Volcanic ash aggregation occurs when particles of

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...

collide and stick together during transport. This process modifies the

size distribution Size in general is the magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions (length, width, height, diameter, perimeter), area, or volume. Size can also be measur ...

of airborne particles, which affects both atmospheric dispersal and fallout patterns on the ground. Aggregation also impacts the dynamics of volcanic plumes, pyroclastic density currents, and their associated

hazards A hazard is a potential source of harm. Substances, events, or circumstances can constitute hazards when their nature would allow them, even just theoretically, to cause damage to health, life, property, or any other interest of value. The probabi ...

Numerical models

There are two main approaches to include the effects of ash aggregation in numerical models of ash injection and dispersal. One is to initialize the model with an aggregated grain size distribution, by moving fractions of the erupted mass into larger size bins (for example, the Cornell model). A second approach is a full theoretical description of aggregate growth through time, based on the

Smoluchowski coagulation equation In statistical physics, the Smoluchowski coagulation equation is a population balance equation introduced by Marian Smoluchowski in a seminal 1916 publication, describing the time evolution of the number density of particles as they coagulate (in ...

. Several methods exist to deal with this equation, including continuous and discrete methods. Continuous methods use the method of moments to track the evolution of a continuous grain size distribution, typically represented by a mean and standard deviation. In contrast, discrete methods use a discretized (binned) grain size distribution. Areas of uncertainty in the numerical schemes include parameterization of the particle sticking efficiency, timescales over which aggregation occurs, and the fraction of erupted mass participating in the aggregation process.

References

{{reflist Volcanic rocks