Ash, the Atmosphere and the Anthrosphere

The Atmosphere

If a Taranaki eruption were to be of sufficient size, the eruption could have effects on the whole world, not just New Zealand. The eruption of Mt St. Helens in May, 1980 is an example of a plinian eruption (VEI 5). The eruption of Mt Pinatubo in June, 1991 is an example of an ultra-plinian event (VEI 6). The Mt Pinatubo event sent volcanic material as high as 35 km into the atmosphere (Newhall, et al., 2005). As a comparison, commercial jet airliners generally fly at altitudes of 10 to 12 km. An altitude of 35 km means that the eruption was strong enough to put the ash into the stratosphere. The aerosols remained in the atmosphere for over 18 months. These aerosols reduced the solar radiation reaching the earth, with reductions of 25 to 30 percent observed at widely distributed stations. Overall, a global cooling of at least 0.5 to 0.7 degrees Celsius resulted from the ash cloud (Self, 2005). 

The eruption column of Mt Pinatubo on the 12th of June, 1991 (Newhall,et al. 2005).

The Mt St Helens eruption in 1980 was not as strong as the Mt Pinatubo eruption, however the explosion was still very large. According to the U.S. Geological Survey, the eruption column of Mt St Helens rose to an altitude of approximately 22 km in under 10 minutes. The eruption cloud succeeded in circling the earth in approximately two weeks (USGS, 1999).

The eruption column of Mt St Helens, on the 18th of May, 1980 (USGS, 1999).

A large eruption like Pinatubo is very unlikely for Mt Taranaki. However smaller eruptions can still send material to a considerable altitude. The 1995 eruption of Mt Ruapehu sent an ash cloud into the atmosphere, which had considerable effects on the East Coast of the North Island. If Taranaki were to experience an eruption comparable to Ruapehu in 1995, the resulting ash clouds would be capable of having wide-reaching effects throughout the North Island. Ohakea Airbase, approximately 150 km southeast of the volcano, has provided high altitude wind direction data. This will help to predict the likely dispersal of the volcanic ash. Tephra distribution is likely to follow a northeast and southeast sector for 91% of the time between 12,000 and 16,300 metres. For altitudes from 3000 to 10,400 metres, ash is likely to be dispersed in the same northeast and southeast sector 73% of the time and 55% of the time for altitudes below 3000 metres (Neall & Alloway, 1991). 

If an event comparable to Mt St Helens in 1980 were to occur, then the effects could be felt worldwide. New Zealand's geographical position on the planet puts it in an area affected by a global wind system known as the Westerlies. These winds, as their name suggests, blow from the west to east. This means that, if Taranaki's eruption column were to reach a significant altitude, the ash cloud could be blown eastward across the Pacific Ocean. This means that it would be capable of affecting South American countries such as Chile and Argentina. A similar effect was experienced by New Zealand in 2011 when a Chilean volcano Cordón-Caulle erupted. The ash cloud was blown east across the Atlantic and the Indian Oceans to reach southern Africa, Australia and New Zealand. Flights across the North Island were cancelled or delayed during the event.

Ash and Aviation

Volcanic ash poses a significant threat to aviation. The abrasive nature of ash means that it can cause severe damage to engines and other mechanical and structural components of aircraft. In 1982, Galunggung, which is located in Java, Indonesia experienced a large eruption. A Boeing 747 jet airliner operated by British Airways was caught in the resulting ash cloud. First the aircraft experienced a luminous glow on its windscreen and wings, similar to an electrical effect known as St. Elmo's fire. Then all four of the aircraft's engines lost all power. Luckily the pilots were able to restart the engines and make an emergency landing at Jakarta airport. This was a feat in of itself, since the ash had essentially sandblasted the cockpit windscreens so that it was like looking through frosted glass. This incident highlights the abrasive nature of airborne volcanic ash. The engines were found

Since the 1982 British Airways incident, there have been instances where volcanic ash has caused significant disruptions to the commercial airline industry. In  2010, a volcano in southeastern Iceland called Eyjafjallajökull (pronounced "Eh-yah-fyaht-la-yur-kuetl") caused severe disruption to commercial air traffic throughout Western Europe.

