The density (mass per unit volume) of individual ash particles varies. For example, 700-1200 kilograms per cubic meter (kg/m3) for pumice, 2350-2450 kg/m3 for glass shards, 2700-3300 kg/m3 for crystals, and 2600-3200 kg/m3 for lithic particles (see table below). Pumice fragments may form temporary mats of floating material if deposited on water. Since larger and more dense particles are deposited close to source, distal ashfall deposits are relatively enriched in fine glass and pumice shards.
|Type of ash particle||Density of particle|
|Pumice fragments||700-1,200 kg/m3|
|Volcanic glass shards||2,350-2450 kg/m3|
|Crystals and minerals||2,700-3,300 kg/m3|
|Other rock fragments||2,600-3,200 kg/m3|
Grain-size, composition (proportions of crystal, lithics, glass shards and pumice fragments), particle shape, and moisture content determine the bulk density (total deposit density including all constituents) of ash deposits. Less spherical particles (more angular or irregularly shaped) will pack relatively poorly, resulting in higher porosity and lower bulk densities. Particle aggregation prior to deposition will result in higher particle packing and therefore higher densities.
The dry bulk density of newly fallen and slightly compacted deposits ranges from 500 to 1500 kg/m3, whereas the bulk density of wet ash ranges from 1,000 to 2,000 kg/m3. Distal ashfall deposits most commonly show slight decreases in bulk density with distance from the volcanic vent source.
Ash particles commonly have sharp broken edges, which makes volcanic ash a very abrasive material. The degree to which volcanic ash will be abrasive, however, is a function of the hardness of the material forming the particles, as well as the shape of those particles. Hardness values for the several common minerals are shown in the table below.
|Scale Number||Mineral||Metal||Minerals in volcanic ash and their hardness|
|mica (H 2-3)|
|5||Apatite||Steel||volcanic glass, pyroxene, amphibole (H 5-6)|
|plagioclase, alkali-feldspar (H 6-6.5)|
|7||Quartz||olivine (H 6.5-7)
quartz (H 7)