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May 8, 1902 began as a sunny day in Martinique, an island in the Caribbean, with only a column of steam rising above its Mount Pelée. Until 7:50 a.m., when the mountain, an active volcano, exploded.
The first rescuers arrived on the site twelve days after the
explosion, accompanied by British, French and American geologists. In
the city of St. Pierre, almost all of the buildings had been destroyed
and estimated 20.000-40.000 people killed. But how? The main cause of
destruction during a volcanic eruption, lava flows, were not in or near
the city. Havivra Da Ifrile, a girl who survived by hiding inside a cave
near the shore, reported this strange geological phenomenon:
“…I looked back and the whole side of the mountain, facing towards the town, seemed to open and topple down on the screaming people. I was burned … by the stones and ashes that came flying …, but I got to the cave,…“
Was this a landslide? Geologist Edmund Hovey of the American Museum of Natural History noted that “In
many places the limit [of the devastation] passes single trees, one
side is dark and burned, the other green as if an eruption never
happened.” A landslide could not explain the burned trees nor could
it explain the sharp boundary between the destroyed and untouched
areas.
So if it wasn’t lava, and it wasn’t a landslide, what destroyed the city?
Two months later, geologists Tempest Anderson and John S. Flett of the Royal Society of London survived a smaller eruption of Mount Pelée. (Yes, it erupted again.) “The cloud had a spherical form and resembled rounded protuberances amplifying and doubling with terrifying energy. They extended to the sea, in our direction, boiling and changing shape at every moment. It didn’t spread laterally. It didn’t rise up in the atmosphere, but it descended on the sea as a turbulent mass….“
For the very first time geologists observed a deadly “nueé ardente” – an “incandescent cloud” or “glowing avalanche” (as the phenomenon was later named by French volcanologist Alfred Lacroix). A nueé ardente, today referred as pyroclastic density current, is a mixture of volcanic material and hot gases. Because its density is greater than air, it sinks downward, flowing like an avalanche along the slopes of a volcano. Pyroclastic flows can originate from the collapse of the eruption column, from a lateral blast or from the partial collapse of a volcano.
So what makes pyroclastic density currents so dangerous? The first evidence to explain their deadly effects was found in the ruins of St. Pierre. Researchers were able to estimate temperatures inside the pyroclastic flow that destroyed St. Pierre based on the observation that bottles melted (glass melts at ~700°C/1,300°F), butcopper tubes were not deformed (copper melts at 1,100°C/2,000°F). The geologist therefore concluded that temperatures of a pyroclastic flows can range between 700 to 1,000°C/1,300-2,000°F. Temperature explains also why and how so many people perished in St. Pierre. Many of the recovered bodies were horribly mangled, as if they’d exploded from the inside. Other victims were suffocated, but their clothes were still surprisingly intact.
For those engulfed by a pyroclastic flow, the heat was so intense that it instantly burnt the outer layers of skin and flesh. Their bodies shrank due to the loss of water, and so their organs were squeezed out from the inner cavities. Even those not hit by the pyroclastic flow directly weren’t spared after inhaling the still hot gases (300°C/570°F) nearby, which burned their lungs (yet left their clothes untouched).
The photo above of a 200 million year old ignimbrite (from the Latin term of fire, as lithified deposits of a nueé ardente are named) summarizes in one picture why pyroclastic flows are so dangerous. Some of the larger clasts in the photo show an outer rim, indicated that the temperature inside the pyroclastic flow was high enough to alter the mineralogical composition of the rock. These larger rocks were embedded into a matrix of fine ash and tiny crystals, making these “unsorted facies” a typical appearance of density current deposits. Pyroclastic flows - a mixture of rocks, overheated gases and vapour - are able to transport even large boulders at a speed of 160km/h (99mph). As a result, the impacting mass destroys everything in its path.
