Mount Pelée

Mount Pelée, located on the island of Martinique in the West Indies, gained notoriety on May 8, 1902, when all but two of about thirty thousand people in the town of St. Pierre were suddenly killed by the eruption of a hot cloud of ash, rock fragments, and gases called a nuée ardente—the first time such an eruption was recognized.

Nuées Ardentes

Mount Pelée, on the island of Martinique, is one of many volcanoes located along the sinuous arc of islands of the Lesser Antilles in the West Indies. One oceanic plate is being thrust or subducted underneath another oceanic plate along the Lesser Antilles, resulting in the generation of silica-rich magma such as andesite and dacite. The silica-rich magma is very “stiff,” or viscous. Therefore, it will not readily flow up the conduit of the volcano, which may result in a plug of magma in the volcano crater that prevents the escape of magma and gas. Therefore, the gas pressure inside the volcano may build until it explodes at its weakest point.

The explosions of this type often blast horizontally below the volcanic plug, resulting in an exceedingly hot cloud of glowing ash (fine dust), rock fragments, and gases called nuée ardente, which means burning cloud in french, that cascade rapidly downhill. Initial temperatures may be approximately 1,200 degrees Celsius (2192 degrees Fahrenheit), but the temperature may rapidly drop off as the nuée ardente moves downslope. The rapid expansion of gases after eruption is believe to cause much of the cooling, but the nuée ardente of May 8, 1902, at Mount Pelée was still hot enough to soften glass (about 700 degrees Celsius, or 1292 degrees Fahrenheit) eight kilometers (five miles) from the volcano. The speeds of nuées ardentes average 100 kilometers (62 miles) per hour. Lava does not extrude during a nuée ardente eruption.

This type of eruption was first recognized on May 8, 1902 when Mount Pelée erupted, instantly killing all but two of nearly thirty thousand people in the town of St. Pierre. The nuée ardente eruption is therefore often called a Peléan type of eruption. The journalists and scientists who descended on the area shortly after the eruption recorded many fascinating eyewitness accounts.

Warning Signs

The many signs of unusual volcanic activity that occurred prior to May 8 are now recognized as warnings of an imminent eruption. The emission of steaming gases was first noticed on April 2 in the upper portions of a stream draining off of Mount Pelée. There were minor earthquakes and an odor of sulfur, and ash was noticed in St. Pierre on April 23. Explosions in the cone of the volcano threw ash and rocks into the air on April 25. People who climbed up to the crater on April 27 said that it contained a small cinder cone, a lake, and columns of steam that were not previously noticed. The fine particles of dust, called ash, thrown out of the volcano during loud explosions gradually became heavier and eventually blocked some roads and forced residents indoors. Many residents left St. Pierre in fear. Many from the countryside, however, migrated to the city. As a result, the population of the city was much larger than normal by early May. Authorities issued assurances that residents were in no danger and that they should not panic.

Volcanic Weather

Often, the hot updraft around active volcanoes produces thunderstorms. This often results in a large amount of water suddenly mixing with the abundant volcanic ash. Heavy rains occurred on the southwest slope of Mount Pelée on May 5, which resulted in water that was ponded in one of the craters bursting through the crater walls and forming a scalding torrent that rushed down the river valley at speeds of up to 90 kilometers (56 miles) per hour. As the mudflow, carrying huge trees and boulders, hit the ocean, it produced a large wave that overturned boats and washed up on the low-lying areas of St. Pierre. Residents of the lowlands panicked and fled to higher areas, which caused more of an exodus from St. Pierre. Explosions on May 6 were quite loud, and the authorities had to block the roads to keep people from leaving the city. The newspaper and governor issued assurances that there was no cause for alarm. By May 7, there were continuous downpours and clouds of ash that produced muddy floods in the swollen streams.

