Volcanoes as a Hazard - Natural Hazards - page 232

tried on one tube feeding a pahoehoe flow that threat-
ened Hilo, Hawaii, in 1935. The flow was disrupted
enough that it turned into a more viscous aa flow, and
the tube became blocked. The bombing might not
have been the reason for the flow termination, because
it was already in the final stages of expulsion. However,
there are still contingency plans for bombing lava flows
if they threaten Hilo in the future. The sides of a flow
can also be breached, thus diverting lava to a safer
path. However, what one person considers safe,
another may consider a threat. This problem beset the
unfortunate inhabitants of Catania in the 1669
eruption of Mt Etna. The citizens of the walled city
made the first recorded attempt to alter a lava flow.
They clad themselves in wetted animal skins, and dug
into one of the lava levees controlling the route of the
flow towards their city. Unfortunately, the diversion
sent the flow towards the town of Paterno. Five
hundred enraged residents of this latter city raced up
the slopes to attack the citizens of Catania, and termi-
nated their efforts after winning a pitched battle. The
lava flow clogged the breach, proceeded to Catania,
overflowed its 20 m high walls, and inundated large
sections of the town. A similar attempt in Hawaii in
1942, rather than diverting the flow, only sent it on a
parallel path.
Barrier construction has also been used to divert aa
flows, or give more time for effective evacuation. Most
lava flows will pond behind the flimsiest obstacle, and
then simply overtop it, as eventually occurred in
Catania. Barriers should not be used to stop a flow, but
simply to divert it. Because lava flows can become very
viscous as they cool down (106 times more viscous than
water), they may only be flowing over very low grades
and exerting very little force. They can thus be diverted
successfully by putting guiding barriers diagonally in
their paths, as long as the diversion route is steep.
However, the rapid movement of some flows rules out
time-consuming barrier construction. Even construc-
tion of barriers long distances in front of the advancing
lava may be impractical, because the flow could easily
stop naturally before reaching the barrier. Barriers
must also consist of denser material than the lava, be
attached to the ground, or be three times wider at their
base than the flow thickness itself. Less dense material
will simply be buoyed up by the lava and incorporated
into the flow. If barriers only divert a flow, there is
always the problem of finding a safe diversion path that
does not affect someone else's property or life. Only

additional study in the mechanics and behavior of
lava flows will permit barrier construction to be used
effectively.

Ball istics and tephra clouds

(Prabaharan, 2002)

The eruption of Vesuvius in 1944 illustrates another
major hazard of volcanic eruptions. At that time the
Allied war effort in the area was severely hampered
by the bombing of airfields, not by the Germans,
but by liquid lava blobs tossed out by the volcano.
Strombolian-, Vulcanian-, Surtseyan-, and Plinian-
type eruptions all shoot out ash to heights greater than
30 km. The larger particles consist of boulder-sized
blobs of fluid magma and remnant blocks of the
volcano walls. Measured velocities are in the range of
75-200 m s -1 , and maximum distances are obtained
with trajectory angles of 45°. Measured distances for
boulder material rarely exceed 5 km, although
material weighing 8-30 tonnes has been projected
over distances of 1 km or more. This type of debris can
be voluminous and hot, and can fall over a small area.
It can also be extremely destructive. The density of
projectiles varies considerably so that the kinetic
energy of impact is wide-ranging. Larger bombs can
behave exactly as human-made projectiles in their
explosive effect when hitting the ground. Panoramic
photographs of vegetated landscapes subject to
sustained projectile bombardment look exactly like
war bombing scenes.
The production of tephra or ash can also be destruc-
tive (Figure 11.2). The eruption of Mt Hekla, Iceland,
in 1947 ejected 100 000 m 3 s -1 of material, dropping ash
as far away as Finland two days later. Tephra can rise at
velocities of 8-30 m s -1 , with drift rates downwind of
20-100 km hr -1 . While much of this dust falls out locally,
fine material < 0.01 mm in size can be thrown up to
heights in excess of 27 000 m into the stratosphere.
Here, residence times may be two or more years. The
eruption of Tambora in Indonesia on 5-10 April 1815 is
the largest recorded tephra eruption. It blasted 151 km 3
of material into the atmosphere as fine ash, which was
responsible for a cooling of the Earth's surface temper-
ature by 0.5-1.0°C. As already mentioned in Chapter 9,
volcanic dust exerts a dramatic control on the Earth's
temperature, with many of the changes in temperature
over the last three centuries correlating well with
fluctuations in volcanic dust production.