The ash cloud from Eyjafjallajökull is blown southeast from Iceland toward mainland Europe. Credit: Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC (Choi, 2012)

An eruption of Mount Taranaki would mean that most if not all commercial airports in New Zealand would need to be shut down for some time. This is because it would just be too dangerous to fly while the ash concentration in the air is high. Airports in the North Island would be most strongly affected, especially those around the central North Island and the Hawkes Bay/Gisbourne areas when taking the most probable wind directions into account.

Ash Loads on Roofs

According to AS/NZS 1170.3:2003, which is the building standard in Australia and New Zealand which governs how to build structures to withstand snow and ice loads, a large portion of the Taranaki Region is classed as "N1," meaning it is considered that there will be no significant snow below 400 metres above mean sea level (MSL). This means that in settlements such as Stratford, where the highest part of town is approximately 340 metres above MSL, there are no enforced snow-loading requirements for buildings.

The bulk density of dry, newly fallen or slightly compacted volcanic ash can range from 500 to 1500 kilograms per cubic metre. This density increases when the ash becomes wet, with the densities now ranging from 1000 to 2000 kilograms per cubic metre.

As a comparison, in N1 snow areas, the average density of snow is 2.9 kN per cubic metre. 500 kilograms per cubic metre translates to 4.9 kN per cubic metre. Therefore the minimum ash load is approximately 1.7 times heavier than a snow load of equal volume. Therefore, if 25 cm of dry ash were to settle on a roof, the equivalent depth of snow is 42.5 cm. The equivalent depth of snow for the minimum wet ash load is 85 cm. These ash loads would be more than capable of causing considerable structural damage to buildings and collapse of roofs.

Further corrosion damage could also affect metal roofs. The metal salt coating on ash particles can react with steel roofs. Due to the angular shape of the ash, it would likely be capable of scratching through galvanisation. After this has occurred, the metal salts would be able to react with the metal. The most rapid corrosion appears to be connected to sodium and chlorine ions in the salts, as well as, to a lesser extent, sulfate, magnesium, ammonium and potassium ions. Moist, humid conditions (which are fairly common in the Taranaki region) would exacerbate a corrosion problem, because the moisture in the air helps to dissociate (break down) the salts into their more reactive forms (Oze, et al., 2014).

Ash and Electricity

Volcanic ash also has the capability to have detrimental effects to electrical systems. High voltage transmission pylons tend to be at particularly high risk. A major problem for these transmission lines is insulator flashover. This phenomenon is caused by the coating of soluble salts on the surface of the ash particles. The severity of flashover is dependent on the ash moisture content, the concentration of mineral salts on the ash particle surfaces, ash compaction, and, to not such a great extent, the size of the ash particles (Wardman, 2013)
The ash, when dry, has a high resistivity of greater than 1.56 × 10⁷ Ωm (15.6 billion Ohm metres) (Wardman et al. 2012). Resistivity is a measure of how strongly a material will resist the flow of electric current. A high resistivity means that a material will strongly oppose an electric current. When dry, the salt coating on the ash is in an inert form. However, when water is introduced, the salts will dissociate into charged ions which are capable of transmitting an electrical current. This causes the resistivity of the ash to drop dramatically to less than 100 Ωm (Wardman et al. 2012).

Insulator Flashover under laboratory conditions at the University of Canterbury. The insulator is contaminated with a layer of volcanic ash approximately 2 millimetres thick (Wardman et al. 2012).

Due to the ash now being wet and therefore capable of carrying a current, flashover of insulators can result. This often means that electrical services will be disrupted. The arcing of electricity can also be disruptive and damaging to equipment if it occurs on transformer installations. Flashover on or over the external insulation (bushing) can irreparably burn, etch or crack the bushings and potentially cause damage to internal componentry (Wardman, 2013). Newer style insulators appear to be more vulnerable to ash adhesion due to having a rougher surface than the older glass and porcelain insulators (Wilson et al. 2012).

Following the Mt St Helens event of 1980, it was found that heavy rain only washed off approximately two thirds of ash that collected on insulators. This left a considerable residue that could still cause flashover. Light rain had little to no effect on settled ash (Wilson et al., 2012). The Taranaki region is recognised as a high rainfall region. Depending on the amount of ash to fall and the length of time taken during the cleanup, wetting of ash on transmission pylons would be highly likely. Therefore, insulator flashover is a very real possibility.