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  1. GEOLOGIC BACKGROUND

 

Every kind of natural disaster has its particular characteristics. As I said before, a natural disaster is a not human caused disaster, and it could be geological, but also meteorological, biological or astronomical. Mt Pelée's disaster was caused by a volcanic eruption, and it is the worst volcanic disaster since the beginning of the twentieth century. It occurred on May 8, 1902, into the northwest coast of the French island of Martinique, and its principal target was the town of St. Pierre, with a population around of 25 thousand people. The Martinique Island is part of the Lesser Antilles islands, an arc of islands in the Caribbean Sea. This island is located on a typical subduction zone, in this case where the Caribbean Plate meets Atlantic Oceanic crust belonging to the South American Plate and, like most islands in the Lesser Antilles, Martinique was built by volcanoes. The main cause of this disaster was the pyroclastic flow expelled during the eruption. Pyroclastic flows are fluidized masses of hot gases and rocks (known as tephra) that move rapidly in response to gravity, and these can reach temperatures of about 1,000 °C (1,830 °F). The main factor of the damages caused by pyroclastic flows is the heat, but there are others factors that also contributed to the high destruction that this kind of phenomena provokes, among them its velocity of displacement. In the 1902 eruption of Mt Pelée, the initial speed of the cloud of pyroclastic surge that hugged the ground and sped down towards the city of Saint-Pierre, was calculated over 670 kilometers (420 mi) per hour, and the specialists say that this was the first factor that began the destruction, before the heat, the poisonous gases, ashes and others. Saint-Pierre was located about 6.4 kilometers (4.0 mi) south of the summit, but because of the high speed of the pyroclastic flow, it reaches the town in less than one minute. Based on the records, had three stages of unequal duration: intensification of fumarolic activity (January to mid-April); thirteen days of phreatic eruptions (23 April to 5 May); and magmatic eruptions during the last 2 days before the cataclysmic pyroclastic flow. The scientists are not yet sure what exactly happened during the process of formation of the pyroclastic flow. Some say the eruption emerged through the newly formed lava dome and moved out of the lowest point of the crater, which faced St. Pierre, but others say a massive column first rose, and then collapsed under its own weight. This eruption, as I said before, completely destroyed the town of St. Pierre. It also killed around of 30 thousand of people (only two survived) and the devastation in the ecosystem covered an area of about 8 square miles (21 km2). Two of the local conditions that helped the occurrence of this disaster and increased its effects were, besides being located in a geologically active zone, was the proximity of the town to the volcano, and the steep slope of the mountain towards the side of the town, which increased the speed of the pyroclastic flow. Volcanic eruptions are associated with many hazards that these can cause. Among of them we can refer the already mentioned pyroclastic density currents (pyroclastic flows and surges), lahars, structural collapses (debris flows or avalanches), dome collapses, lava flows, tephra falls and ballistic projectiles, volcanic gases, tsunamis, and volcanic lightnings. In the case of the Mt Pelée eruption, some explosions occurred, minor earth tremors shook the town of St. Pierre, and the town was soon covered in ash and sulfuric gas. Insects and snakes moved down from the towering volcano.  Animals were bitten by red ants and around 50 people and 200 animals died from snake bites alone. A crater lake, known as Etang Sec, was heated and its water began to boil.  This crater rim collapsed on May 5 and created a gigantic mudflow destroying everything in its path and killind at least 150 people which were buried under a layer of mud between 60 and 90 meters thick. Also, when this flow reached the sea, it created a 3 meter high tsunami which crashed into the waterfront of St. Pierre. Human behavior influenced in many ways with the occurrence of this disaster. First, because of the construction of a human settlement in a high risk zone. But, in this specific case, maybe the most important factor was the unconcern, due to the ignorance on the subject, disinformation, and mainly the politic interests. There were elections in those days, and the government convinced the many panicky citizens considering leaving St. Pierre to stay put. The governor released a report saying that the safety of St. Pierre was completely assured. Also troops forcibly retained those who tried to leave, sentencing thousands of people to certain death. A scientific commission was reunited and finally determined that there was not any threat about the volcano. When the monstrous wave of red-hot gas, rock, and ash burst reaches the town, the residents of St. Pierre were heading for church to celebrate Ascension Day on May 8. Everything was obliterated in its path and most people were cremated instantaneously.

Tag(s) : #Geology, #Pelée, #Martinique, #Pierre, #Volcano, #disaster, #eruption, #volcanic eruption
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