Volcanic ash is really shattered magma, glass and crystal smaller than 2 mm
Volcanic ash is not soot. Each grain, smaller than 2 mm, is a shard of rock, crystal or glass, formed when gas bubbles rip rising magma apart or when magma meets water and flashes it to steam. Hard, angular and abrasive, it can ride the wind for thousands of kilometres.
The main factory is decompression. As magma climbs, dissolved water and carbon dioxide come out of solution as bubbles, turning the melt into a foam that speeds up the conduit. Once bubbles make up roughly 70 to 80 percent of the volume, the foam bursts apart and the fragments freeze in the air. Water offers a second route. When magma hits sea, lake, groundwater or ice, a thin insulating layer of steam forms first, the Leidenfrost effect, then collapses, and the sudden heat transfer blasts the magma into fine particles, which expose more surface and feed further shattering. Collapsing lava domes and eruption columns grind out still more ash as particles smash together in fast-moving pyroclastic currents.
What comes out depends on the magma. Gentle basalt eruptions yield dark ash rich in iron and magnesium with about 45 to 55 percent silica, while the most violent rhyolite eruptions produce pale, silica-heavy ash above 69 percent. Low-viscosity basalt tends to form rounded droplets, some near-perfect spheres; sticky, gas-rich magmas leave angular pumice shards whose broken bubble walls create troughs and tubes. Grains can be as fine as a thousandth of a millimetre.
Ash also carries chemistry. Around 55 kinds of ions have been found washing off fresh ash, mostly simple salts such as sodium chloride and calcium sulphate that form as acid gases in the plume attack the particles. When researchers leached Mount St. Helens ash from 1980 step by step, they found chloride salts dissolved most easily. Dry, those salts insulate; add fog or light rain and the ash turns corrosive and electrically conductive, which is bad news for power networks.
With a hardness of about 5 on the Mohs scale, silica-rich ash is especially abrasive. Its reach explains the wide list of harms: breathing and livestock problems, grounded aircraft, damaged crops and buildings, and disrupted water, telecoms and transport.
Source: Volcanic ash
