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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 119

Table 8.2 Parameters of pyroclastic fountains forming pyroclastic density currents. For a range of pre-eruption magma water
contents dissolved in the magma chamber, n , values are given for the amount exsolved from the magma emerging from
dis
the vent, n ; the pressure at which the gas emerges, P ; the speed at which the gas and entrained pyroclasts emerge, U ;
exs v v
the bulk density of the emerging mixture, β ; and the bulk density β in the gas–clast mixture after the gas has decompressed
v e
to reach equilibrium with the atmospheric pressure.
−1
−3
−3
n dis (%) n exs (%) P (MPa) U (m s ) β β (kg m ) β β (kg m )
v
e
v
v
1.57 1.00 1.93 89.4 360 13.7
2.81 2.00 3.91 126.4 365 7.7
4.00 3.00 5.93 154.8 369 5.4
5.16 4.00 7.99 178.7 373 4.2
6.31 5.00 10.08 199.8 376 3.4
a pressure of up to 10 MPa must exist at the vent, of the 1980 eruption of Mount St Helens. In that
the value increasing with the amount of gas in the case magma was intruded beneath the surface of
magma. The consequence is to reduce the upward the steep flank of the volcano. This caused the flank
speed into the fountain to values in the range 100 to to bulge, oversteepening it until it collapsed as a
−1
200ms , leading to revised fountain heights of 1 landslide, thus uncovering the intrusion and almost
to 8 km for magma water contents in the range 1 to instantly relieving the pressure exerted on it. Bubbles
5 wt%. The smallest of these fountain heights, espe- of gas that had already exsolved from the magma
cially for even smaller magma gas contents, prob- abruptly expanded and the magma exploded.
ably corresponds to what has been described as No matter what the triggering mechanism, the
“magma boiling over from the vent” in the few very subsequent events are probably very similar in all of
small-scale eruptions of this kind that have been these types of eruption. The pressure on a layer of
observed. trapped gas bubbles is released and the outer walls
of these bubbles break, freeing the pressurized
trapped gas which starts to expand and carry with
DIRECTED BLASTS AND COLLAPSES FROM LAVA
it the bubble-wall fragments that have been formed.
DOMES AND FLOWS In other words, this first layer of bubbles explodes

The common factor in these three processes in a kind of Vulcanian explosion. But this releases
seems to be that a body of viscous magma is being the pressure on the next exposed layer of trapped
erupted, or has recently been erupted, to form a gas bubbles, and these too explode. A wave of
lava dome or a short lava flow in a way that has decompression, called an expansion wave, travels
allowed a continuous, stable, cooled surface layer into the exposed lava body, and this feeds the
to form on the lava. For a while, this cooled shell or expanding mixture of gas and fragments until, in
carapace has enough strength to resist the outward the extreme case, the entire lava body is destroyed.
pressure exerted on it by gas trapped in vesicles The speed of the expansion wave will be a large
within the lava body. Then, something disturbs the fraction of the speed of sound in the lava which,
stability of the shell. This may be a sudden increase because of the presence of the gas bubbles, will
in the rise rate of magma in the dike beneath the be a few to several tens of meters per second. The
lava body, the increased deformation rate causing speed reached by the expanding gas and pyroclasts
the rheological response to change from plastic to will depend, as we have seen for all explosive erup-
brittle; it may be the fact that some part of the edge tions, on the gas mass fraction and the difference
of the lava body becomes too steep and collapses between the pressure in the trapped gas and the
under its own weight – this might happen where the atmospheric pressure. The description of Vulcanian
dome edge or flow front advanced onto a steeper activity in Chapter 7 provides the closest approx-
slope. A third option is the one that caused the start imation to what is happening, with the strength of