Pyroclastic surges

Now to focus on pyroclastic surges. When thinking about relatively small block-and-ash PFs in places such as Montserrat, it makes good sense to distinguish clearly between the avalanche of shattering blocks that slides down the slopes and the swirling clouds of hot gas and flying lava fragments that both rise upwards from the PF and swirl out sideways in places. Because the gas is very hot, it mostly rises, forming a row of dusty clouds above the flow (Figs 23-24). The rock fragments caught up in these clouds progressively fall out of them. Afterwards you can see the block-and-ash deposits concentrated in the ghauts and thin fine-grained surge deposits covering surrounding areas (Fig. 26).

Fig. 23  Pyroclastic flow, Montserrat (Marco  Fulle, Stromboli online).	Fig. 24  Pyroclastic flow, Augustine volcano, Alaska (Betsy Yount, USGS).

Fig. 25  Another PF trundles down the bed of White's Ghaut (by now almost filled) as its associated surge floods outwards across green vegetation on the far side of the gully. (More brilliant SHV photography in January 2010 by Marco Fulle of Stromboli Online)	Fig. 26  Block-and-ash PF of 21 Sept 1997 concentrated within White’s Ghaut, while its accompanying surges have covered both flanking areas with thin ash (Copyright NERC).

The deposits of ash from pyroclastic surges are distinctive. They are relatively thin; usually no more than about 10 cm thick, except in local "drifts" (like snow). From above you can see their fine grainsize and the distinctive slight ridges and grooves that show which way the mass of hot dusty air was moving. Often their surface is dotted with small spheres of fine ash stuck together, rather like ashy marbles. These are called "accretionary lapilli" and form when static electricity inside the surge cloud causes moist ash particles to stick together. The photos below were taken a few days after the huge partial summit dome collapse and accompanying vulcanian explosions of Montserrat's volcano on 11 February 2010.

Ash deposited from the 11 February 2010 pyroclastic surges NE of SHV. Chicken and goat tracks for scale. The surge cloud clearly moved roughly from top to bottom. Bob Thompson

These little powder spheres are so fragile that they collapse to dust if you touch them. Nevertheless, a further fall of ash may cover them and then they may eventually harden, as underground water seeps through them and deposits minerals between the ash grains. The result is that sometimes you can find these in extremely old rocks, and know for sure that you are dealing with ancient fallout from a damp ashy cloud,. There are beautifully- preserved ones in fossil surge deposits about 400 million years old (!) in the Lake District, NW England.   Bob Thompson

The next two photos (Figs 27-28) show clearly that pyroclastic surges are hot! You can see how the Unzen surge billows upwards into the sky, as its very hot air rises. In contrast, the man-made “surge” created when the Twin Towers tragically collapsed in New York on 9/11 rushed outwards but showed little tendency to rise upwards because the air powering it was relatively cool.

Seen from a distance, the swirling dust clouds of a surge look quite harmless – sometimes almost romantic in the right light. So why are they so dangerous? The answer lies in three of their features:

1.  The swirling cloud moves extremely fast.

2.  The fast-moving cloud is loaded with dust, vegetation and even rocks, making it a dense fluid.

3.  Surges can be up to hundreds of degrees centigrade hot.

You can get a general idea, on a miniature scale, of the difference between a surge cloud seen from a distance and too close by the next two photos Figs 29 and 30). These show a characteristic feature of deserts on hot days, called a “dust devil”. From a distance they look harmless but, when you get close, you find that the hurricane-force winds within them are picking up and throwing vegetation, and even small stones. Likewise everyone in the Caribbean knows that major hurricanes can do immense damage by blowing down trees etc and throwing them around. Therefore it’s obvious that the high speed of air in a surge is a major threat to property and people.

But why should a surge cloud be denser than pure air and why should that add to its lethalness? This is mostly to do with the dust. To do an experiment that shows this point, start by taking some pure water and then stir in more and more very small rock, mineral and dust particles. Eventually you will have some sticky and dense mud. Similarly, a fast-moving pyroclastic surge cloud, laden with dust, has a higher density that pure air. Density affects the “hitting power” of a moving fluid. Stand in fast-moving pure air and all you experience is a windy day. Stand in water moving at the same speed and you are in a raging torrent – a dangerous place to be. If the water is dense mud and full of picked-up boulders, you are in a lahar and nobody has to go further than the Belham Valley to see what damage they can do. The image of reinforced concrete pillars in Indonesia flattened by the 2004 tsunami (Fig. 31) looks so similar to images of severe pyroclastic surge damage around St Patrick's on Montserrat (see Damage to Buildings topic) or the forests flattened by the huge sideways-directed surge around Mt St Helens in 1980 (Fig. 32).

