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19 January, 2001
Dominica's Volcanic Past and Present

Roseau, capital of Dominica, has been identified as lying within a potential zone of hazard in the event of a volcanic eruption at Morne Canot near Bellevue Chopin, part of the Morne Plat Pays volcanic complex.

This information is contained in a paper presented at the University of the West Indies, Dominica Country Conference that was held at the Fort Young Hotel from 8-10 January 2001. Dr. Shepherd, Director of the UWI Systemic Research Unit at St Augustine, Trinidad, presented a detailed report on the matter to members of Cabinet on Friday 5 January. He then presented it to the conference on 9 January. Other authors of the paper are Jan M. Lindsay, and Mark V. Stasiuk, also of the Seismic Research Unit.

The paper, entitled "Volcanic Hazards in Dominica" explains that Dominica has one of the highest concentrations of live volcanoes in the world. A major volcanic eruption affecting Roseau is more likely than not to occur within the next century. The paper discusses the probable effects of such an eruption. Dominica has one of the highest concentrations of potentially active volcanoes in the world. It is the only island in the Eastern Caribbean that has more than one major volcano. In fact it has eight volcanoes and six of these are located in southern Dominica within 10km of the capital. Plat Pays, Anglais, Micotrin, Trois Pitons, Watt, Valley of Desolation and Grande Soufriere Hills have now been confirmed as " live". All are possible locations of future eruptions.

SWARMS

During the last two years there has been continuous monitoring of earthquake swarms using a nine-station seismograph network. Seismographs are instruments that are placed at different points around Dominica to record even the mildest of quakes. By correlating all of the signals recorded by all of the seismographs, the scientists can identify the source of the earthquakes. In this way they now know that swarms are reaching closer to the surface, producing stronger earthquakes, and lasting longer with each re-occurrence since the 1960s. This movement, or seismicity, is happening at a depth of one to five kilometres beneath southern Dominica and it reflects magma movement. Magma is rock material in a melted state. It is made up of silicates (silicon, oxygen and metals), water and gases at high temperatures. An eruption occurs when this material blasts out onto the surface.

South Dominica is known to have experienced at least nine intense swarms of volcanic earthquakes during the last 100 years. The most recent major earthquake swarm occurred between 1998 and early 2000. This was associated with the Morne Plat Pays and Anglais volcanic centres. This was the most energetic swarm ever and it approached the surface more closely than any other swarms in the last 50 years. The scientists are watching this area carefully, for with each new swarm the chances of a major eruption becomes ever more likely.


ERUPTION

The Seismic Research Unit's report makes it clear that, based on all of its findings, the most likely next magmatic eruption is a dome eruption (one similar to Montserrat) in the Bellevue Chopin-Point Michel-Loubiere area. This could occur at any time within the next one hundred years. With every passing year from now, the likelihood of such an eruption rises. This eruption may form a new dome, or reactivate an existing dome.

The most likely next hazardous volcanic event is a phreatic explosion in the Valley of Desolation. Phreatic eruptions occur when water, trapped beneath the surface, is heated to form steam under pressure. This then explodes. Such eruptions eject jets of hot steam, old rock debris and ash into the air. In the case of the Valley of Desolation the affected area will only be forest since the area of high hazard expected is only within a radius of two kilometres.

The annual report of the Unit for the year 1999-2000 also discloses that preliminary versions of eruption scenarios and hazard maps for the five most likely magmatic eruption cases has been completed. A preliminary volcanic hazard map for southern Dominica has also been done.

HAZARD MAP

At the conference Dr. Shepherd showed the audience the preliminary eruption scenario map for an eruption centred on Morne Canot. The zone of greatest danger extends from the dome in a circle encompassing the south of the island in a radius of some five kilometres or just over three miles. This would take the extent of serious danger to a point in central Roseau, south of the Roseau River. Taken together with the environs of Roseau, particularly to the south of the capital, this zone houses some thirty-percent of the island's population.

In the south, this danger zone would extend to Soufriere including Pointe Michel and around the south coast to Grand Bay and the Geneva Valley. To the north it takes in Giraudel, Eggleston and all of the Roseau Valley. Beyond that, the zone of ash and other possible lesser hazards could extend to Canefield in the area of the airport and out to sea around the south of the island. Bagatelle to Petite Savanne could be protected in such a situation because the Foundland or Pedi Temps mountains shelter them.

In effect, however, a rough line can be drawn across the island from Canefield to La Plaine. It would be likely that everything south of that line would be affected to a greater or lesser extent. This would depend on the power of the eruption and on what particular type of eruption takes place. The related hazards in this area would continue for 10-50 years after the start of the eruption.

Dr.Shepherd assured his audience that some of the best monitoring facilities has been installed on Dominica. The equipment and scientific expertise that is being focused on the island will enable warning to be given of any disaster. It is most likely to occur within this century. But one thing that cannot be determined exactly at this time, is just when such an event will actually take place in the years ahead.
The question therefore is not " if" but " when".

Seismic Activity in our History

The swarm of tremors that were felt during October 1998 was related to seismic activity in the southern part of the island. It created great concern among residents of the area and several persons have sought an opinion on the situation. Not being a seismologist or volcanologist I am very cautious. It would be folly to make uninformed
assumptions. However there is the concern that Dominicans should be made more aware about the formation and geological composition of their island home. It is part of a wider concern about the lack of general knowledge
about Dominica as a whole.

In so many basic areas such as agriculture, natural history, climate, environmental studies and local geography, we have not been prepared for life on the island of our birth. When natural phenomena occur therefore we
do not fully understand them. The scientific terms which were being used by the scientists in their meeting with the villagers in the affected areas showed that the language communications gap seemed very wide indeed. But
this should not detract from the painstaking and invaluable work which these volcanologists perform on our behalf for they have to tread a delicate balance between simplying the information while at the same time
maintaining their scientific accuracy without causing undue alarm.

