Geoscience Reference
In-Depth Information
acidic, and give off large blocks of very viscous, hot
magma with few if any flows. This very viscous material
may be tossed up to heights of 10 km (40 km in
extreme cases). Activity is explosive for periods ranging
up to several months. These volcanoes build up tephra
and block cones. Usually the lava freezes in vents
between eruptions, so that the strength of the plug ulti-
mately determines the amount of pressure that can
build up before the next eruption. 'Surtseyan' erup-
tions are violently explosive because hot, fluid magma
encounters seawater. These types of eruptions are
termed 'phreatomagmatic' and make up 8.8 per cent
of all volcanic events. Because of the interaction of
magma and water, large clouds of very fine dust are
produced that can drift several kilometres upward into
the atmosphere. Coarser particles tend to form rings
around the vent, rather than cones or large ash sheets.
The rubbing together of fine dust particles builds up
static electricity, leading to numerous and spectacular
lightning displays around the rising column of ejecta.
'Plinian' events are more explosive, with wide
dispersal of tephra. Over 1 km 3 of magma may be
driven straight up, at velocities of 600-700 m s -1 , to
heights of 25 km by a continuous gas jet and thermal
expansion over the course of several hours. As the
magma chamber slowly empties, the character of the
magma changes and the eruption becomes starved for
material. Usually, the volcano then collapses internally
under its own weight to form a caldera. The Mt
Vesuvius eruption of 79 AD was of this type, and will
be described later in more detail. While such events
have been dramatic on Earth, on other planets with
lower gravity and thinner atmospheres, Plinian
eruptions have been truly cataclysmic.
If the eruptive ash column from an explosive volcano
collapses under the effect of gravity, it can produce a
debris avalanche of ash and hot gas that can race
downslope at speeds reaching 60 km hr -1 . This type of
eruption is termed a 'Peléean' eruption after the explo-
sion of Mt Pelée in 1902. The debris clouds are called
pyroclastic flows or nuées ardentes , and the resulting
debris deposits are termed ignimbrites . The large death
toll of Vesuvius in 79 AD can be attributed to an
associated pyroclastic flow event. Pyroclastic flows will
be examined later in this chapter when secondary
volcanic hazards are examined in detail. The most
destructive and largest Peléean eruptions are termed
the 'Katmaian' type, after the Katmai eruption at the
eastern end of the Alaska Peninsula on 6 June 1912.
This eruption ejected ten times the material of the 1902
Mt Pelée event. As the magma chamber emptied,
a 2500 m high mountain collapsed into a caldera
5 km across and 1 km deep. The eruption was heard
1200 km away, with dust being deposited over a
1500 km distance. The pyroclastic flow from this
eruption flooded into a valley 20 km long, leaving an
estimated 11 km 3 of hot, fused ash spread to a depth of
250 m.
Peléean eruptions should not be used to refer to all
volcanic events that generate hot ash flows. Many
destructive pyroclastic flows have not originated as ash
columns collapsing under the effect of gravity, but have
been preceded by a ground or basal surge caused by
the lateral eruption of the volcano. This type of
explosion is termed a 'Bandaian' eruption. The 1980
eruption of Mt St Helens in the United States was of
this type, which is more properly known as a base
surge . Because the force of the eruption shoots out
particles at velocities in excess of 150 km hr -1 , the
blasts have been likened to cyclones of ash. Objects in
the paths of such hot and violent blasts are destroyed
by sandblasting. Base surges often precede a large and
slower moving pyroclastic flow, and represent the most
destructive of all types of volcanic eruptions.
VOLCANIC HAZARDS
(Bolt et al., 1975; Whittow, 1980; Hays, 1981; Tazieff &
Sabroux, 1983; Blong, 1984; Wood, 1986; Ritchie &
Gates, 2001)
There are many hazardous phenomena produced
directly, or as secondary effects, by volcanic eruptions.
The association of earthquakes and tsunami with
volcanoes has already been covered in Chapter 10, and
will be discussed in detail for particular events at the
end of this chapter. Excluding these two phenomena, it
is possible to group volcanic hazards into six categories
as follows:
• lava flows,
• ballistics and tephra clouds,
• pyroclastic flows and base surges,
• gases and acid rains,
• lahars (mudflows), and
• glacier bursts ( Jökulhlaups ).
Lava flows
Since the sixteenth century, 20 km 2 of land per year has
been buried by lava flows. There are about 60 lava flow
 
 
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