what is volcanic ash?
Volcanic ash consists of particles smaller than 2 mm in diameter produced by volcanic activity.
The particles may consist of fragmented magma (juvenile material), fragmented rock (lithics) or a mixture of the two.
Ash particles are typically composed of crystals and volcanic glass formed within magma, but may also include fragments of rock forming the volcano.
Ash particles are transported in the atmosphere by wind patterns and are highly susceptible to regional weather systems. Ash that reaches at least 10 km above sea level and enters the jet stream can be transported great distances, sometimes encircling the earth.
how is it formed?
Volcanic activity varies from explosions to inflation to dense, hot flows and there are also different ways that volcanic ash is generated (primarily fragmentation); these ash particles interact then interact, changing their shape and size (secondary fragmentation)
As magma rises it loses gases, and bubbles trapped in the magma can become highly pressurised. During an eruption, gas in the bubbles rapidly decompresses and expands, breaking the bubble walls and fragmenting the magma, just like opening a champagne bottle. The ash particles are highly angular and contain curved remnants of the bubble walls.
Low viscosity magmas flow like a thick liquid, while high-viscosity magmas behave more like a rock. When magma rises it experiences shear and friction against the stationary wall rock. Under intense shearing magma can break and fragment, producing particles that have a flat, platy appearance.
At shallow depths magmas may encounter a water table or a hydrothermal system. Liquid water flashes to steam, dramatically increasing in volume and causing the partially molten magma to fragment in a steam explosion (called a phreatomagmatic eruption). The particles produced are blocky and angular with characteristic quenching features and particle sizes.
Particles can also be produced through brittle deformation of rock-like magma at shallow depths. The fracture, grinding and faulting of high-viscosity magma produce ash particles that may be ejected during an eruption. These sorts of particles were common during the 2004-2008 eruption of Mount St. Helens.
This can occur through particles colliding and splitting in a dense flow, or particles chipping and rounding as they grind and mill. Particle interactions can happen within the volcano, in an explosive ash plume and within pyroclastic flows - dense, ground-hugging flows consisting of hot particles and gases. Secondary fragmentation increases the proportion of fine particles and also rounds particles, reducing their angularity.
more volcanic ash properties...
Volcanic ash particles that are ejected from a volcano react with the volatile gases in the plume, forming a surface enriched in S, F, Cl and other volatile species. This affects the toxicity of the ash and the volatile elements can leach into waterways after deposition. The size, shape and chemistry of the ash particles determines the propensity to scavenge volatiles from the ash plume.
Within an ash plume, collisions result in particles sticking and cementing together to forms aggregates. This is particularly effective in humid environments, and causes ash to fall out from the plume earlier than expected as the aggregate mass and size increases. Aggregation is particularly effective at removing fine ash particles from an ash cloud.
Volcanic ash also provides a site for ice nucleation and which strongly affects cloud dynamics and longevity, important for climate interactions, meteorology and ash plumes. This will be explored in detail in this project co-hosted at the University of Leeds and LMU Munich
Finally, a spectacular result of particle interaction is volcanic lightning. You can learn all about it in this project by colleagues at LMU Munich.