Violent Blasts, Sudden Death

Then, on May 8, the volcano erupted with four loud blasts. One black cloud full of lightning rose upward from the crater. A nuée ardente moved rapidly down the volcano, over the town of St. Pierre, and out over the ocean, killing nearly everyone in its path within two minutes. People died suddenly either from burns or from breathing the heated gas and ash. Houses lost their roofs, and walls one meter (three feet) thick were ripped apart. Most of the ships in the harbor were destroyed. The two ships that were not capsized had a few survivors who were in inner cabins with the doors closed. There were two survivors in St. Pierre, including a prisoner in a windowless cell. A small grate in the door had allowed enough heat into the cell to burn parts of his body. The second survivor was located in an inner room of a house in which all other occupants died. Much of the city had burned after the initial blast and the city itself resembled an ancient ruin. Foundations of houses were left standing mixed with mangled bits of metal and furnishings. Everything was covered with a mantle of ash nearly one meter (three feet) thick. A few tree trunks remained, devoid of their bark and scorched by the heat.

Continuing Eruptions

Mount Pelée continued to erupt more or less continuously for several hundred days. Nuées ardentes were observed on May 20, June 6, July 9, August 30, and December 6. The nuée ardente of August 30 moved down a different slope and killed several thousand people. The amount of ash given off in these continuous ash eruptions was prodigious. One conservative estimate of the quantity of ash given off in twenty-four hours was that it was about equal to one and one-half times that of the sediment carried by the Mississippi River in one year. Brilliant afterglows and modified sunsets were observed for several years after the eruptions. Deflections of the earth's magnetic field related to a volcanic eruption were observed for the first time.

A giant obelisk, or spine, of stiff lava gradually projected upward from the crater of Mount Pelée. This spine first appeared in photos taken in June 1902, and it pushed upward out of the summit for more than one year before it broke up and shattered. The obelisk reached its maximum height of about 330 meters (1083 feeet) above the crater floor and about 160 meters (525 feet) across at the base in June and July of 1903. It continued to crack and break at the top as it continued to grow in irregular spurts at rates of up to 17 meters (56 feet) per day. The obelisk was reduced to a height of approximately 160 meters (525 feet) by August 1903. The continued eruptions from August to November reduced it to a pile of rubble.

Mount Pelée was quiet until September 1929. Then, it erupted in a pattern similar to its previous eruptions. Fortunately, the one thousand people living in St. Pierre were evacuated at the first signs of volcanic activity, so there was no loss of life. During this period of activity, there were a number of nuées ardentes during the early stages of eruption. None of them, however, were as violent as those that occurred in 1902. As the volcanic activity waned, a new spine grew in the crater near the old spine and gradually crumbled. The new spine never reached the height of the 1903 spine.

Similar Eruptions

The volcano La Soufrière exists on St. Vincent Island, 150 kilometers (92 miles) from Martinique. It also erupts a silica-rich magma and has a pattern of eruption similar to that of Mount Pelée. La Soufrière erupted in 1718, in 1812, only one day before Mount Pelée in 1902, and in 1971. The eruption of La Soufrière on May 7, 1902 had a much smaller and more vertical initial blast than that of Pelée because the blast from La Soufrière occurred through an open vent rather than one that was blocked. The resultant cloud, consisting of sand-sized particles, descended down the slope induced only by gravity. The speed of the flow (about 40 kilometers per hour, or 25 miles per hour) was not nearly as great as at Pelée. The initial destruction of buildings in this area was much less than that caused by Pelée. Fewer buildings caught on fire than at Pelée because the cloud was much cooler from its longer contact with the atmosphere. The area covered by the cloud was much larger at La Soufrière than at Pelée because of the vertical nature of the eruption. The people on one side of the mountain in the town of Soufrière noticed that the volcano was erupting, and most evacuated. Clouds covered the other side of the volcano; the people there did not evacuate in time. Approximately two thousand people lost their lives. This variation of a nuée ardente is sometimes called the Soufrière type of eruption.

