Page 144 - Fundamentals of Physical Volcanology

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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 121

the gas. However, several processes act to prevent changing; thus vertical, as well as lateral, variations
this happening. The separations between the mov- in the texture and grain size of the deposit are
ing particles may be small enough that collisions readily explained. Furthermore, to the extent
between particles moving at different speeds are that irregular topography will have its greatest
common. As clasts settle downward, i.e., undergo influence on the motion of the densest part of the
sedimentation through the gas–clast mixture, current, it is easy to understand how lateral gra-
they displace upward the gas and the very small dations between ignimbrites and surge deposits
particles effectively locked to the gas by their can occur.
extremely small terminal velocities – we speak of A final comment concerns deposits from very
a dusty gas. These processes collectively lead small pyroclastic density currents. Many of the
to strongly hindered settling. When particle– deposits formed by currents produced in the dome-
particle collisions dominate the interaction the term collapse events at Mount St Helens in 1980 were
granular flow is used to describe the motion. somewhat reminiscent of lava flows insofar as
In contrast, at least in the upper parts of the flow, they had distinctive levées, flow fronts, and central
collisions between particles may be much less fre- channels (Fig. 8.13). The levées and flow front
quent, and the bulk motion of the gas–clast mixture deposits were dominated by coarse pumice clasts
is turbulent, which leads to a constant stirring of the and the central channels contained particles that
mixture, also reducing particle settling. were very much finer and supported occasional
The nature of the deposit formed by a pyroclastic much larger pumice clasts at the surface. Examin-
density current appears to be determined by the ation of these deposits with instruments very shortly
relationship between the upper turbulent zone and after their emplacement showed that the central
the lower hindered-settling zone. The deposit con- channel materials had extremely nonNewtonian
sists of all of the clasts which have segregated to the rheological properties, and attempts were made
base of the current and ceased to have any lateral to link the structures of the deposits with the way
movement. Thus the boundary between deposit the moving density currents had interacted with
and current is constantly moving upward relative to the atmosphere at their flow fronts and margins.
the pre-eruption ground surface. In some currents, On the scale of these small deposits (lengths of
it appears that the turbulent dilute zone extends a few kilometers, widths of ∼30 m, thicknesses of
essentially all the way to the base of the current. ∼2 m) this was probably valid, but it seems unlikely
There is a very large gradient in the horizontal that incorporation of atmospheric air has much

velocity between the current and the deposit, and influence on the depositional processes at the
tractional forces cause clasts at the boundary to be bases of large-scale pyroclastic density currents;
rolled, dragged or bounced (saltated) along the the only significant effect is the production of
interface before they come to rest. This appears phoenix clouds.
to be what produces the internal stratification in
pyroclastic surge deposits (Fig. 8.9). In contrast, if
the base of the current is dominated by a laminar 8.5 Summary
zone of hindered settling and granular flow, the
gradient in the horizontal velocity between the cur- • Eruption columns contain a wide range of clast
rent and the deposit is small, traction is minimal, sizes, and some clasts of all sizes can fall out of
and stratification is essentially absent. the column at all heights. However, whereas
Figure 8.12 shows these two extreme velocity small clasts can be carried right to the top of the
distributions near the base of a pyroclastic density column, there is a maximum height to which
current. They represent end-members of a con- large clasts can be carried, and the larger the clast
tinuum of possible configurations. Which type of the smaller the maximum height.
deposition will dominate at a given location in • The well-defined shapes of eruption columns
the deposit can change with time if conditions at (Fig. 8.1) means that the lateral distance at which
the vent (mass flux, magma volatile content) are a clast of a given size and density is released can