Hazards associated with Soufriere Hills
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| There are a lot of hazards associated with eruptions of the stratovolcanoes on Montserrat. Since 1995 the island has experienced pyroclastic flows, lahars, rock falls, dome collapses, and earthquakes in addition to health hazards caused by fine ash falls. Not only do these hazards effect humans, but they also kill a large amount of animals and vegetation on Montserrat. |
| Pyroclastic flows are the most deadly volcanic processes because they often strike without warning, move swiftly, and destroy everything in their path. These flows carry pyroclastic materials (hot volcanic rocks and ash) down the mountainside to surrounding areas at speeds up to 400 kilometers per hour. Temperatures in the middle of a pyroclastic cloud can reach up to 700° C. Over the past eight years, pyroclastic flows on Montserrat have buried entire cities, including the former capital, Plymouth. Pyroclastic flows have also killed a large number of cattle that graze off the land surrounding the volcano. |
Left: pyroclastic flow on the north face of Soufriere Hills, September 2002.
Right: downtown Plymouth, June 17, 2002.
| In addition to pyroclastic flows, Montserrat experiences pyroclastic surges. Pyroclastic surges are not as constrained by the land topography as pyroclastic flows. This phenomenon is very dangerous because of its combination of high temperature, high speed, and high mobility. Often no living organisms in the path of pyroclastic surges survive. |
| Lahars are mudflows that are often triggered by heavy rains after a volcanic eruption. In the case of Montserrat, ash and other material from an eruption mixes with rain, creating "mud floods." Lahars can uproot trees, sweep away homes, and destroy entire communities. Like pyroclastic flows, lahars are often unpredictable, and they move very fast. One of the most dangerous qualities of lahars is that they can occur long after a volcanic eruption, so people may already be back in their homes when they strike. |
The former capital of Montserrat, Plymouth, has been buried by mudflows and pyroclastic flows.
| Rock falls do not pose a high threat to human life, but they can cause road blocks and damage property. Sometimes rock falls are triggered by earthquakes. Earthquakes are common in volcanically active areas. Depending upon the magnitude and frequency of earthquakes, they can be used to predict volcanic eruptions. |
Small rock fall on Soufriere Hills, February 13, 2001.
| Volcanic landslides are not common on Soufriere Hills, but they have happened in the past. A landslide is not likely to occur on a stable side of the mountain, but is more likely to occur on a flank with variable material. A pyroclastic surge could be initiated by a volcanic landslide, and could even generate tidal waves if the debris fell into the ocean. On Montserrat, a landslide followed by a vertical blast on the northern flanks would devastate what is left on the island, but the probability of such an event is 1 in 2,000. |
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Lava flows can travel fast or slow, long distances or short distances depending upon the silica content of the magma. Shield volcanoes (ie. Hawaii) are usually made of magmas that contain low amounts of silicate minerals, while calderas contain magmas with a high silica content. Stratovolcanoes fall between these two extremes and have magmas with intermediate levels of silica. The lower the silica content, the faster the lava will flow on the Earth's surface. Before Soufriere Hills erupted in 1995, much of the land around the base of the volcano was very fertile because of certain minerals contained in the soil from previous volcanic eruptions. In the future, this may be a benefit to the island and its agricultural production. |
| Tephra falls can be very deadly. Tephra is known as pieces of volcanic rock that are ejected from a volcano. These pieces can range from millimeters in diamater (ash) to 1 meter in diameter. The amount of damage caused by these tephra falls depends upon the energy of the explosion, the height of the eruption plume, and the total mass of rock ejected. Large amounts of ash can cause roofs to collapse, while larger pieces of tephra can put holes through roofs, and even kill innocent bystanders. However, larger pieces of rock do not travel as far from the volcano as smaller pieces, so the risk to human life and property correlates to the distance from the volcano. |
small tephra (ash)
| Ash is incorporated into the air when Soufriere Hills has pyroclastic flows or explosions. Fine ash is a health hazard to all living on the island depending upon weather conditions and some volcanic hazards. Ash is a hazard to human health and vegetation, especially on volcanoes that generate large amounts of fine ash, have long-lived eruptions, and where forms of toxic crystalline silica are a large component of the ash. |
Fine ash is easily breathed in by people,
causing lung problems and sometimes death. The more human activity in an
area (ie. children playing, driving cars), the more hazardous the ash can
be. Rain dampens deposited ash, reducing the chance it could become
airborne again. Volcanic explosions inject ash into higher altitudes which is
not as hazardous to human health, but allows ash to travel long distances.
