Pelean eruptions and nuees ardentes
When Montagne Pelee annihilated St Pierre on Ascension Day 1902, a previously unrecognized kind of eruption was thrust upon the scientific world. Pelean eruptions are characterized by high degrees of fragmentation and thick accumulation, but dispersal is limited to about 50km2. They develop chiefly on strato-volcanoes mainly from rhyohtic and dacitic, but also from trachytic and andesitic magmas. What sets Pelean eruptions apart from others is the dom- inant combination of dome-formation, nuees ardentes, and laterally directed blasts, which are very destructive because they often concentrate their energy at ground level.
Pelean eruptions are separated by decades when no activity is
displayed. Warning ejections of ash begin a few weeks before the Pelean
culmination and continue in spasms of decreasing intensity for several months
afterwards. The climax develops very suddenly when nuees ardentes at 700'C or
more blast from the volcano. Subsequently, the viscous domes that often rise out
of the vents may be partly destroyed by further eruptions. As the eruptions
become weaker, the domes solidify, strengthen and eventually survive as solid
corks capping the vent until the next eruption. The dome at the summit of Lassen
Peak has an exceptional height of 500m, which, no doubt, enabled it to resist
the small eruption of 1915 almost intact.
Nuees ardentes have captured the imagination ever since St Pierre was destroyed. If purely aesthetic considerations could be divorced from their effects, then nuees ardentes would be as attractive as their name suggests, but they are often the greatest killers in the volcanic repertoire. The direct translation of nuee ardente is "incandescent cloud", but "glowing avalanche" is, perhaps, a more graphic and accurate description. Close investigations have shown that they encompass a continuum of variations, and many different terms have been used to describe them. They are, in fact, prominent features of both Pelean and Plinian eruptions. Nuees are a type of ashflow, propelled down slope in avalanches of a hot, frothy, dull-red, aerosol-like emulsion of gas, fluid lava, old blocks, and densely packed fragments of molten and chilled ash and pumice. They are commonly accompanied or preceded by similar, but less dense, ground surges and hot blasts, propagated at supersonic speeds and frequenty spreading 10km from the vent. In Pelean eruptions, many nuees ardentes may succeed each other during several months, although the first, like the one that destroyed St Pierre, is usually the most powerful. Nuees ardentes move fast. Billowing clouds of ash and gas rise above them as they roll over the ground in enormous bursts at speeds of up to 500km an hour. Photographs often give a misleading impression of nuees ardentes because they usually reveal only the billowing clouds and mask the damage-dealing, thin, surging avalanche below them. They are brief, lasting only a few minutes from initial blast to smoking wreckage.
By coincidence, a different nuee ardente had erupted from the Soufriere of St Vincent on 7 May 1902, the day before St Pierre was destroyed, but it only killed 1565 people and was immediately overshadowed by its rival. The St Vincent nuee was first directed vertically, perhaps because it was tunnelled upwards by a deep confining crater, in an eruption that was more violent, in fact, than that at Montagne Pelee. It rose in a column high into the air, cooled, waned and collapsed, and then spread in a radiating base-surge all around the flanks of the Soufriere. This type of nuee ardente seems to be the most common generated in Plinian eruptions. Because the St Vincent type of nuee ardente travels up through the air, its fragments are often sorted into layers of similar size when they fall back to Earth, unlike the unsorted mixtures typically thrown out by Montagne Pelee. The base-surges spread outwards in hot gusts, forming hummocky, dune-like landscapes. Not to be outdone, Montagne Pelee itself produced a St Vincent type of nuee ardente on 30 August 1902 that devastated an area of 114 km2, and killed about 1000 people who had hoped they were safe on the eastern slopes of the volcano.
Most nuees ardentes are erupted only infrequently, but Merapi in Java regularly emits a third, less explosive and less dangerous type of nuee ardente. Nuees are continually released from the base of the dome growing at the summit of Merapi. As it swells, the dome bulges over the western flank of the volcano like a souffle, and finding nothing to rest on, its outer shell collapses and crumbles down slope, causing the sudden relief of pressure that releases the nuees ardentes. It is the topographic position of the Merapi dome that determines the frequency of its eruptions. If the dome were supported, it would probably grow larger and would only be destroyed by more violent, but less frequent, truly Pelean nuees ardentes.
Thus, the most dangerous types of nuee ardente are influenced by the relief of the volcano. The paths that they take are dominated by gravity. They hug the ground. That is why they can kill so many people so quickly.