Volcanoes as a Hazard - Natural Hazards - page 231

m 3 s -1 . The actual velocity of flows is in a range of
9-100 km hr -1 , with average values around 30 km hr -1 .
While most documented flows rarely exceed 4 km 3
in volume, some have caused impressive damage. The
Laki fissure eruption of 1783-1784 in Iceland covered
550 km 2 , had a length of 88 km and a total volume of
12.3 km 3 . This eruption caused melting of glaciers and
flooding of valuable farmland. The sulfurous vapors
from the lava hung over the countryside for weeks. The
'haze famine' that resulted killed 20 per cent of
Iceland's population, and wiped out 50 per cent of
its cattle and 75 per cent of its sheep and horses.
Extensive flows sweeping down the slopes of Mt
Vesuvius engulfed the town of Bosco Trecase at its base
in 1906 and the town of San Sebastiano in 1944. Lava
from Mt Etna in 1669 reached the wall of the feudal
city of Catania 25 km away, eventually overtopping the
walls and moving slowly through the city. In 1928,
the town of Mascali was also destroyed by lava from
Mt Etna. Parícutín in Mexico, in 1946, buried 2400
hectares of forest and agricultural land before com-
pletely covering the town of San Juan Parangaricutiro,
5 km away from the main vent. Eruptions from Mauna
Loa and Kilauea in Hawaii have consistently flooded
agricultural land, destroyed small villages surrounding
the cones, and threatened the major city of Hilo
on several occasions. In January 2002, lava from Mt
Nyiragongo in the Democratic Republic of the Congo
buried the town of Goma under 1.5 m of lava and
caused over 500 000 people to flee for their lives.
The easiest way to stop lava movement is to enhance
the processes that slow it down. Flow depends upon
yield strength, which can be increased by lowering the
temperature of the melt, increasing the rate of gas
escape from the flow, stirring, and seeding the lava
with foreign nuclei. In some cases, increasing the yield
strength of the flow may only turn a fluid flow into a
more viscous one, making it thicker and more difficult
to stop. Because of the insulating properties of solid
lava, cooling by water is not always effective. It has
been observed that lava flows can continue to move
even when submerged under the sea. One notable
exception appears to be the attempts in 1973 to stop
a block lava flow, which threatened the town of
Vestmannaeyjar in Iceland. Water was intensively
pumped for months onto, and just behind, the moving
lava front, thus increasing the viscosity of the magma
and slowing its rate of progress. Agitation of the flow
can be difficult to achieve except by bombing. This was

events per century that result in severe social and
economic disruption. The main reason for this
disruption is the fact that lava flows in volcanic areas
are very fertile, and have attracted intense agricultural
usage and dense settlement. Re-eruption of volcanoes
in these areas has, thus, led to the destruction of
this agricultural land and to loss of life. Much of this
destruction can be prevented simply by mapping and
avoiding those areas most frequently inundated by lava
flows. Techniques also exist to modify flow behavior,
especially for pahoehoe and aa flows.
Based upon the more than 1000 flows that have
occurred historically, low-viscosity flows have a median
length of 4.1 km and an average thickness of 10 m, while
high-viscosity flows have a median length of 1.3 km and
an average thickness of 100 m. There is a 1 per cent
probability that high- and low-viscosity lava flows will
exceed, respectively, 11 km and 45 km in length. The
fastest moving, hottest and most mobile flows are the
low-viscosity Hawaiian- and Icelandic-type eruptions.
These flows would appear to be dangerous; however,
they are thin and often consist of pahoehoe-type lava,
which cools very rapidly. Pahoehoe lavas may develop
large diameter lava tubes extending several kilometres
under the flow. These can provide conduits for subse-
quent lava venting once the original flow has cooled at
the surface. Such flows can travel great distances, but
are the easiest to divert. Aa flows are thicker and more
viscous. They tend to channelize to depths up to 30 m.
Both aa and block lava flows are more difficult to divert.
The characteristics of flows can vary depending
upon temperature, viscosity, yield strength , and expul-
sion rate. Because of the high temperatures of flows
(880-1130°C), all carbon materials such as cloth,
paper and wood are easily ignited upon contact with
lava. Once temperatures drop below 800°C, a skin
forms on the magma that effectively inhibits further
heat loss. Lavas have been known to have high internal
temperatures 5-6 years after they have stopped
flowing. As temperature decreases, viscosity increases,
and it is this factor that stops most flows. While the
velocity of a flow depends upon gravity or the slope of
the terrain, the rate at which lava is expelled also
controls the speed of movement. Most expulsion rates
range between 15 and 45 m s -1 , but values above
1000 m 3 s -1 have been measured. The distance
reached by lava is a direct function of this expulsion
rate, such that the distance of travel in kilometers is
about three times the expulsion rate measured in