Geoscience Reference
In-Depth Information
restart three of the engines. Impact with the ash pitted
the cockpit windscreen so badly that the pilot had to
land the plane by peering out a side window. Two
weeks later, a Singapore Airlines 747 flying the same
route strayed into another ash cloud and dropped
2500 m before the pilot regained control. Only after
this second incident was a warning issued for planes to
avoid the area. Since then more than 80 commercial
airplanes have flown into clouds of volcanic ash.
Mt Pinatubo in 1991 damaged 20 commercial planes in
transit at a cost of $US100 million. Explosions on
Redoubt, Galunggung, Mt Spurr, and Mt Pinatubo
sent clouds of ash into the atmosphere that damaged
planes 150, 200, 1200, and 1740 km, respectively, from
the site of the eruptions.
Tephra ejecta from Mt St Helens, 22 July 1980. Pyroclastic
flows generated the lower ash clouds to the right. Note
the spreading of the ash cloud at the tropopause and the
fallout of coarse particles downwind (photograph courtesy
of the United States Geological Survey).
Fig. 11.2
Pyro
clastic flows and base surges
(Branney & Zalasiewicz, 1999)
Mention has already been made of the collapsing ash
columns - termed 'nuées ardentes' or 'pyroclastic
flows' (Figure 11.3) - and the basal surges produced by
the lateral explosion of volcanoes. Base surges are akin
to the lateral blast waves accompanying a nuclear
explosion, and can pick up considerable dust in
addition to carrying the ash originating from the
volcano itself. The Bandai eruption of 1888 saw a
300 m high volcano pulverized and converted to a
debris avalanche in this manner. The blast wave can
reach speeds in excess of 150 km hr
-1
, and the dust can
sandblast objects. The Taal eruption in the Philippines
in 1965 ablated 15 cm of wood from the sides of trees
within 1 km of the eruption. The basal surge may be
followed by a pyroclastic flow. The Mt St Helens
eruption followed this sequence. The classic descrip-
tion of a pyroclastic flow is 'an avalanche of an exceed-
ingly dense mass of hot, highly charged gas and
constantly gas-emitting fragmental lava, much of it
finely divided, extraordinarily mobile, and practically
frictionless, because each particle is separated from its
neighbors by a cushion of compressed gas' (Perrett,
1935). In pyroclastic flow, fine particles are suspended
and transported by turbulent whirlwinds of gas. While
appearing as choking clouds of ash, the particles are so
widely spaced that they rarely collide. As these turbu-
lent eddies pass over a spot, they can deposit alternat-
ing layers of coarse and fine sediment as the velocity of
the current waxes and wanes in a similar fashion to
gusts of wind. The destructive force of the flow can
The area covered by tephra deposits can be consid-
erable because most tephra falls from atmospheric
suspension within a short distance of the volcano. Fine
ash settling out from Krakatau in 1883 covered an
area of 800 000 km
2
. This volcanic material can be
extremely fertile once incorporated into the soil as a
fine dust; however, it can have severe environmental
consequences. Only a small proportion of volcano-
related deaths can be attributed directly to ash fallout,
nevertheless, ash fallout 70-80 km from Krakatau was
still hot enough to burn holes in clothing and vegeta-
tion. Tephra can also contain toxic fluoride compounds,
which are poisonous to animals attempting to eat
contaminated fodder. The 'haze famine' of 1784 in
Iceland, and the death by famine of 80 000 people
following the eruption of Tambora in 1815, can be
attributed to the destruction of vegetation by tephra.
The glassy tephra from Mt St Helens in 1980 had a
severe impact on road conditions and on motor
vehicles. Tephra from the 1982 eruption of Galung-
gung volcano in western Java almost brought down, in
separate incidents, two 747 passenger jets bound for
Australia. The initial volcanic eruption had not been
noticed nor reported to aviation authorities. The first
plane to succumb was a British Airways 747 flying
between Singapore and Australia, when it entered the
ash cloud at 11 300 m. All four engines cut out, one
after the other, sending the plane into a 13-minute
silent descent, through 7300 m, before the pilot could
