In 1902, Mt. Pelée had three major eruptions, on the dates of 8 May, 20 May, and 30 August, all of which released powerful pyroclastic flows and surges. The figure below shows a map of the region surrounding Mt. Pelée, where a heavy line marks the "zone of devastation" for the 8 May eruption (the May 20 eruption did not surpass this boundary) and a dotted line marking the increase in the zone resulting from the eruption of 30 August. [LaCroix 1904, p. 223]:
|A map of the region surrounding Mt. Pelée. The heavy line marks the "zone of devastation" resulting from the 8 May and 20 May eruptions of 1902. The dotted line marks the expansion of the zone resulting from the eruption of 30 August. [LaCroix 1904, p. 223].|
Comparing this figure with the isopach map of ash deposits (An isopach is a contour marking constant thickness of a geologic formation), shown below [Fisher 1982, p. 344], reveals the respective role of pyroclastic surge and flow in the May eruptions.
|A map showing the extent of ash deposits resulting from the combined effects of the 8 May and 20 May eruptions of Mt. Pelée. The dotted lines indicate contours of constant thickness of the deposit measured in meters. [Fisher 1982, p. 344].|
The isopach map reveals that the dense deposits of pyroclastic flow are limited to the valley surrounding a river that ran south-west from the crater, while the ash deposits of pyroclastic flow are much more widespread because of the greater motility of the less-dense surge clouds. Although the city of St. Pierre was near the fringe of the surge area affected by the 8 May and 20 May eruptions, with deposits ranging from 1 m (3.3 ft) to zero, it was nonetheless completely destroyed. The 8 May eruption resulted in the death of all but two of the City's estimated 28,000 residents [Sigurdsson 1982, p. 41]. The images below show overall views of St. Pierre before and after the eruptions. The second image, taken in 1904, shows the combined effects of the 8 May, 20 May, and 30 August eruptions of 1902.
|The city of St. Pierre, before the 1902 eruptions. [LaCroix 1904, p. 232].|
|The city of St. Pierre, after the 1902 eruptions, June 1904. [LaCroix 1904, p. 233].|
Looking at a more local scale, the image below shows a before and after image of a theatre building. In addition to the complete destruction of the building, note the defoliation of the area behind the building and the small building in the background in the upper right, which lost its roof in the eruption. The thin deposit of ash reflects the relatively low concentrations of solid material in a pyroclastic surge.
|Before and after images of a theatre building in St. Pierre. Note the defoliation of the hillside in the background. [LaCroix 1904, p. 247].|
Despite its stout appearance, this building probably had relatively low resistance to the lateral loading induced by pyroclastic surge. The front wall is very open, and the interior probably included a large span space with few internal walls.
|Next: Mt Pelée: Comparing the 8 May and 20 May Eruptions|