VOLCANICITY OF MARTINIQUE - GEOLOGICAL HISTORY OF THE PELÉE MOUNTAIN
- HISTORICAL ACTIVITY - THE VOLCANIC RISK OF MARTINIQUE - FUTURE SCENARIOS

Historical eruptions of the Pelée Mountain

It was in 1635 that the first Europeans established in Martinique and in particular at the site of the town of Saint-Pierre. The Pelée Mountain had just known an eruption, and a dome had risen in the summit crater from which a certain number of pyroclastic flows discharged into the surrounding valleys including the Rivière des Pères near Saint-Pierre. The vegetation was destroyed on the sides the summit zone of the volcano, from where probably came the name of Pelée Mountain that the first inhabitants gave to this volcano. Two phreatic eruptions (without magma flow to the surface) occurred in the 18th and 19th century. The first, in 1792, generated some explosions which affected only the summit zone of the volcano and did not disturbed the habitants of the north part of the island or those of Saint-Pierre. The second, in 1851, was preceded by fumaroles activity at the Etang Sec crater for several months. When the phreatic explosions occurred, they were more violent than those of 1792 and in two or three of them ashes fell on the cities of Prêcheur, Morne Rouge and Saint-Pierre.
Following the eruption of 1851, fumaroles activity continued in the summit zone, gradually declining and disappearing completely after four years.

Volcanic cloud.

The eruption of 1902-1905:

We can consider that the first signs of reactivation of the Pelée Mountain occur in 1889, with the appearance of fumaroles at the Etang Sec summit crater. But it is only in 1900 and especially at the beginning of the year 1902, when the number and the intensity of the fumaroles increased regularly, culminating in April 23, 1902, when the first phreatic explosion occurs (Lacroix, 1904; Christian and Bush, 1990). Many phreatic explosions followed one another involving abundant ash fallout on the western side of the volcano. On May 5, the lake occupying the summit of the Etang Sec crater was drained into the white valley after the rupture of its containing walls, carrying the ashes accumulated on the side of the volcano. This produced a lahar that destroyed the Guerin distillery located at the mouth of the river and makes the first 23 victims of the eruption. During the night from the 5 to June 6, the incandescence at the top of the volcano signals the arrival of magma to the surface. A lava dome starts to build in the crater.

Saint-Pierre, before the eruption.

Saint-Pierre, after May 8, 1902

On May 8, at 08h 02 AM, a violent explosion occurred at the top of the volcano. The testimonies mainly collected by Alfred Lacroix (Lacroix, 1904) and the detailed study of the deposits (Boudon and Lajoie, 1989; Boudon et al, 1990; Bourdier et al, 1989; Lajoie et al, 1989) make possible to reconstitute the eruptive phenomenon.
The explosion was consequence of the retaining followed by the brutal relaxation of the magmatic gases contained inside the dome and at the top of the supply conduit; it is a surface blast. It occurs at the base of the dome and is directed laterally towards south-west. The direction of the explosion is due to: -the existence of a weakness zone in this part of the building, - a probably oblique orientation of the supply pipe in the summit zone - the presence of a deep breach in the western part of the crater of the Etang Sec

All this contributed to the side direction of the explosion and the concentration of energy in this direction. The explosion occurred with a very broad aperture, about 120°, and generated a pyroclastic flow - mixture of ashes, rocks and gas - extremely diluted and turbulent, moving at very high speed - about 120 to 150 m/s - and very destructive. It is what is called a "Pelean volcanic cloud". It traversed the slopes and reaches the town of Saint-Pierre in less than one minute, killing 28 000 people present except for two survivors. The height of the flow could be estimated, taking into account the limits of the destruction, to 190 m. The temperature of the volcanic cloud, very low, probably did not exceed 200 or 250°C, mostly due to the incorporation of air in the flow. The habitants were killed by mechanical effects and especially by burns and asphyxiated by ingestion of gas and hot ashes.

A total of 7 comparable volcanic clouds followed one another until August 30, 1902 all of them reaching the town of Saint-Pierre. We can cite the one on May 20, that completed the destruction of the city -as testify the numerous quarters entrapped in the deposits- and those of May 26, June 6 and August 30. The latter, directed at the same time towards the west and the south, destroyed part of the city of Morne Rouge, increasing by 1000 the number of victims. This volcanic cloud attained the paroxysmal phase of the eruption. A total of approximately 30 000 people perished.

Geographical distribution of deposits from the 1902 eruption.

The volcanic activity continued until the beginning of 1905. The growth of the viscous dome of lava was punctuated by the collapse of unstable parts generating pyroclastic flows of another type (flows of ashes and blocks called "volcanic clouds of avalanches"), with less energy than the precedents, which run out into the White valley filling it partially. It is during this period that at the top of the dome grew a needle, piston of viscous lava, that reached 350 m height before collapsing completely (Lacroix, 1904). We can presume that its diameter, of around fifty of meters, corresponds to that of the supply pipe.

The volcanic spine, called the 'Needle of Pelée'

The 1929-1932 eruption

After the crisis of 1902-1904, a new eruption took place between 1929 and 1932. Again an important phreatic activity precedes the arrival of magma to the surface. When this last reached the top of the volcano, it built a new dome of viscous lava, next to that of 1902. The explosive activity, less violent this time, does not generate any Pelean volcanic cloud. The dome continues to grow and become unstable, exploding regularly and causing volcanic clouds of avalanches. As previously, they run down into the valley of the White River and finish filling it. Following this eruption, the fumaroles activity in the Pelée Mountain declines slowly. The last fumaroles localized between the two domes, disappear in 1970.