Page 192 - Fundamentals of Physical Volcanology

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VOLCANIC HAZARDS AND VOLCANO MONITORING 169

Fig. 11.7 A Roman column knocked down by the first
pyroclastic density current that reached the city of
Herculaneum during the AD 79 eruption of Vesuvius.
The column is embedded in the deposit and is exposed
in a tunnel. (Photograph by Lucia Gurioli, University
of Hawai’I.)

Fig. 11.8 A dilute pyroclastic surge cloud crossing a ridge.
The mixture of water and solid clasts is called a
The top of the ridge is at the top of the slope covered with
lahar (Fig. 11.9). If most of the solid material is
bright deposits from earlier surges. The topography of the
fine-grained, the lahar is sometimes called a mud- ridge has deflected and channeled the main pyroclastic

flow, but of course mudflows can also be formed density current from which the surge cloud is derived so
from nonvolcanic materials. Lahars generated from that it is traveling down the valley behind the ridge and
pyroclastic density current deposits can be quite cannot be seen in this view. (© NERC, 1997. Montserrat
hot: mixing equal volumes of pyroclasts at, say, Volcano Observatory photograph.)
650 K with water at 300 K yields a mixture in
which the water is just boiling. However, lahars
involving fall deposits are generally cold. An altern- the Nevada Del Ruiz volcano in Colombia in 1985
ate mechanism to produce lahars is for pyroclasts to which swept ∼70 km down a river valley on the
be emplaced on top of a layer of ice or snow; if the flank of the volcano at up to 15 m s −1 killing more
clasts become waterlogged and sink through the than 23,000 people in the town of Armero.
liquid water as it is being produced by melting, this Although lahars can flow almost as fast as water,
maximizes the rate of heat transfer. their densities are typically 1.5 to 2 times greater
Lahars have rheological properties somewhat than the density of water, so that they cause more
similar to those of lava flows, being nonNewtonian destruction than similar-sized water floods. Also,
fluids, but they generally have much lower visco- after it is emplaced, the water–ash mixture sets
sities and can move at speeds up to a few tens of rather like concrete, and so both rescue and clean-
meters per second. They can travel considerable up operations are extremely difficult. An added
distances – at least tens of kilometers. A terrible problem is that formation of lahars is often likely to
example was the lahar from the summit region of continue to be a threat long after an eruption is