Page 132 - Fundamentals of Physical Volcanology

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PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 109

1000

100 Lithics

Terminal velocity (m s –1 ) 0.1 Pumice
10

1

0.01

Fig. 8.4 Terminal velocities in the
atmosphere at sea level of typical 0.001
dense lithic clasts and vesicular 0.01 0.1 1 10 100 1000
pumice clasts. Clast diameter (mm)

the column, are quite complex. So far it has been shows that certain characteristics are common to
shown how they depend on the eruption condi- them all.
tions (magma gas content and mass flux from the
• The size of the largest clast found at any location
vent) and the general atmospheric properties, espe-
in the deposit decreases with increasing distance
cially the decrease in atmospheric pressure with
from the vent. This is because the largest clasts fall
height. However, the properties of the atmo-
out from the lowest heights in the eruption column
sphere vary with latitude and season, and the water where the column is narrowest, and therefore land

content of the lower atmosphere – the troposphere closest to the vent (Figs 8.1 & 8.2).
– varies enormously with local weather conditions.
• At any given site a fall deposit exhibits a range of
All of these factors influence the details of eruption
clast sizes. This is mainly the consequence of turbu-
column behavior and particle fallout, and so to simu-
lence carrying clasts to the column edge, allowing
late a particular eruption of a specific volcano one
them to fall out prematurely, before reaching the
needs to use appropriate values for the properties
maximum height that they could have attained in
of the local atmosphere. If it is a historic eruption
the column, and so land closer to the vent than
in a populated area the weather conditions may be
expected. There are other causes. Large clasts have
known extremely well, but if it is a prehistoric erup-
a high terminal velocity in the atmosphere and so
tion and there is no information on the time of year
may land at such a great speed that they break. Hot
when the event took place, the best we can do is to
clasts may cool so much that thermal stresses cause
use the appropriate annual average conditions.
them to crack. Both of these processes give a false
impression of the maximum clast size, and this is
8.2.5 Common features of fall deposits why volcanologists always measure size and den-
sity of clasts. It is very common for a fall deposit to
Study of fall deposits from recent steady eruptions contain not only clasts derived from the erupting
that produce eruption plumes, including Hawaiian magma but also pieces of dense rock torn from the
eruptions as well as subPlinian and Plinian events, walls of the dike and vent system feeding the erup-