Page 143 - Fundamentals of Physical Volcanology

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120 CHAPTER 8

the confining layer defining the pressure in the the gas–pyroclast mixture will be in the range 360–
−3
trapped gases. The equivalent of the calculations 370 kg m . This is much greater than the density of
−3
given in Chapter 7 suggests that for the magmatic the Earth’s atmosphere, ∼1kgm . However, as the
volatile contents of order 1–3 wt% that commonly mixture accelerates laterally away from the vent, its
apply to these evolved magmas, and for carapace bulk density will rapidly decrease as the gas decom-
strengths of a few megapascals, the initial speeds in presses to atmospheric pressure. The bulk density
these kinds of explosions could easily be at least will become similar to that expected from the dis-
−1
100ms . integration of a lava dome or lava flow; the values,
The fact that this process causes the develop- obtained from eqn 6.2, range between ∼3 and
ment of a pyroclastic density current, rather than a ∼20 kg m −3 (Table 8.2), depending on the magma
vertical eruption column, must be strongly influ- volatile content. Even the smallest of these densi-
enced by the bulk density of the mixture of gas and ties, corresponding to a very volatile-rich magma,
pyroclasts that is produced by the lava disintegra- is greater than that of the atmosphere. This ensures
tion; it will also be at least partly dictated by the that a ground-hugging density current is always
detailed geometry of the lava body and the location formed, whether from a pyroclastic fountain or
where the disintegration starts. Of course, if the the disintegration of a dome or lava flow, but clearly
erupted lava is connected to hot magma in the there is the potential for pyroclastic density
underlying dike system, it may well be that, as soon currents to have a wide range of initial particle
as the overlying erupted lava is removed, the subja- concentrations.
cent magma explosively erupts upward and forms As soon as a pyroclastic density current moves
a Plinian eruption column (as at Mount St Helens away from its source, its top begins to entrain the
in 1980) or, if this would not be stable, forms a air above it in the same way that the edge of an
pyroclastic fountain. Certainly, numerous examples eruption column entrains adjacent air. This process
have been observed of vertical eruption columns starts even sooner if the current is fed by a fountain,
being produced very shortly after dome collapse since some mixing with the surrounding atmo-
events. sphere must happen on the outer surface of the
fountain as material descends toward the ground.
This inclusion of air must decrease the bulk density
8.4.3 Ignimbrite emplacement mechanisms
of the part of the current involved, and so it begins
Our understanding of the way pyroclastic density to cause a density stratification within the current.

currents emplace their characteristic deposits has As part of the interaction between the density cur-
advanced enormously over the past two decades, rent and the overlying air, a mixture of heated air
although there are still many details that are not and volcanic gas elutriates some fraction of the
fully understood. One of the most important issues smallest clasts from the main body of the current to
relates to the density of the currents while they are form a growing convection cloud above it. This is
moving and the motions within them – especially called a co-ignimbrite cloud, or phoenix cloud
the extent to which those motions are turbulent (see Fig. 8.10). These clouds may be carried by the
or laminar. These factors control the way clasts are wind in a quite different direction from that of the
deposited on the ground as the current passes a density current, eventually depositing their small
given location. clasts to form a characteristic co-ignimbrite ash
Because of the way they are formed, the currents fall deposit.
are likely to start out as homogeneous mixtures of There is another important source of stratifica-
gas and pyroclasts, but they may have a wide range tion within the main body of the density current:
of bulk densities. If a current is formed from a the tendency of clasts to fall through the gas phase.
pyroclastic fountain, the analysis described in the In principle every pyroclast will try to reach its
previous section shows that the pressure in the equilibrium terminal velocity in the gas, which
gas emerging from the vent will be 1 to 10 MPa eqns 8.1 and 8.3 show to be a function of the clast
(Table 8.2) and as a result the bulk density of size and density together with the properties of