Page 118 - Fundamentals of Physical Volcanology

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TRANSIENT VOLCANIC ERUPTIONS 95

negligible which means that magmatic gas bubbles 7.2.1 Modeling transient magmatic
rise upwards through the overlying magma and explosions
accumulate beneath the vent “plug”. In Strombolian
eruptions it is common for this accumulating gas
RELATING VENT PRESSURES TO ERUPTION VELOCITIES
to cause an updoming of the cooled magma skin
on the top of the magma column (Fig. 1.14). This We saw in the last chapter (section 6.3) that in
“skin”, although cooled, is still plastic and can steady-state eruptions it is the expansion of gas
stretch and deform just like the rubber of a balloon which provides the energy to accelerate and erupt
being blown up. Eventually the bubble “skin” tears, the gas–magma mixture. The energy use in an erup-
releasing the gas within and ejecting the gas and tion can be defined by the energy equation (eqn
fragmented “skin” upwards and outwards (Fig. 6.4). A similar energy equation likewise forms the
1.14). If the magma forming the “skin” is cooled basis for any treatment of the dynamics of transient
to a greater degree then it may form a brittle plug explosions. During a transient explosion a certain
within the vent which has much greater strength mass of gas is trapped at a certain starting pressure,
than a plastic “skin”. In this case more pressure will P , behind the vent “plug”. This pressure is higher
i
need to build-up before the plug fails and an erup- than atmospheric pressure, P , so when the plug
a
tion occurs. In the case of Vulcanian eruptions the fails the gas will expand in order to reduce its pres-
plug material may consist of cool magma and/or sure to atmospheric. A finite amount of energy will
material from previous explosions which has fallen be released, the actual amount being a function of
back into the vent blocking it. The higher velocities the initial mass of gas and the initial pressure it was
in Vulcanian explosions suggest that the strength of under. The released energy is shared in the explo-
the plug is considerably greater in Vulcanian events sion by the expanding gas, the mass of plug material
than in Strombolian ones, so that a much higher that it expels, and the mass of atmosphere that is
pressure is reached before failure. The fact that the displaced as the expansion takes place.
two types of eruption are associated with different There are two extreme ways in which gases can
magma compositions suggests that a link exists expand: adiabatically or isothermally. As they
between plug strength and composition. Such a expand, their temperature will decline unless heat
link may arise because the higher viscosity of more is supplied to them. If no heat is supplied and the
evolved magmas means that bubbles rise more temperature declines, then the expansion is said to
slowly through them. Slow rise and accumula- be adiabatic. If an infinitely large amount of heat

tion of bubbles means that more cooling of the can be supplied to the gas there is no temperature
skin/plug will occur prior to an explosion. In the change and the expansion is said to be isothermal.
extreme case, bubble movement in high viscosity In the steady-state case treated in Chapter 6 it was
magma is so slow that segregation and rise of bub- assumed that expansion occurred isothermally.
bles is likely to be negligible. This fact leads to the This assumption is valid in the steady-state case
suggestion that meteoric water is involved in many because the gas represents only a small part of the
Vulcanian events. Cooling of the magma by contact total mass being erupted, and the much greater
with external water stops it rising, and at the same mass of magma forms a heat reservoir which is able
time that water is boiled to steam and trapped, thus to maintain the gas temperature almost constant as
providing a lot of the gas to drive the explosion (see long as the contact between the gas and magma is
below). sufficiently good. In the case of transient explo-
In purely magmatic events, the upward and out- sions, however, the contact between the gas and
ward movement of gas and fragmented magmatic magma is not as good – the short time scale of the
material is driven by the expansion of the magmatic explosion means that not much heat can travel
gas which occurs when the plug fails, exposing the from the inside of a pyroclast to its surface to reach
compressed trapped gas to the lower pressure envir- the gas. Also, because gas has been accumulating
onment of the surrounding atmosphere. under the trapping lid, the gas to magma mass ratio