Science at Work:
Volcanic Hazard Prediction at Soufriere Hills Volcano
by Stacy Clifford, April 1997
Volcanology is one of the few areas of science in which technical research toward a practical end can gain extreme public visibility. Millions of people around the world live near active volcanoes and sometimes even on their slopes. The power of volcanoes to destroy what humans have built and to kill thousands of people has been demonstrated repeatedly throughout history in such famous examples as Vesuvius, Tambora, Krakatoa, Mont Pelée, and Nevado del Ruiz. When a volcano in a populated area wakes from a long slumber, it is simply impossible to ignore. Part of the science of volcanology is to attempt to understand and be able to predict what an erupting volcano might do beforehand in order to save lives and property that might otherwise be destroyed. An excellent modern example of this process at work is Soufriere Hills Volcano on the island of Montserrat in the West Indies.
The Soufriere Hills Volcano had never erupted in historical times before 1995. In fact, the most recent eruption that can be verified by Carbon 14 radiometric dating was over 16,000 years ago. This does not mean that it would never erupt again, but that it was simply remaining dormant for a long period of time. Three times in the last hundred years, however, there have been swarms of earthquakes underneath the southern part of the island where the volcano lies, indicating that hot magma was moving toward the surface here from deep in the Earth’s crust. Geologists reasoned that if this were indeed the case, then there was a strong possibility that there could be an eruption in the next few decades to a century or two. Since it could just as easily happen sooner as later, a few geologists in the 1980s decided to try to assess the risks that might be faced by the people living on the island.
In order to make a guess about what the volcano might do in the future, they had to find out what it had done in the past by studying the rocks and ash deposited by old eruptions and the shape of the volcano itself. The name Soufriere Hills refers to the French word soufriere, meaning fumarole, which is where hot water, steam, and other volcanic gasses come out of the ground. The hills are a series of five lava domes around the summit where previous eruptions spewed up lava so viscous that it could not flow anywhere, so it simply piled up and hardened where it came out. The youngest lava dome is inside English’s Crater at the summit, a horseshoe-shaped caldera, or collapse depression, in which the eastern wall gave way and fell down the slope.
All of these lava domes are surrounded by low aprons of pyroclastic materials, formed when gases dissolved in the lava deep underground are released into the low-pressure environment of the atmosphere and cause the lava to froth up and explode like a well-shaken soda, scattering the quickly cooled fragments everywhere. The slopes of the volcano down below the domes were all formed by pyroclastic surges and flows, the boiling clouds of hot gasses and pyroclastic fragments that flow down the sides of the volcano, filling in the valleys without leaving much material on the ridges in between. There is not, however, much evidence for airfall deposits from past eruptions, which rain out of the sky from the tall eruption columns that spread upward instead of downward, leaving a layer of even thickness over everything. Mud flows, made of pyroclastics mobilized by water either in stream channels or from heavy rainfall, are also an important constituent of the volcano’s flanks.
Based on this evidence, the geologists reasoned that typical eruptions for this volcano meant the formation of lava domes with occasional to frequent explosions that did not eject material high into the atmosphere, but kept it close to the ground. These explosions could be caused by groundwater flashing to steam due to rapid heating, called a phreatic eruption, or by the sides of a lava dome breaking off as it grows and releasing pressure on the gassy magma underneath. Also, since all of the most recent eruptions occurred in about the same area, it made sense that there was a zone of weak, fractured rock underneath it where magma could move, so the most likely place for the next vent to occur was in the same area as the other five, and particularly in the most recent crater.
Once they had a reasonable model for what the next eruption might be like, the geologists could try to predict which areas around the volcano would be the most dangerous to be in if an eruption occurred. P.E. Baker of the University of Nottingham predicted in 1985 that by far the most dangerous place on the island during the type of eruption he had reconstructed would be the east side of the island below the open end of English’s Crater, where the Tar River flowed out. This was the easiest and most direct path for pyroclastic flows to move downhill. Areas to the northeast of the volcano and directly to the west over the back wall of the crater could also be at high risk.
Although the areas east and northeast of the crater have relatively few people living in them, directly west is the capital, Plymouth. The knowledge of where these potentially dangerous areas are is very important to the government of Montserrat, since it allows them to make plans for evacuation that will send people to safer parts of the island.
In 1988, G. Wadge of the University of Reading in Britain and M. C. Isaacs of the University of the West Indies, St. Augustine, Trinidad and Tobago published their own study of the volcanic hazards at Montserrat. They used computer modeling based on the same evidence to predict the hazards for more and less powerful eruptions within a certain acceptable range that would still produce the kinds of deposits seen in the rock record. They basically agreed with Dr. Baker that the Tar River valley area was the most dangerous, but they also included narrow strips along stream beds running down the other sides of the volcano that could easily be affected by mud flows. For the lowest-energy eruptions, this was all of the danger area. As the eruption models increased in energy, though, the danger area moved farther down the side of the volcano until it covered nearly the entire southern half of the island. Wadge and Isaacs also predicted that the real eruption would begin with phreatic explosions, followed by lava dome formation, explosive activity, and pyroclastic flows, followed eventually by a gradual decrease in the intensity of activity until the eruption ended.
The ultimate test of these scientists’ abilities to predict both the type of eruption and the dangers to the people began in July of 1995, when steam and ash began venting from English’s Crater. More vents opened up in the crater in late July and late August. On August 21, the first large phreatic eruption occurred, blanketing Plymouth with ash and causing about fifteen minutes of darkness. A lava dome began to form in the crater in late September, and on October 17 a mud flow emanated from the southeast side of the crater. Thirteen days later another large phreatic eruption occurred. So far, the eruption was following the general pattern that their work had forecast, although the phreatic eruptions may have produced somewhat more ash than expected.
The lava dome continued to grow through March 1996, by which time southern Montserrat had already been evacuated twice. Pyroclastic flows began occurring on April 3 and reached the sea below on May 12, Mother’s Day. Throughout the summer dome growth moved around to different parts of the crater and the rate of magma extrusion increased. As the sides of the dome oversteepened from the growth, parts of it would collapse and form pyroclastic flows. This again matched well with the geological models.
Around midnight on September 17, however, a series of dome collapses led to a large magmatic explosion, in which gasses escaping from the magma are the primary explosive mechanism. A plume of ash rose to about 14 km high and spread to the west, interfering with civilian air traffic. Rocks and pumice fell over most of the southern part of the island, with some pieces large enough to form impact craters 5 meters wide and 1.5 meters deep and hot enough to start fires and destroy a number of buildings. About one fourth of the dome was removed. This was more violent than had been expected, but more powerful eruptions had been accounted for in Wadge’s and Isaacs’ computer model, and the southern part of Montserrat was successfully evacuated after the initial blast.
During November and December the dome growth continued, filling in the scar left by the blast and more. The bulk of all the loose rock tumbling off the sides of the dome had begun to pile up against the southwest wall of the crater and large expanding cracks appeared, threatening to collapse the wall. To date there have been no reports of this actually occurring, but the idea causes great concern because it would allow pyroclastic flows to threaten more populated areas and significantly alter the conditions of the eruption model. This serves as an important reminder of the unpredictable nature of volcanic eruptions. The most recent reports from Montserrat indicate that the dome is now larger than ever before and is visible above the rim of the crater and still growing. Another violent eruption may be possible at any time.
Overall, these scientists seem to have been remarkably successful at predicting the general nature of the eruption, if not so much the specific details. Relatively little has been lost other than homes and property destroyed by the explosion and the people seem to be prepared to handle the disaster much better than in many other places around the world. This could be considered one instance in which scientific research has provided a great public service when it was needed most.