Abstract
Volcanic ash clouds are carefully monitored as they present a
significant hazard to humans and aircraft. The primary tool for
forecasting the transport of ash from a volcano is dispersion modelling.
These models make a number of assumptions about the size, sphericity and
density of the ash particles. Few studies have measured the density of
ash particles or explored the impact that the assumption of ash density
might have on the output of a dispersion model. In this paper, the raw
apparent density of 23 samples taken from 15 volcanoes are measured with
gas pycnometry, and a negative linear relationship is found between the
density and the silica content. For the basaltic ash samples, densities
were measured for different particle sizes, showing that the density is
approximately constant for particles smaller than
100 μm. There is a deviation in
density of up to 25% from the
operational model currently used by the London Volcanic Ash Advisory
Centre (VAAC); by inputting the measured density-size relationship into
a numerical simulation, up to 18%
difference in ash fallout time was found, with the VAAC model
overestimating ash removal times.