Page 191 - Fundamentals of Physical Volcanology

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168 CHAPTER 11

currents. No living thing can survive being engulfed
by pyroclasts at these temperatures. Surges may
have lower temperatures, given that they are likely
to have involved more interaction of the juvenile
materials with the atmosphere, but they are still
often hotter than 300 K.
The speeds of density currents and surges, com-
−1
monly at least 100 m s , mean that it is essentially
impossible to imagine running – or driving in any
surface vehicle – away from one. A jet-engined
aircraft (but not a helicopter) can fly faster than a
pyroclastic density current can travel, but without
access to a vertical-take-off type of aircraft with its
engines already running it would be hard for any-
one to make use of this fact. These high speeds of
pyroclastic density currents also mean that they
exert high pressures on obstacles such as buildings
that they encounter. The pressure exerted on a wall
at right angles to a pyroclastic density current with
a bulk density of ρ = 1000 kg m −3 and a speed of
2
u = 100ms −1 will be 0.5 ρ u = 5 MPa, hundreds
of times greater than typical load-bearing strengths
of the strongest building materials and so it is not
surprising that even much more dilute pyroclastic
Fig. 11.6 Damage to the interior of a jet engine from a DC-8 surges cause great structural damage (Fig. 11.7).
aircraft that encountered ash from an eruption of Hekla in
Perhaps the only possible protection against a
February 2000. (Photograph courtesy of Thomas Grindle
pyroclastic density current is topography. We saw
(NASA DFRC), Frank Burcham (ASM, Inc.) and David Pieri
in Chapter 8 that high-speed currents can surmount
(JPL).)
ridges several hundred meters high if there is
no alternate. However, given the opportunity, a

in the fallout area. Humans and animals require an density current will travel down a valley rather than
input of oxygen just as much as aircraft engines do. climb a ridge. So, given a fast vehicle and enough
In the cases of living creatures, the air must be able advance warning, one can imagine driving to the
to reach the alveoli in their lungs. A layer of moist top of a hill or ridge in the hope that the density cur-
ash on the lung surface stops this happening – it rent will travel down a valley adjacent to the ridge,
behaves like a layer of wet cement – and can cause or flow around the hill. Unfortunately, as will be dis-
death as easily as asphyxiation or drowning. Even if cussed later in this chapter, it is not likely that the
death does not occur, there can be long-term dam- general population in a volcanic area will have the
age to the lungs comparable to that associated with required early warning. And even if the pyroclastic
exposure to asbestos fibers or coal dust. density current proper does not reach the top of
the hill or ridge, it is quite likely that the associated
surge cloud, almost equally lethal to people, will do
11.2.4 Pyroclastic density currents
so (Fig. 11.8).
and surges
These are by far the most dangerous of volcanic
11.2.5 Lahars
hazards in terms of their immediate effect on peo-
ple, animals, and buildings. The obvious feature is Whenever rain falls onto pyroclastic materials,
the high temperature of the gas and clasts, at least whether in a fall or a density current deposit, the
600 K and as much as 1000 K in pyroclastic density potential exists to fluidize the volcanic material.