VOLCANICITY OF GUADELOUPE - HISTORICAL ACTIVITY - THE VOLCANIC RISK IN GUADELOUPE - FUTURE SCENARIOS

Future scenarios

The eruptive scenarios for Soufrière of Guadeloupe

The risks at la Soufrière of Guadeloupe are in general of similar nature to those of the Pelée Mountain in Martinique. If the eruption of 1976-1977 was finally benign, independent of the human consequences and economic disorders, it made the authorities aware of the importance of the monitoring networks of the volcano. Thanks to several research programs, a better geological knowledge of the massif of la Soufrière (Grande-Découverte, la Soufrière, massif of La Madeleine) and a more precise reconstitution of the last activity of the volcano we can now identify the main types of eruptions that could take place in the future.

The different types of activity are in fact composed of a sequence of volcanic events (inductive hypothesis) some of which can be common to several eruptions at a given stage of a volcanic crisis. For example, since 1995, the eruption of Soufrière Hills in Montserrat had several types of eruptive activity, most very different in their nature, intensity, and the associated risks for the residents. We can draw the following eruptive scenarios:

1. Phreatic eruptions

Example: Soufrière 1680, 1797-1798, 1836-1837, 1956, 1975-1977
Frequency: 20 to 50 years

They are the most frequent, at least on the last 500 years of activity of Soufrière. It should be précised that any magmatic eruption will also begin with a phreatic phase.

2. Eruptions with erection of a viscous lava dome

Example: Soufrière 1440 AD
Frequency: 500 to 1000 years

Can be accompanied by pyroclastic flows of various nature ("volcanic clouds" whose distribution could be controlled by the relief). For approximately 3000 years, there have been at least 3 eruptions at Soufrière with formation of a dome and pyroclastic flows. The last dates back to approximately 1440 AD and led to the formation of the current dome of Soufrière of Guadeloupe.

3. Eruptions with side collapse

Example: Soufrière 8500 BC, 3600 BC
Frequency: 2000 to 5000 years

Research in progress have suggested a frequency of at least 5 events of this type every 10 000 years, accompanied by the creation of products known as "debris avalanche" of low volume (a few hundreds of million m³) on a zone of 40-60 km² extending from Soufrière to the Caribbean coast and mainly in the old valleys draining the slopes of the volcano. At least one of these eruptions (approximately 3100 years ago) was associated to magmatic activity accompanied by a catastrophic side explosion that destroyed a zone of 60-100 km².

4. Explosive eruptions with formation of cinder cones

Example cones: Echelle, Citerne
Frequency: 5000 to 10 000 years

This type of eruptions was not very frequent in the history of the Soufrière massif but the scoria cones of Echelle and Citerne were formed approximately 1500 ago to 2000 years.

5. Effusive eruptions with lava flow

Example: Plateau du Palmiste
Frequency: 10 000 to 20 000 years

This type of eruptions was very frequent in the erection of the Soufrière volcanic massif and of Madeleine until approximately 15 000 years ago.

6. Catastrophic eruptions (Plinian)

Example: Formation of Grande-Découverte - Soufrière
Frequency: 50 000 to 100 000 years

Characterized by explosive emission of several km³ of magma in the form pumice ejection volcanic ash, and pyroclastic flows covering several km², it is by far the type of eruptions that can have the most catastrophic consequences. Several eruptions of this type are known since the beginning of the formation of the volcanic massif of Grande-Découverte-Soufrière more than 200.000 years ago. The last of this eruption, of an age range between 100 000 and 40 000 years, emitted the pumice deposits of Pintade, achieving a volume of several km³ and distributed all around the Soufrière massif

On the last 10 000 years of activity the eruptions of the type 1, 2 and 3 were the most frequent.

The eruption taking place in Montserrat since 1995 is very representative of the various types of activity which are likely to occur at Soufrière of Guadeloupe in the event of reactivation (phreatic explosions, installation of lava domes, side destabilization). This shows that during the same eruption, various types of activity can follow each other and that it is important to define scenarios taking into account these various types of activity.