We have done the first-ever-made attempts to sample ash clouds of erupting volcanoes. Sampling
device was carried into eruptive clouds of Karymsky volcano by tethered balloons filled with helium.
GOAL
To know grain-size distribution of pyroclasts in eruptive cloud.
PROBLEM
One of important problems of volcanology is REAL grain size distribution of pyroclastic
material in eruptive cloud. Without knowing that it is impossible to calculate precisely erupted
volumes of magma, to understand processes of magma fragmentation, to predict effects of eruptions
on climate, and to estimate volcanic danger to aircraft. Currently the problem is solved only
approximately, mainly by means of various calculations of the original grain size basing on grain
size analyses of ash-fall deposits collected on the ground. The calculations are necessary because
ash-fall deposits result from complex gravitational settling of the original population of pyroclasts
from turbulent eruption cloud through layer of air. As a result larger particles fall out in more
proximal areas to the source than
do smaller ones, while finest particles can stay suspended for years in atmosphere, and thus their
percentage can not be calculated precisely. Percentage of the finest fraction is the key question.
Rough estimations show that mass of fines can be at least equal to, or up to ten times exceed the
mass of larger particles, which form ash-fall deposits. Volume of fine-grained pyroclastic material
ejected by one large eruption can reach hundreds cubic kilometers.
APPROACH
Obviously, the only reliable solution of the above problem, is DIRECT sampling of eruptive cloud. Although
sounds unrealistic, for small-to-moderate-scale persistent eruptions that can be done easily using
rather simple equipment. There are no principle obstacles to sample also clouds of big eruptions.
To determine the original grain-size distribution it is necessary to catch and isolate some volume of
the eruptive cloud and after complete settling of pyroclasts, make standard analysis of the sample
(sieve, pipette, laser particle counter, SEM). Such as the most interesting question is content of finest fraction, volume
of the sample should not be very large - fines will be statistically represented even in several
liters - several tens liters of eruptive cloud, depending on concentration. To carry sampler into the
cloud, the unmanned aircraft can be used. Seems strange, but to our knowledge, until now no
attempts to do that were made.
EXPERIMENT
In summer 2003 we have made the fist ever made attempt to sample eruptive cloud.
Karymsky was selected as relatively safe volcano producing regular small-to-moderate
scale explosions of vulcanian type. Cone of the volcano is 1540 m. high, with relative elevation
about 700 m. In the time of the experiment, explosions occurred every
5 - 45 min, ejecting incandescent bombs up to 0.5 km high, and convective
ash cloud rose up to 1,5 km above the crater (3 km a.s.l.).
To carry the sampler into eruptive cloud we used a tethered balloon filled with helium. The
tether was rolled in and rolled out with a hand winch. Two types of balloons were used:
Zeppelin-shaped plastic aerostat and standard latex
weather balloon. Each balloon contained
about 6 cub.m. of helium. Two basic schemes of sampling were tried: launch from location
downwind from the crater (simple lifting of the balloon in drifting eruptive cloud) and launch
from upwind location (when wind tilted the balloon toward the crater). In the first scheme,
sampling device was working in already cooled and strongly diluted part of the eruptive cloud.
Conditions inside the cloud were not destructive for the balloon, thus sampler was directly
attached to it. The low-temperature sampler was a 30-liters plastic
box like a big mouse trap, which had electronic
triggering device based on a dust sensor. In the second scheme the eruptive cloud was thought to
be still hot and containing high concentration of large strongly abrasive particles. Thus the
balloon in this configuration ought to fly higher than the eruptive cloud, and sampler was hanging
down from the balloon on a long (500m) metal line. The 10-liters
high-temperature sampler was also like a mouse
trap, but smaller and made of metal. Triggering mechanism represented a plastic filament which
kept the lid of the sampler in opened condition. When the sampler entered hot cloud the filament
melted allowing the lid to be closed.
RESULTS
Totally 4 launches were made with the maximum achieved altitude about 2,5 km above the launching
site and 1,7 km above the crater (3300 a.s.l.). Although the sampling itself was not achieved, the
experiment have demonstrated principle validity of the designed samplers as well as sampling
procedure. Main reason of sampling failure was that we used rather cheap types of balloons, which
were not sturdy enough to withstand harsh weather conditions on the volcano. Apart from budget
shortages, cheap balloons were used because risk to loose an expensive balloon due to tether
breaking seemed rather high. Our experience have shown that main danger to balloons is their
breaking in strong wind. Even with cheap balloons we were very close to our goal. The experiment
allowed us to receive valuable experience in operating balloons above erupting volcano. Also we
understood how to improve the equipment for the future success. The experiment has been continued
at 2004 and 2005.