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104 CHAPTER 7

strongly influenced by how rapidly one explo- results. The most explosive interactions are those
sion follows on from the previous one. in which water forms 25–30% of the mixture
• The main differences between the two types of volume.
explosion are the difference in their violence and
in the chemistry of the erupting magma. Strom-
bolian events are generally very mild and are 7.5 Further reading
associated with basaltic magmas while Vulcanian
events are more powerful and associated with
Fagents, S.A. & Wilson, L. (1993) Explosive volcanic
more evolved magmas.
eruptions – VII. The range of pyroclasts ejected in
• The basic mechanism in both types of explosion transient volcanic explosions. Geophys. J. Int. 113,
is thought to be similar with gas rising through a 359–70.
near stationary magma column to accumulate Morrissey, M. & Mastin, L.G. (2000) Vulcanian erup-
beneath a vent cap or plug. The difference in vio- tions. In Encyclopedia of Volcanoes (Ed. H. Sigur-
lence between the two eruption types seems to dsson), pp. 463–75. Academic Press, San Diego, CA.
be linked to the strength of this plug, and hence Morrissey, M., Zimanowski, B., Wohletz, K. & Buettner,
the amount of pressure build-up. R. (2000) Phreatomagmatic fragmentation. In Encycl-
• Modeling work shows that the eruption veloci- opedia of Volcanoes (Ed. H. Sigurdsson), pp. 431–
ties of the two types of eruption are linked to the 45. Academic Press, San Diego, CA.
vent pressures developed prior to each explo- Wilson, L. (1980) Relationships between pressure,
sion and hence to the strength of the vent plug, volatile content and ejecta velocity in three types of
the typical velocities in Strombolian explosions volcanic explosion. J. Volcanol. Geotherm. Res. 8,
−1
of < 200ms being linked to the weak vent plug 297–313.
formed between explosions, the higher velo-
−1
cities (200–400 m s ) of Vulcanian explosions
reflecting the greater strength of vent plugs 7.6 Questions to think about
involved in these eruptions.
• Modeling can be used to predict the range of large 1 In what two main ways can transient volcanic
blocks thrown out in transient magmatic explo- explosions occur?
sions and, conversely, the position of blocks can 2 What determines the violence of a transient vol-
be used to infer the initial eruption conditions. canic explosion as judged by the eruption speed

Study of a number of eruptions shows that the of pyroclastic fragments?
vent conditions thus inferred are consistent with 3 How would you expect an explosive eruption
the link between vent pressure and plug strength under water to differ from one in air?
just described. 4 Imagine two eruptions, both of which release
5
−1
• Modeling of plumes generated in transient explo- magma at an average rate of 2 × 10 kg s (which
sions shows that the plumes are usually only of corresponds to about 80 m 3 of bubble-free
limited height but that when explosions occur magma per second). The first eruption proceeds
in close succession the eruption becomes more steadily forming a Hawaiian lava fountain with
like a sustained event (from the point of view of a constant plume above it. The second eruption
the plume) and so much higher plumes can be is Strombolian and has one explosion every ten
generated. seconds, forming a series of transient plumes.
• Transient explosions are also common in hydro- Use eqns 6.7 and 7.5 to deduce the height of
magmatic eruptions. In these events the relative the plume in each case.
quantities of magma and water involved and the 5 What is the difference between vents and root-
details of how the magma and water are brought less vents?
into contact with each other have a profound 6 What is it that makes maar-forming explosions so
influence on the character of the eruption which violent?