Heat and hot volcanic ash

 

Because surges travel fast they interact with individual buildings for quite short periods. Brief exposure to hot air at 300-400oC (typical temperatures in the major 1997 Montserrat surges) can melt tarred roof tiles and set dry timber and furnishings smouldering. But the main agent for setting light to houses and their flammable contents is not hot air but the lava dust and larger fragments the surge carries. These are dropping to the ground throughout a surge event, leaving a characteristic dusty deposit everywhere. If the surge enters a house via windows or doorways, the rooms inside will also be coated with hot ash.  If the coated material (such as fabric or wood) is flammable at 300-400oC and the ash is thick enough, the whole house will burst into flames, regardless of any blast or missile damage. Thus a characteristic feature of the 25 June 1997 surge that hit the Streatham area was that even houses with little blast damage caught fire, if the surge broke into them through doorways or windows.

 

 

Fig. 14  Thin ash coat inside a Montserrat building after a surge.

(R Roscoe, photovolcanica.com).

Fig. 15  Thick ash in a Montserrat basement. This is a similar situation to the seaside caves at Herculaneum in 79 AD where many sheltering people were killed (Copyright NERC).

 

Variable damage

 

Baxter and his team were able to write a general account of the damage to buildings inflicted by a surge, from the margins to the centre of an affected zone. But they also stressed what had been well known ever since the rescuers first reached Streatham village on 25 June 1997, that damage was extremely variable within even a very small area. This is just what you would instinctively guess about blast damage from experience of any stormy day. In any storm you can often shelter from the worst of the wind by going behind a sturdy building or slight bank in the land. Surge clouds behave in the same way except that, as density currents, they seek out locally lower places (just like a water flood). Likewise the deadly hot flying ash particles behave rather like snowflakes; they will form a deep drift in one place and a much thinner layer in another; they will get into one room of a house through a broken window (perhaps causing a fire) but fail to enter the rest of the house.

 

 

 

Will surge damage be relevant to Montserrat in the future?

 

After the traumatic experiences of Montserratians and their former homes in the south, plagued by pyroclastic surges (dilute PDCs), it would be wonderful to be sure that this will never happen again. Unfortunately this is definitely not certain at the time of writing in 2010. There are two obvious threats:

 

1.  Since its re-growth after the almost total collapse in 2006, the lava dome has reached higher and wider than ever before. I shall summarise the changes in 2009-2010 in an expanded version of Topic 10, as soon as I can find the time to do so. As the dome gets bigger, so the law of gravity dictates that its debris from collapses will spread wider.

 

3.  There is a very remote possibility that renewed future volcanic activity might even pump magma into a new reservoir beneath the NW slopes of the volcano, much as happened at Mt St Helens in 1980.

 

The Main Report number 8 of the Scientific Advisory Committee in March 2007 (see MVO website) considered the possible results of the sorts of scenarios listed above. They showed by 3-D modelling of potential events that maximum-sized (in 2007) dome collapses and/or explosions down the Belham Valley might generate PFs big enough to reach the sea, accompanied by surges that “escaped” the Valley  and spread out northwards across Salem and the surrounding area, eventually stopping close to the Nantes River. In the four following SAC reports, 9, 10, 11 and 12, the risk of such an event has been judged to be even less likely than in March 2007 but certainly not zero. To put the matter bluntly, there remains a very small but non-zero possibility that part or all of the inhabited area south of the Nantes River (and maybe even a little further north) may be ravaged by a pyroclastic surge formed if the present lava dome collapses wholesale or explodes (this scenario corresponds with Hazard Level 5 in the new GoM/DMCA/MVO Hazard Level System). This possibility might increase substantially, if lava extrusion resumes above the NW flank of the lava dome. So far the volcano has been quite obliging in signalling its intentions in advance by seismic and other means, allowing time for evacuation when necessary. However, it’s certainly not written in stone (or lava for that matter) that the present (2007-10) lava dome will make ample warning noises before it collapses, as was shown by the unannounced explosions in early December 2008 and January 2009, and the huge collapse on February 2010.  Therefore everyone living south of the Nantes River has been taking a very small gamble since 2007 that the lava dome will remain intact, or at least collapse in a direction away from residents, as on 11 February 2010.

 

The preliminary statement 14 of the SAC, following their meeting in March 2010, is clear about this point.

"Preliminary estimates based on current conditions indicate that the risks faced by people in Zones A and B are slightly higher than six months ago. As we saw on February 11th, extremely dangerous events, impacting hazard zones A and B without warning, remain a very real possibility."

 

 

What can be done to protect houses from surges?

 

Here the research of Baxter and his team has produced a simple answer that supports previous MVO work on the damage caused to buildings by the big 1997 surges. Hurricane boards nailed over windows do wonders to keep out hot air and ash, preventing fire inside houses. Ideally, you should shelter fully clothed and with shoes on (reduces burns) in a well-sealed room facing away from the volcano, within a strongly shuttered house. Better still, the house and its roof are built (or re-built) of strong reinforced concrete, adequate to survive bombardment by missiles such as uprooted trees.

 

All this is fine and dandy but it’s not Montserrat. A hillside covered by concrete bunkers might make sense on Mars but is it realistic for the Salem-Old Towne area? Likewise sleeping fully clothed behind boarded windows during the Montserrat summer sounds like an endless nightmare, rather than a realistic precaution. So what is the best way forward for the many Montserrat residents south of the Nantes River? Only the residents themselves can think this through and then persuade their government to accept their joint decision.  That's what democracy is all about. In its communal battle against the eruption, Montserrat is showing itself to be one of the planet's most vibrant democracies. What other country has lost two thirds of its land to a volcano and still carried on almost regardless?

 

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