Interested in reading more? Try:
DAVIS, L. (2008): Natural Disasters. Facts on File Sience Library. Infobase Publishing: 464
HEILPRIN, A.(1903): Mont Pelée and the Tragedy of Martinique. J.B. Lippincott Company, Philadelphia and London: 335.
LACROIX, A. (1904) : La Montagne Pelée et ses éruptions. Masson et Cie, Paris.
MORRIS, C. (2006): The San Francisco Calamity by Earthquake and Fire. Librivox.
“…I looked back and the whole side of the mountain, facing towards the town, seemed to open and topple down on the screaming people. I was burned … by the stones and ashes that came flying …, but I got to the cave,…“
Photographs of the city of St. Pierre before
and after the eruption of Mount Pelée, the volcano is seen in the
background (from LACROIX 1904).
Sequence showing a pyroclastic flow, photographed December 1902 by French volcanologist A. Lacroix (from LACROIX 1904).
Two months later, geologists Tempest Anderson and John S. Flett of the Royal Society of London survived a smaller eruption of Mount Pelée. (Yes, it erupted again.) “The cloud had a spherical form and resembled rounded protuberances amplifying and doubling with terrifying energy. They extended to the sea, in our direction, boiling and changing shape at every moment. It didn’t spread laterally. It didn’t rise up in the atmosphere, but it descended on the sea as a turbulent mass….“
For the very first time geologists observed a deadly “nueé ardente” – an “incandescent cloud” or “glowing avalanche” (as the phenomenon was later named by French volcanologist Alfred Lacroix). A nueé ardente, today referred as pyroclastic density current, is a mixture of volcanic material and hot gases. Because its density is greater than air, it sinks downward, flowing like an avalanche along the slopes of a volcano. Pyroclastic flows can originate from the collapse of the eruption column, from a lateral blast or from the partial collapse of a volcano.
So what makes pyroclastic density currents so dangerous? The first evidence to explain their deadly effects was found in the ruins of St. Pierre. Researchers were able to estimate temperatures inside the pyroclastic flow that destroyed St. Pierre based on the observation that bottles melted (glass melts at ~700°C/1,300°F), butcopper tubes were not deformed (copper melts at 1,100°C/2,000°F). The geologist therefore concluded that temperatures of a pyroclastic flows can range between 700 to 1,000°C/1,300-2,000°F. Temperature explains also why and how so many people perished in St. Pierre. Many of the recovered bodies were horribly mangled, as if they’d exploded from the inside. Other victims were suffocated, but their clothes were still surprisingly intact.
For those engulfed by a pyroclastic flow, the heat was so intense that it instantly burnt the outer layers of skin and flesh. Their bodies shrank due to the loss of water, and so their organs were squeezed out from the inner cavities. Even those not hit by the pyroclastic flow directly weren’t spared after inhaling the still hot gases (300°C/570°F) nearby, which burned their lungs (yet left their clothes untouched).
The photo above of a 200 million year old ignimbrite (from the Latin term of fire, as lithified deposits of a nueé ardente are named) summarizes in one picture why pyroclastic flows are so dangerous. Some of the larger clasts in the photo show an outer rim, indicated that the temperature inside the pyroclastic flow was high enough to alter the mineralogical composition of the rock. These larger rocks were embedded into a matrix of fine ash and tiny crystals, making these “unsorted facies” a typical appearance of density current deposits. Pyroclastic flows - a mixture of rocks, overheated gases and vapour - are able to transport even large boulders at a speed of 160km/h (99mph). As a result, the impacting mass destroys everything in its path.
Interested in reading more? Try:
DAVIS, L. (2008): Natural Disasters. Facts on File Sience Library. Infobase Publishing: 464
HEILPRIN, A.(1903): Mont Pelée and the Tragedy of Martinique. J.B. Lippincott Company, Philadelphia and London: 335.
LACROIX, A. (1904) : La Montagne Pelée et ses éruptions. Masson et Cie, Paris.
MORRIS, C. (2006): The San Francisco Calamity by Earthquake and Fire. Librivox.
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