi.

Special attention must be paid to wind direction, especially when it is perpendicular to the flow direction, as it can significantly alter the trajectory of a gas cloud increasing the impacted area The potential areas affected by this phenomena has been identified as “Secondary Surge” in the hazard map (see Hazard Map Series).

Due to their rapid velocities and capacity to transport large blocks, hot gases, and ash, pyroclastic flows are highly destructive and can obliterate or bury any structures in their path. The lava blocks, gases, and ash carried by pyroclastic flows are typically extremely hot, ranging from 300 to 600 °C.

At Soufrière Hills Volcano, pyroclastic flows are typically formed in one of two ways. The first method involves the collapse of part or all of a lava dome as we mention before (See lava dome). Another method of generating pyroclastic flows is through the collapse of an eruption column (See Vulcanian eruptions). In this case, pyroclastic flows are less dense and more mobile than those generated by dome collapse.

Due to the large volumes of material they transport, pyroclastic flows can significantly alter the landscape by filling in valleys and creating new land along the coast when flows reach the sea.

Debris Avalanches

As special case of PDC, debris avalanches, also known as volcanic landslides, are rapid and highly destructive events involving the movement of rock, soil, and snow or ice downslope. They can occur as a result of the large-scale collapse of a volcano and can vary widely in volume, ranging from less than 1 cubic kilometer to more than 100 cubic kilometers. The size of the debris avalanche is often proportional to the size of the volcano.

These avalanches can travel significant distances from the volcano, covering hundreds of square kilometers and extending tens of kilometers away. If they contain a substantial amount of water, such as from snow and ice or entrained from rivers, they can transform into debris flows, which can travel even farther from the volcano, sometimes reaching hundreds of kilometers.

Several factors can contribute to the large-scale collapse of a volcano and the generation of a debris avalanche. Intrusion of magma into shallow depths or extrusion of a lava dome onto steep slopes can cause parts of the volcanic edifice to become displaced or bulge outward, creating highly unstable slopes prone to collapse under gravity. The collapse event may be triggered by a strong earthquake that shakes the unstable slope or by heavy rainfall that saturates the volcano’s slopes.

For example, the massive debris avalanche during the 18 May 1980 eruption of Mount St. Helens was triggered by a large earthquake that destabilized the northern slope of the volcano, which had been made unstable by magma intrusion into the upper part of the volcano edifice. In contrast, the debris avalanche during the 26 December 1997 Boxing Day collapse at Soufriere Hills was caused by the overloading of Galway’s Wall (part of the old English’s Crater) due to the extrusion of a lava dome onto and against the steep slope.

Debris avalanches at active volcanoes can sometimes be associated with explosive eruptions. Both the collapses at Mount St. Helens and Soufriere Hills were followed by violent explosions that directed horizontally instead of vertically, resulting in what is known as a lateral blast. These blasts are accompanied by very violent and fast-moving pyroclastic flows that can overrun valleys and ridges, as well as destroy buildings and knock down extensive areas of forest.

Prior to the onset of activity at Soufriere Hills in 1995, a large collapse scar, called English’s Crater, existed. Measuring approximately 1 km wide by 1.5 km long, it was generated by collapse of an older Soufriere Hills volcano about 4,000 years ago.

Main Volcanic Hazards – Tephra Fallout (Ashfall)

In Montserrat, due to the short distance to the volcano, distinguishing and measuring the small particles generated by magma fragmentation inside the conduit during phreatic or Vulcanian eruptions from those produced by other external processes, such as rockfalls and pyroclastic flows, is challenging. As a consequence, the deposited material varies in size, ranging from fine particles to larger rocks (several millimeters in diameter). Because of this, the scientific analysis of this hazard has been approached more broadly, referring to tephra fallout instead of ashfall, as do our hazard maps. Hence, the terminology we use includes ashfall within it.

Volcanic ash particles can be very small, measuring less than 1 mm in diameter. This makes them very hazardous to people and animals who breathe them in. Due to the very small size of ash particles, it is possible for them to be transported very long distances via regional winds. This means that communities living many kilometres downwind of a volcano may be severely affected by volcanic ash fall. Also, aircraft that fly through an ash cloud can be severely affected by the ash, causing significant damage to the engines and outer surfaces of the wings.

A slightly less vigorous method of ash production also occurs at Soufriere Hills volcano. Termed ash venting, it is defined as the continuous emission of ash, producing eruption plumes that typically reach 3-6 km above sea level. The ash venting originates directly from the lava dome.