From an historian's point of view I have been collecting as many reports as possible on seismic activity recorded on Dominica during the historic period as well as the various scientific studies undertaken, particularly
over the last one hundred years. It was in this spirit that the GIS television team of Felix Augustin, Anthony 'Brother Me' Richards and I got together to produce a feature on the seismic situation some weeks ago. The
displays which can be seen at the Dominica Museum on the Bayfront provided a useful backdrop to this presentation and I would encourage anyone who is interested to visit the Museum to view these maps and dioramas.

It may be some reassurance to know that such "swarms" of earth tremors recently associated with the south have been recorded at several periods during our history: in 1765, 1841, 1849, 1893 , 1937-1938 and 1986.
Instrumentally recorded swarms (with a few felt shocks) have occurred in 1967, 1971, 1974, 1976, 1986. The recent experience of the volcano on Montserrat has certainly been responsible for heightening our tension this
time around. It may also be of interest to note the effect which the earliest recorded swarm of tremors had on the population, as recorded by Thomas Atwood in the eighteenth century. Their reactions to the tremors in
1765 appear somewhat similar to that of Dominicans a couple weeks ago:

"It is asserted by some of the first inhabitants, that earthquakes happened here formerly very frequently; especially soon after the English first took possession of the country; when they were felt severely, several times in a
day, for the space of some weeks together, which so terrified the inhabitants, that they were on the point of quitting the place, but happily they soon subsided. These people say likewise, that although no material
damage happened at that time, yet that the island was split in several places; and in particular a large chasm was made in a mountain there called Desmoulins, so very deep that ...they were unable to fathom it."

A French archaeologist, investigating an "Arawak" site at Soufriere some years ago, found one layer of clay pots lying below a layer of volcanic ash. This indicated that an eruption had occurred while Amerindian people
had been living there, before, or at about the time of, Columbus' arrival. This may fit in with the findings of scientists, Roobol, Wright and Smith, that the last eruption of Morne Patate took place 450 years ago (give or
take 90 years). Hence we find the information of two different disciplines: Archaeology and Volcanology, complimenting one another.

An engraving of the Boiling Lake made in the 1880s
An engraving of the Boiling Lake made in the 1880s


It is thanks to the investigations of the eminent "uncrowned king of Dominica" Dr.Henry Nicholls in the last century, that we have such a detailed report of the eruption of 4 January 1880 which emanated from the area of the Boiling Lake and Valley of Desolation. It was a "phreatic" eruption, caused by the contact of ground water with hot rocks. There is no evidence that molten rock (magma) was involved. The eruption sent up a plume of ash which was blown by the wind down the Roseau Valley and over the town to the consternation of its inhabitants. Dr. Nicholls collected some of this ash that can still be seen at the Museum today.

Although the first chapter of my "The Dominica Story" history book also contains text and diagrams explaining the volcanism of our island, not everyone is convinced. A couple years ago, I was in conversation with a teacher at one of the fundamentalist religious schools. He told me that although he appreciated the book as a whole, he never taught chapter one, or took it seriously, because my account of volcanic formation did not conform to The Creation as detailed in the Book of Genesis. In such situations I never argue, I only quietly regret the self-imposed barriers which some people place on the scope of their knowledge. I also always recall the persecution of the great astronomer Galileo Galilei by the Vatican, when he showed that the Earth was not the centre of the universe but that it moved around the Sun. The fact is that we live on a volcanic island on the edge of one of the many "tectonic plates" which make up the earth's crust. In geological time we are a "young" island and, as in the past, we will continue to be subject to this activity at varying levels of intensity as our island continues to grow.


The Stages of Dominica's Formation over 25 million years.
Eruption piled upon eruption to build the island.

1. Eruptions first occurred around Morne Concorde, Morne Fraser and Morne
au Delices - these have left the oldest exposed volcanic rocks along the
Windward Coast and are estimated to be about 13 -25 million years old.
2.Next is Morne Couronne between 1 -12 million years ago.
3.Next, the arc of mountains: Morne Anglais, Morne John, Morne Watt to
Grand Soufriere Hills
4.Morne Negres Marron grew up beside Morne Couronne within the last one
million years.
5. Eruptions centred on Foundland peak (Pedi Temp) built up the southeast
corner of Dominica at about the same time.
6. Early magma from Trois Pitons formed coastal areas from Mahaut to Loubiere.
7.Early magma from Morne Diablotin produced the remaining coastline from
Mahaut to Morne Espagnol.
8.Plat Pays centre was next to erupt.
9. Morne Anglais erupts again producing pyroclastic fallout and flows over
a wide area in the south central zone.
10. Eruptions centred on Morne Aux Diables less than half a million years
ago created the northern headland of Dominica.
11. The peak of Morne Micotrin (Macacque) formed and produced pyroclastic
flows and fallout that cover the Roseau valley. Roseau is built on this
flow.
12. Morne Trois Pitons erupts again, forming the present enormous land mass
covering parts of zone 7 and zone 6 in the west as well as towards Rosalie
and Castle Bruce in the east.
13. Ash covered the Mang Peak area.
14. Morne Diablotin erupted again over 40,000 years ago and produced a
gigantic ash flow covering the entire north central zone to the north and
northwest coast.
15. Morne Patate has been the last to be formed, of which the partially
weathered crater remains today. It last erupted about 450 - 540 years ago.
Sources: D.Lang (1967), J. Tomblin (1970), G.Wadge
(1985).





 

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