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.
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
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?
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:
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.
There is a very remote possibility that renewed future volcanic activity might
magma into a new reservoir beneath the NW slopes of the volcano, much as
happened at Mt St Helens in 1980.
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
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
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.
statement 14 of the SAC, following their meeting in March 2010, is
clear about this point.
What can be done to protect
houses from surges?
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.
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?