Main Volcanic Hazards – Ballistic

Ballistic

Ballistics, also known as volcanic bombs, are large blocks of lava, sometimes still partially molten, that are ejected during an explosion. Ranging in size from tens of centimeters to more than 1 meter, they can travel several kilometers from the vent. If the blocks are still partially molten when ejected, they can take on different shapes due to aerodynamic effects while traveling through the air. Consequently, various types of bombs can form, including breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and ‘cow-dung’ bombs (so-called when partially molten bombs become flattened and squashed upon impact with the ground). Breadcrust bombs are a special type characterized by a cracked outer surface resembling a freshly baked loaf of bread. This texture develops as the surface of a partially molten lava block cools in flight, forming a brittle surface that subsequently cracks due to expansion of the interior as gas trapped inside the lava block continues to form bubbles.

At Soufriere Hills, ballistics have been associated with powerful Vulcanian explosions, such as the 17th of September 1996 explosion. During this event, blocks measuring up to 1.2 meters in diameter created impact craters up to 6 meters in diameter, extending up to 2.4 kilometers from the vent. Ballistics pose a significant hazard close to the volcano, capable of puncturing through wooden roofs and igniting building interiors.

Main Volcanic Hazards – Lahars or Mud Flows

Lahars or Mud Flows

In Montserrat, the public term used to describe gravity-driven mass flows of volcanic rock fragments mixed with water is “lahar”. This term, originally from Indonesia, was adopted by the scientific community after some debate. The accepted definition at the time was as follows: A lahar is a general term for a rapidly flowing mixture of rock debris and water (other than normal stream flow) from a volcano. Lahar is an event and it can refer to one or more discrete processes, but does not refer to a deposit.

Montserrat experiences a subtropical maritime climate characterized by substantial rainfall, primarily linked to larger tropical weather systems. Rainfall is distributed throughout the year, but there is a distinct seasonality. The wet season typically extends from July to November, followed by a transitional period from December to January, and a dry season from February to May. In Montserrat, lahars are primarily associated with heavy rainfall and can occur independently of ongoing eruptive activity. However, the likelihood of their occurrence increases during the rainy season, not only because the heavy rain, but also because other factors like ground saturation.

One of the more sensitive areas threatened by lahars is the Belham Valley, which must be crossed to access the southwest of Montserrat. Currently, there is no bridge in place (the previous one was destroyed years ago), so the crossing must be done through the riverbed. Lahars make this crossing impossible, and the dirt road has to be restored using heavy machinery.

Main Volcanic Hazards – Tsunamis

Tsunamis

The term “tsunami” refers to the displacement of a large mass of water in a short time. The causes that produce such displacement are movements of the seabed or the sudden entry of a large amount of material into the water. Common phenomena associated with these processes include earthquakes, volcanic eruptions, landslides, or even meteorite impacts.

Small tsunamis have occurred twice in Montserrat since 1995, in both cases associated with large dome collapses. The displacement of water caused by the landslides entering the sea triggered these events. The first instance occurred after a catastrophic failure of the lava dome on December 26, 1997, in White River. It was reported that waves reached up to 2 meters above sea level, inundating 80 meters inland in the Old Road Bay area, located northward of the source. Some minor destruction was observed along its path. The second occurrence of a tsunami was after a massive dome collapse on July 13, 2003, in the Tar River area. While 15 fishing boats were reported destroyed in Guadeloupe, Montserrat experienced only limited impact.

Main Volcanic Hazards – Shockwaves

Shockwaves

The sudden release of energy during eruptive explosions can generate shockwaves. This release of energy increases temperature and pressure, converting part of the emitted material into hot compressed gases. These gases then rapidly expand, initiating pressure waves known as shockwaves in the surrounding medium (air, water or earth). An example of this phenomena can be seen in the following video during the Tonga Volcano eruption in 2022.

Hunga Tonga eruption On January 15, 2022 and shockwave propagation by NOAA.GOV.

The Vulcanian explosion that occurred on July 13, 2003 produced shockwaves for the first time since 1995. This event impacted Harri’s Village, located 3 km north of the Soufrière Hills Volcano. The observed damage, including blown doors and windows, as well as collapsed buildings, was presumably linked to this shockwave, as only the bottom of the village was previously affected by pyroclastic density currents (PDCs). Other shockwaves were also detected on January 3, 2009, associated with four Vulcanian explosions, but there is not information about a similar impact.