What is volcanic ash?

Sample 1

The density of individual particles may vary from 700-1200 kg m^-3 for pumice, 2350-2450 kg m^-3 for glass shards, 2700-3300 kg m^-3 for crystals and 2600-3200 kg m^-3 for rock particles. Pumice fragments may form mats of floating material if deposited on water.

The bulk density of any pyroclastic fall deposit can be variable, with reported dry bulk densities of newly fallen and slightly compacted deposits ranging from between 500 and 1500 kg m^-3.

The abrasiveness of volcanic ash is a function of the hardness of the material forming the particles and their shape. Ash particles commonly have sharp broken edges, which makes them a very abrasive material.

Freshly fallen ash grains commonly have surface coatings of soluble components (salts) and/or moisture. It is these components that make ash mildly corrosive and potentially conductive. These soluble coatings are derived from the interactions in an eruption column between ash particles and aerosols, which may be composed, of sulphuric and hydrochloric acid droplets with absorbed halide salts. This process is most active close to a volcano (i.e. <50 km), although the amount of available aerosols varies greatly even between eruptions of similar volumes. The release of soluble components (leachates) can also result in changes to local water chemistry and hence quality.

Ash fall predictions

Scientisits from the Institute have developed computer programmes to estimate the ash that may fall from a volcanic ash plume. To model these ash falls one has to know the eruption column height, volume of ash erupted and information on the wind at various altitudes. Each day we are modelling a typical moderate scale eruption from both White Island and Ruapehu volcanoes. Maps showing the outputs of these models are shown here.

Volcanic Ash and Aviation

This threat came to wide public attention in 1982 when two 747 passenger jets encountered ash at night from separate eruptions of Galunggung Volcano in Indonesia. In these incidents, volcanic ash extensively damaged exterior surfaces, instruments, and engines, resulting in the loss of thrust and powerless descents of nearly 25,000 feet before the pilots of both aircraft restarted their engines and landed safely at Jakarta. The Galunggung encounters occurred for two main reasons. First the pilots were unable to see the ash or to otherwise detect it using on-board instruments, and second, no warnings about the activity of the volcano were contained in the aeronautical information generally available to pilots, such as notices of significant meteorological events-SIGMET's — or in notices to airmen — NOTAM’s. These incidents led in 1982 to the formation of a volcanic ash warning group under leadership of the ICAO.

Eruptions and aircraft encounters with ash clouds during the past 15 years have prompted several other important international efforts to mitigate the volcanic hazard to aviation safety. Because volcanic ash clouds are carried by upper-level winds and often cross national boundaries as well as boundaries separating flight-information regions, efficient and prompt communications between regions are essential to avoiding encounters.


Compaction, erosion and redeposition of ash

The bulk density of an ash deposit will increase with time by up to 50% (within a few weeks) and thickness will correspondingly decrease.

Mount St. Helens' 1980 ash showed initial resistance to wetting and water beaded on its surface. However, this resistance lasted for only a few hours in light rain and was eliminated by heavy rain in minutes. After initial wetting, an undisturbed ash layer may remain persistently wet due to the inefficient water drainage from between the angular surfaces of grains. Raindrops impacting on an ash layer contribute to rapid compaction as porosity decreases. Pore space saturation will then occur relatively rapidly during heavy rain. In the 1964 Irazu eruption fine-grained soft, loose ash formed a hard impervious surface crust thought to be a result of precipitation of soluble salts by evaporation

When dry ash falls onto areas without vegetation cover or on paved surfaces it may be reworked by the wind. On moistening by rain, ash usually exhibits cohesive properties that dramatically decrease its reworking potential. The erosion resistance of compacted ash will increase as grains nest more tightly together. Mount St. Helens' 1980 ash was almost completely stripped from slopes of 50^o or steeper, with redeposition nearly always local and immediate. It is during severe rainstorms that ash is readily eroded from the land surface to be ultimately deposited in streams or rivers. Such events are little different to the behaviour of soils on non-vegetated land during similar severe rainstorms.