The hotness of both pyroclastic flows and their associated surges is vividly illustrated by any film or photo taken immediately after the 25 June 1997 disaster on Montserrat, when countless buildings etc were on fire. Both the gas and the rock fragments carried by it can be at temperatures around 300-400^oC within a Montserrat surge. This is too cold for the fragments to glow at night but amply hot enough to ignite wood and cause devastating burns to living animals. You can show this point by means of a simple experiment (not a popular one!) at home. Most domestic ovens reach about 250^oC at maximum temperature. Put a bread roll or something similar into a hot oven and “forget” it for a day or so. This temperature will convert the bread to charcoal (Fig. 33). Keep the oven tightly closed throughout this experiment because otherwise you risk the contents catching fire. Lastly repay your debt to those who run the kitchen by cleaning both the dish and the oven! If you are still not convinced about the hotness of a surge, then stick your hand into the hot oven and touch the metal, but please don’t blame me for the outcome (when you return from hospital)!

Obviously the temperatures will be lower near the margins of a pyroclastic surge and we shall explore this topic in more detail in a later article. For the moment, the most important practical matter to note is that, although both the air and its suspended load of rock fragments are hot, they retain their heat for very different times. Although hot air can travel very quickly sideways in a moving surge, it rises upwards as soon as it slows down and is replaced by cooler air flowing inwards from the surroundings. By contrast, the rock fragments (dust/gravel) carried by this air tend to fall to the ground as soon as the air slows down and to form a layer of sediment that can remain extremely hot for hours to days (even after relatively small surges).

Is it really necessary to treat pyroclastic flows and surges separately?

So far this summary has been aimed exclusively at Montserrat residents and the present eruption. But scientists worldwide are of course studying a great variety of both modern and ancient pyroclastic flows and surges. When they look at enormous examples, produced by volcanic explosions that continued for hours or longer, it becomes clear that you cannot neatly divide the products thundering down the sides of volcanoes and outwards across the surrounding region neatly into pyroclastic flows and surges. When you decipher (by features of the preserved volcanic sediments) what happened at a given point within the chaos of the collapsing prolonged eruptive columns, it appears that you could theoretically say that flows and surges were endlessly alternating at that point, depending on small changes to and fro of the speed and ratio of air to solid fragments (dust to rocks) there. Therefore leaders in this field of research now tend to ban the terms “pyroclastic flow” and “pyroclastic surge” from their reports and refer to both together as “pyroclastic density currents” (PDCs). This approach is particularly helpful for scientists in places that experienced brief but extremely violent volcanic events, such as St Helens, where there has been a big controversy as to whether the “thing” that caused such devastation during the 1980 lateral blast was a pyroclastic surge or flow. Similar controversy has focused on the 1902 eruption of Mt Pelée in Martinique.

During the current Montserrat eruption, the only similar event so far was the “thing” that followed the initial debris avalanche, across the St Patrick’s area after the collapse of Galway’s Wall on Boxing Day 1997, which has been described in various reports using both the pyroclastic flow/surge and the PDC nomenclature. There are three practical reasons why the flow-surge nomenclature should remain in use on Montserrat:

1.  Everyone on the island is accustomed to it. Academic debates about PF nomenclature are a bit above the heads of worried local residents and certainly do nothing to make them either better informed or safer.

2.  Although in many cases the pyroclastic flows and their accompanying surges follow the same tracks down ghauts and valleys on Montserrat, this is not always so. Even when it bends quite sharply, a ghaut can act as a channel confining PFs as they slither downhill (at least until the ghaut is full enough with debris from earlier events to cease to act as an efficient channel). But the fast-moving surge associated with some channel-confined PFs has a very nasty habit of jumping out of such channels at their bends and spreading out across the adjacent area, leaving the PF to rattle on down the channel below (Figs. 25, 26 and 34). This is why it makes sense to treat PFs and surges separately on Montserrat.

3.  Although these so-called “detached surges” may cause terrible devastation, as on 25 June 1997, they are also marginally more survivable than PFs in general. Being buried in a PF by a heap of either red-hot chunks of a collapsed lava dome or a pile of incandescent pumice is obviously a terminal business and the only way to avoid such disasters is not to be there. By contrast, the temperatures and amounts of flying dust etc near the margins of surges become low enough that survival is a possibility, as we shall see in a later section. The 2010 eruption of Merapi, in Indonesia, has made this point very clearly but at a cost of over 250 lives.

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