The area on Montserrat most affected by fine ash is the western part of the island, which is usually the most downwind of pyroclastic flows and low-altitude winds. This is why the city of Plymouth received the most ashfall. The northern side of the island is the least affected by ash, because winds from the south are very infrequent (Baxter, et al., 1999).
It has been found that long-term exposure to crystalline silica in ash (like the ash emitted by Soufriere Hills) can lead to the development of silicosis, an irreversible disease that scars the lungs of its victims. Inhabitants of Montserrat are at risk of developing silicosis because of the ash production which is ongoing. The health risk is increased during dome eruptions because of the production of fine ash, the increased formation of crystalline silica in the dome, and the concentration of it in fine ash because of the crushing nature of pyroclastic flows.
Vegetation is also adversely effected by ash. Ashcover can become so thick tree branches break. The weight on smaller plants is so great there is no way they can survive. Ash kills plants, keeping them from sunlight and crushing them. In addition, the animals that eat the plants may also die due to a lack of food source.
Biologists from the
Sustainable Ecosystems Institute (along with others) have been studying the
forests and vegetation around Soufriere Hills to see changes caused by the
eruptions. One of the places they have been focusing on is the cloud
forest at Chances Peak. Doctor Deborah Brosnan and some of her colleagues
visited Chances Peak in late 1994 and found some of the nicest cloud forests in
the Caribbean. There was diverse wildlife, including plants and
animals. The group returned to the same area in January, 1996 to find that
the cloud forest was gone and almost all of the vegetation had been lost to acid
rain, gases, heat and dust.
Acid rain is being formed from sulfur emitted during eruptions of the volcano. Acid rain is formed when hydrogen sulfide and sulfur dioxide in the atmosphere go through a set of complex chemical reactions, eventually combining with water to produce sulfuric acid. These water drops are known as acid rain, and can damage or kill plants and animals. Acid rain can affect plants directly or indirectly. The amount of indirect damage plants receive depends partly on the soil composition. Typically volcanic rocks and thin topsoils are no match for acid rain and cannot filter out the sulfuric acid, thus failing to protect the plants in the area.
Marine environments can also be affected by acid rain. Scientists can measure pH levels to determine if water is very acidic. A pH of 7.0 is considered neutral and anything below that is acidic. When the pH of a lake falls below a pH of 5.0 the ecosystem is severely effected; fish, invertibrates, and plankton die. In January of 1996 Dr. Brosnan and her colleagues measured the pH of a lake at the top of Chances Peak at 2.0. They are not sure of the consequences on the area due to low pH levels, but they do not think that lakes or streams away from the crater will be affected because of their distance from the summit.
The group only found one kind of wildlife near the summit; hummingbirds. Hummingbirds defend their food sources and often get into territory fights with others. However, they are not sure why the hummingbirds are still in the area, or what they are using as a food source (since almost all the vegetation has been destroyed). This is just one of the many mysteries in nature.
Natural disturbances are part of the earth's processes and nature tends to adapt as disasters occur. New vegetation around Chances Peak can be found sprouting from the ground, but it does not survive due to the ongoing eruptions. When the eruptions are through the cloud forest and other vegetation will most likely fully recover and will flourish once again. Pioneer species will grow in the area first and will "prep" the area for new wildlife that will take over when the environment is suited to their needs.
Until the area is inhabited by new vegetation, erosion will be a problem. When rainstorms move through the area soil is transported from the mountainside to the surrounding river valleys and also ends up in the ocean. The deposition of large amounts of sediment in streams can cause runoff or could potentially act as a dam and block the flow of the river. Some of the sediment is being carried to the ocean and is affecting the coral reefs on the east and southwest part of the island. These reefs are being buried by sediments and ash. Sponges and other reef life can be killed if they are buried by too much sediment, or if they are buried for too long. The sediment can block sunlight which is imperative to all marine life in coral reefs.
To find out more on the impact of the Soufriere Hills volcano on marine and terrestrial life click on the link below. http://sei.org/impacts.html
All pictures from the studies done by Dr. Deborah Brosnan were taken from the SEI webpage. http://sei.org/impactsTN.html#anchor4534