Geoscience Reference
In-Depth Information
in water-rich eruption plumes. Above 6000°C, hydro-
fluoric acid forms calcium fluorosilicate that adheres to
the glassy surfaces of tephra particles. Despite this
culling, acids can still pose a hazard over a large area.
The Katmai eruption of 1912 dispersed a sulfuric
acid rain that damaged clothes hung out to dry in
Vancouver, 2000 km to the south. The Laki, Iceland,
eruption in 1783-1784 filled the skies over most of the
north Atlantic, western Eurasia, and the Arctic with a
dry sulfuric fog that led to a cold winter. Explosive
eruptions can also inject chlorine in the form of HCl
directly into the stratosphere where it can react with
ozone. Tambora in 1815 and Krakatau in 1883 were
explosive eruptions, ejecting a minimum of 2.1 and
3.6 million tonnes of chlorine as HCl, respectively. The
eruption of El Chichon, Mexico, in 1982 released
40 000 tonnes of HCl into the stratosphere between
20 and 40°N latitudes.
case, a pyroclastic flow can easily entrain water from
streams and rivers as it moves down topographic lows.
In the process, the gas-rich flow is slowly converted
to a fast moving, heated mudflow as more water is
entrained in the mix. The Toutle River lahars (see
Figure 11.4) from Mt St Helens in 1980 had this origin.
Volcanoes with crater lakes can produce mudflows at
the time of any eruption, if the crater lake is ruptured.
The size of the mudflow is then related directly to the
volume of water in the lake. The 1919 eruption of
Mt Kelat on Java expelled water from a crater lake,
covering 200 km
2
of farmland and killing over 5000
people. Attempts have been made to control the Kelat
situation by drilling tunnels through the walls of the
crater to lower the level of the lake. The first attempt
was in 1929, when Dutch engineers reduced the lake
volume from 21 to 1 million m
3
. A subsequent
eruption in 1951 blocked the tunnels and, even after
repairs, an eruption in 1966 generated lahars that took
hundreds of lives. Indonesian engineers have since
replaced the tunnels and drained the lake completely
to negate the threat of future lahars.
Secondary lahars are caused by rain falling on
freshly deposited, uncompacted tephra. Such water-
soaked material is very unstable, and can move down-
slope as a mudflow that entrains all loose debris in its
path. If acids have not been leached out of the tephra,
then lahars can become acidic enough to cause serious
burns. Such flows have covered enormous areas. One
Laha
rs
(Newhall & Punongbayan, 1996)
One of the more unusual, though still hazardous, phe-
nomena produced by volcanoes consists of lahars, or
mudflows, that can occur at the time of the eruption
(primary lahars) or several years afterwards (secondary
lahars). About 5.6 per cent of volcanic eruptions in the
last 10 000 years have produced mudflows at some
time. Primary lahars can be generated by pyroclastic
flows or by eruptions of crater lakes. In the former
British Columbia
Mt Baker
N. Toutle R.
Mt Olympus
Washington
Pyroclastic
flow
Lahar
Mt Rainier
Yakima
S. Toutle R.
Muddy R.
Mt St Helens
Mt Adams
Mt St Helens
Kelso
Mt Hood
Columbia R.
Oregon
N
Idaho
0
10
20
30 km
Mt Shasta
Lassen Peak
Location map of Mt St Helens (adapted from Blong, 1984).
Fig. 11.4