Another nuée ardente occurred at the Merapi volcano in Java. Here, a dome or spine built up much like that at Pelée, except that the dome extended out beyond the rim of the crater. The spine would periodically break off, sending a nuée ardente down the side of the volcano. There nuées ardentes moved quickly, like those at Pelée. This variation of a nuée ardente is also known as the Merapi type of eruption.

Study of Mount Pelée

Information about the nature of volcanic eruptions such as Mount Pelée's comes from two types of observation. One is the visual observation of the eruption while it is occurring. The second is the examination of the deposits of erosional features that result from the eruption. These two types of observation were made during and after the eruption of Mount Pelée.

The stiff spine that protruded from the volcanic vent at Mount Pelée has been seen at other locations that also have silica-rich and viscous magma. It is believed that the viscous magma plugs the vent of the volcano in these types of eruption. The gas pressure builds until gas-rich magma is suddenly released at a low angle. During the sudden decompression, the magma froth—broken up into ash, fine dust, or large rock fragments—discharges like a cannon. Observers report deafening explosions during these eruptions.

In a 1930 eruption of Pelée, one trained observer, standing only 35 meters (115 feet) away from the eruption, confirmed that a lateral explosion did occur. In lateral explosions, the solid particles and magma are believed to release water vapor rapidly and continuously. In this way, the magma freezes, so little or no magma is left after the eruption's initial release of pressure. Much of the flow's initial speed is a result of this lateral blast. The lower portion of the gas-particle mixture takes on speed and moves rapidly and fluidly down the low portions of valleys on the cooler cushion of air beneath it.

Thus, nuées ardentes behave much like running water or avalanches, except that they move faster. The continual release of hot water vapor from the particles may cause rapid expansion and compression of the gas-particle fluid and add to its downslope speed. Some of the dust-sized particles rapidly escape upward from this moving gas-particle fluid and rise in a glowing, black cloud. These glowing avalanches erode gullies along the steep flanks of the volcano and scratch rocks parallel to their flow. Little material is deposited until the avalanches reach the lower slopes of the volcano and slow down. The deposits usually consist of a random mixture of various-sized particles—from fine ash to blocks several meters wide. Apparently, there is enough turbulence to keep the particles stirred up. Some deposits, however, have coarser material concentrated at the bottom, with the finer material near the top. A thin layer of dust sits atop the main deposit as this material slowly settles from the overlying ash cloud.

Geologists who study volcanoes observe the eruption processes and magma compositions as they are ejected at the surface. One volcanologist, Frank A. Perret, set up an observation shed on the slopes of Mount Pelée during the 1929 eruptions near the area where a number of nuées ardentes descended down the slopes. The edge of one of these flows hit his shed and nearly killed him. He survived to publish his observations and conclusions on the flow mechanisms of nuées ardentes. His theories provided the basis for the first real understanding of these processes.

In 2020, the volcano was labeled as restless due to increased seismic activity beneath its surface, marking the first activity since the eruptions in the 1900s.

Principal Terms

andesite: a volcanic rock of intermediate silica content that contains plagioclase (a calcium, sodium, and aluminum-silicate mineral) and often hornblende (a calcium, iron, magnesium, and aluminum-silicate mineral) and biotite (a potassium, iron, magnesium, and aluminum-silicate mineral)

dacite: a volcanic rock of fairly silica-rich composition with plagioclase, alkali feldspar (a potassium, sodium, and aluminum-silicate mineral), and quartz (a silica mineral)

magma: a naturally occurring liquid that is usually composed of silicate material and contains suspended minerals or rocks

nuée ardente: a hot cloud of rock fragments, ash, and gases that suddenly and explosively erupts from some volcanoes and flows rapidly down the volcano's slope

subduction: the theory of plate tectonics that assumes that the earth's surface is divided into a number of large plates that either are colliding or are being pulled apart

viscosity: how readily liquids flow; silica-rich magmas flow less readily than do silica-poor magmas

volcano: a vent at the earth's surface in which gases, rocks, and magma erupt at the surface and build a more or less cone-shaped mountain

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