Page 189 - Fundamentals of Physical Volcanology

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166 CHAPTER 11

11.2.2 Pyroclastic falls
In the most energetic explosive events producing
eruption columns generating fall deposits, pumice
clasts up to more than a meter in average size can
be erupted. However, as seen in Chapters 6 and 8,
these will be deposited extremely close to the vent,
and so will pose only a very localized threat to
people and buildings. Also, because subsidence to
form calderas, or at least significant depressions, is
common around the vents of large-volume energetic
eruptions, the vent regions often contain lakes, and
Fig. 11.2 Building in the village of Kalapana on the
so are not usually the sites of large population cen-
southeast flank of Kilauea volcano, Hawai’I, set on fire
ters. A major exception to this is the Campi Flegrei
by encroaching lava flow. (Photograph by Jim Griggs,
courtesy of U.S. Geological Survey, Hawaiian Volcano Caldera, which contains part of the suburbs of the
Observatory.) city of Naples in Italy.
The major danger from eruptions generating fall
deposits comes from the accumulation of large vol-
for not only people but also all of their easily mov- umes of fine-grained pyroclasts onto the roofs of
able possessions to be evacuated from threatened buildings over a wide area downwind of the vent.
buildings, and in a few instances some of the build- The clasts will generally have been transported
ing itself may be removed in time. A famous case is high into the eruption cloud and will have taken
that of a small historic church, the Star-of-the-Sea tens of minutes to hours to reach their final destina-
painted church in the village of Kalapana, Hawai’I. tion on the ground. It is not the temperature of the
This building was braced and lifted bodily in time material that matters, therefore, but just the stress
to be saved from the lava flows from Kilauea’s that its weight exerts on the building’s structure,
Pu’u ‘O’o-Kupaianaha eruption in 1991 (Fig. 11.3). causing it to collapse (Fig. 11.4). This is especially
Virtually all the other buildings in this village were true if rain falls on the material before it can be
destroyed, however. removed from the roof. A loosely packed layer of
vesicular pumice may have a bulk density of as little
as 600 kg m ; filling only half the vacant space
−3
−3
with rainwater raises the density to 1350 kg m ,
more than doubling the load on the building beneath.
−3
With a more conservative density of 1000 kg m ,
a layer of damp pyroclasts 1 m thick would exert
a pressure of 10 kPa, which is very close to the
maximum recommended design load for a modern
building such as a reinforced concrete warehouse.
It is not just buildings that are affected by fall
deposits; cars and larger vehicles are also at risk in
various ways. Accumulation of ash around vehicles
near Mount St Helens during the 1980 eruption
melted their tires. And during the 1990 eruption
of Pinatubo volcano in the Philippines, preferen-
Fig. 11.3 The St Mary’s Star of the Sea Catholic Church,
tial accumulation of ash on upper surfaces of the
Kalapana, Hawai’I, being moved to a safe location to
tail structures of aircraft parked in the open at
avoid advancing lava flows. (Photograph by D. Weisel,
courtesy of U.S. Geological Survey, Hawaiian Volcano a USA air base caused them to tip over, so that
Observatory.) their noses rose into the air and their tails hit the