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TRANSIENT VOLCANIC ERUPTIONS 103

eruption when water and magma mix together so great that near the contact surface it becomes
efficiently is analogous to what in an industrial set- solid. If it cools fast enough, the stress caused by the
ting is called a fuel-coolant interaction (FCI). Fuel- large temperature gradient near the surface may
coolant interactions are events in which the rapid shatter the solid. This liberates fragments of solid
transfer of heat from a hot fluid (the fuel) to a cold magma that are stirred into the water and may still
volatile fluid (the coolant) results in the rapid con- be hot enough to boil some more water. Probably
version of the thermal energy into kinetic energy, more important, however, the removal of the frag-
i.e., an explosion. A good example of where such an ments exposes yet more hot magma to come into
FCI might occur is when molten steel is poured close contact with water. A runaway situation can
into a crucible for transfer elsewhere within a steel- develop in which each new pulse of steam genera-
works but rainwater has been allowed to accu- tion provides more than enough energy to drive
mulate in the bottom of the crucible. The resulting the next cycle of deformation, cooling, shattering,
explosion shatters the crucible into high-speed shrap- and stirring. Once this critical state is reached, the
nel. Fortunately such accidents are rare. Because explosive interaction between the water and magma
FCIs have been studied extensively, this wealth of must continue violently until one or other of the
engineering literature can provide insights into two components is completely used up.
the processes which are important in the magma–
water interactions of hydromagmatic eruptions.
7.3.3 Tephra from hydromagmatic
Whether a mixing event between magma and
eruptions
water is explosive or nonexplosive depends crit-
ically on the rate of transfer of heat between the The tephra from these eruptions often differs sig-
fuel (magma) and coolant (water), and this in turn nificantly from that produced in purely magmatic
depends on the surface area over which the two eruptions. For instance, in general hydromagmatic
components are in contact. The initial step in any eruptions produce finer grained material than
FCI is one in which the fuel and coolant first come magmatic eruptions. The tephra usually consists of
into intimate contact with each other. If this inter- magmatic glass, crystals, and lithic material (wall
action is sufficiently gentle the boiling of the water rock and material which has slumped into the vent
will create a stable vapor layer between the fuel and system), often with the lithic component dom-
coolant which limits the transfer of heat between inating. Examination of the juvenile glass under
the two, as in the case of pillow lavas. However, if an optical or electron microscope further shows

the initial interaction is sufficiently vigorous the that the glass typically takes on a number of distinct
vapor layer may collapse or never form properly. forms: blocky, moss-like, platey, and spherical, which
This could happen for a number of reasons, for experimental studies have suggested are linked to
instance because of the passage of a seismic wave the details of how the magma–water interaction
through the mixture, because a local implosion occurs in a particular eruption.
occurs as vapor condenses or because a magmatic
explosion (e.g., a Strombolian bubble burst) sud-
denly accelerates the magma into the water. 7.4 Summary
Once the vapor layer is disrupted the fuel and
coolant come into good thermal and mechanical • There are two main types of purely magmatic
contact and rapid conversion of a small volume transient explosions: Strombolian and Vulcanian.
of water to a large volume of steam occurs. If These share a number of features in common,
the expansion of the steam forces the interface including: short duration; short time between
between the water and magma to deform and individual explosions; they often occur in long
become wavy, the surface area over which they are sequences; they are characterized by the ejec-
in contact increases and this leads to even more tion of large blocks and volcanic bombs; and
steam formation. This is a positive feedback situ- they generate eruption plumes which are usually
ation. The removal of heat from the magma may be moderate in height but with the height being