Volcanoes as a Hazard - Natural Hazards - page 235

1200 km 2 . Eruptions in the Taupo volcanic zone of
New Zealand are the largest to have occurred on
Earth. There is geological evidence that Taupo has
discharged pyroclastic material at the rate of
1km 3 min -1 . The 186 AD eruption of Taupo produced
60 100 km 3 of tephra, an amount five times greater
than that produced by Krakatau in 1883. The tephra
covered all of the North Island of New Zealand with at
least 10 cm of ash, while intense pyroclastic flows
overran 1000 m high ridges around the crater and
flooded an area of 15 000 km 2 .
From ignimbrites - deposits resulting from pyro-
clastic flows - there is some evidence that the flows
have climbed ridges 500-1000 m high up to 50 km
from the source of eruptions. At these high velocities,
the flows can also be erosive, gouging out channels in
less resistant sediments. Ignimbrites produced by
Peléean volcanoes are distinct from ash fallout deposits
resulting from Plinian eruptions. Because particles
settle from suspension in air, tephra deposits from the
latter tend to be well-sorted and spread evenly over
the landscape. Ignimbrites on the other hand consist
of particles of all sizes that have been transported with
air as the interstitial medium. They are made up of
volatile and gas-rich magmas of a rhyolitic or andesitic
nature, are chaotically sorted, and tend to pool in
depressions because their flow is controlled by gravity.
The heat in a pyroclastic flow can weld ignimbrite
material together, whereas Plinian deposits remain
unconsolidated. Some ignimbrites tend to have an
inverted size grading in which finer material is overlain
by coarser blocks. For this to happen, the cloud must
be very dense, travel at high velocities and be thin.
Such a process is called shear sorting , and is analogous
to large lumps in a sugar bag coming to the surface
as the bag is shaken. Alternatively, these well-sorted
deposits may represent evidence of base surges, which
usually produce thin, well-sorted beds less than 10 m
thick. Ground surges, thus, appear to precede a
large and slower moving pyroclastic flow, resulting in
chaotically sorted ignimbrites being deposited on top
of well-sorted ground-surge deposits. Because well-
sorted ignimbrites arc plentiful, basal surge events may
be more common than previously thought.
Gas es and acid rains

untrue, gases cannot be ruled out as a hazard. On
average, explosive volcanoes eject 4 10 6 tonnes of

sulfur dioxide into the atmosphere annually. This is
highly variable from year to year. For example, the Mt
Pinatubo eruption of 15 June 1991 released 20 10 6

tonnes of sulfur dioxide in the space of a few weeks.
The more explosive the eruption, the greater the trans-
port of gas on adsorbed particles. Passively, degassing
volcanoes annually release another 9 10 6 tonnes of

sulfur dioxide. While appearing voluminous, these
amounts represent only 5-10 per cent of the annual
anthropogenic flux to the atmosphere. However,
emissions of sulfur dioxide by volcanoes can have
significant local and regional effects, mainly on crops.
Carbon monoxide emissions are more dangerous,
being toxic to mammals in low quantities, and inhibit-
ing leaf respiration in plants. Even CO 2 can prove
harmful. Volcanoes can produce large quantities of this
gas (20-95 per cent of the gas discharge) which, being
denser than air, can pool in depressions and suffocate
livestock. On 21 August 1986, these gases, plus sulfur
dioxide and cyanide, were released by a landslide from
the bottom sediments of Lake Nios, a dormant
volcanic crater in Cameroon, Africa. Clouds of deadly
gases, reaching concentrations of 20-30 per cent CO 2 ,
hugged the ground and flowed under gravity into
topographic lows killing all animal life in their path.
Over 1700 people died within a matter of minutes,
while 10 000 survivors suffered skin burns. A gas
discharge in neighboring Lake Monoun, Cameroon,
killed 37 people in 1984, while a similar disaster on the
Dieng Plateau in Java in 1978 killed 180 people.
Many gases react with water vapor under the high
temperatures of volcanic eruptions to form hydro-
chloric, sulfuric, carbonic, and hydrofluoric acids.
Passively degassing volcanoes release annually into
the atmosphere 0.05-4.7 million tonnes of fluorine and
0.3-10 million tonnes of chlorine as hydrofluoric
and hydrochloric acid, respectively. Icelandic eruptions
are notoriously high in fluorides. The hydrofluoric acid
content in the Hekla, Iceland, eruption of 1970
reached 1700 ppm. Notable eruptions containing
high amounts of HCl have been Agung, Bali
(1.5 million tonnes), in 1963; Augustine, Alaska
(525 000 tonnes) in 1976; Soufrière, Guadeloupe
(1 million tonnes) in 1979; and Mt Erebus in the
Antarctic (370 000 tonnes) in 1983. In the troposphere,
acid can be precipitated out of the atmosphere
through condensation, scavenged by ash, or dissolved

(Symons et al., 1988)
Many pyroclastic flows were believed at first to consist
solely of hot gases. While this has been found to be