Page 123 - Fundamentals of Physical Volcanology

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100 CHAPTER 7

Table 7.1 Parameters obtained for a number of transient explosive eruptions (S: Strombolian; V: Vulcanian) using the model
represented by eqn 7.1: D, diameter of the largest volcanic bomb measured; R, maximum range to which bombs were
observed to be thrown; U , calculated maximum speed of the ejecta at the end of gas expansion; P , inferred pressure in the
f i
gas at the start of the explosion; n, implied weight fraction of the explosion products that consisted of gas; KE, fraction of all
of the explosion energy that appears as kinetic energy; PE, fraction that appears as potential energy; DE, fraction that appears
as energy used to displace the atmosphere.
−1
Volcano Type D (m) R (m) U (m s ) P (MPa) n (%) KE (%) PE (%) DE (%)
f i
Arenal (1968) V 1.3 5000 300 10 6 39 4 57
Ngauruhoe (1975) V 0.8 2800 250 5 4 47 2 51
Stromboli (1975) S ? 25 150 0.1 20 20 5 75
Heimaey (1973) S 0.2 500 200 0.35 20 20 1 79

that in all cases more than half the available energy time to complete its formation, let alone disperse,
goes into pushing the atmosphere out of the way before the next ejection of gas and clasts took
during the expansion phase of the explosion. place. As a result, the plume was able to be main-
tained in a way more analogous to that of a steady-
state eruption, and the resulting plume heights
PLUME HEIGHTS IN TRANSIENT ERUPTIONS
averaged 6–10 km. Typical masses of material
5
We saw in Chapter 6 that the height of the plume ejected at Heimaey were about 5 × 10 kg, so that if
formed in a steady eruption depends primarily on the explosions had been widely spaced in time the
the rate at which heat is supplied to it and thus on expected plume height would have been just over
5
the mass flux of the eruption. In a transient explo- 1 km. Instead, the ejection of 5 × 10 kg typically
sion it is the total amount of heat released, rather once every second corresponded to an average
5
−1
than the release rate, that matters, and so it is the release rate of 5 × 10 kg s , and eqn 6.7 shows
total mass of erupted material that controls the that this should have led to a plume height of
plume height. The relationship for the Earth’s 6.3 km, in reasonable agreement with what was
standard atmosphere is observed. Needless to say, the fact that the plume
rose six times higher than if the explosions had
H = 0.042 M 1/4 (7.5) been less frequent means that the smaller ejected
e
clasts were deposited from the plume over a much
where H is the plume height in kilometers and M is greater area around the vent.
e
the total mass of solids and gas ejected in kilograms.
However, another factor also influences the
7.3 Transient eruptions involving
heights of the eruption plumes generated in tran-
external water
sient eruptions: the time gaps between individual
explosions. For instance, individual explosions
occur at time intervals of ∼10 minutes at Stromboli Some Vulcanian explosions are thought to result
which allows the plume from any one explosion to from the interaction of magma with groundwater
disperse before the next event takes place. Typical rather than from the exsolution and segregation of
masses of ejecta are up to 500 kg and these gener- magmatic volatiles, and these explosions are thus
ate plumes with maximum heights of ∼200 m. By hydromagmatic eruptions. Interactions between
contrast, Strombolian activity at Heimaey in 1973 magma and an external source of water are com-
consisted of individual explosions which occurred mon, and show a wide variety of eruption styles of
every 0.5–2 s. The short time gap between these which Vulcanian explosions are just one example
individual explosions meant that the plume gener- (see Chapter 1). Many of these types of eruption are
ated by any one explosion would hardly have had transient in character like the magmatic eruptions