Roger Hanson: A menace waking up after a century of slumber
In Iceland there is a menacing volcano which for the first time in 98 years is beginning to stir. It is only 1,512m high (Mt Taranaki is 2,518m) but what makes this potentially nasty is that it is sitting under a glacier.
History shows that its sub-glacial eruptions are powerful and fed by a large magma chamber. The volcano is called Katla and is only 25kilometres west of the Icelandic volcano that erupted in April 2010, the unpronounceable – Eyjafjallajökull volcano.
What has volcanologists worried is that in the past 1,000 years the three known eruptions of Eyjafjallajökull triggered an eruption of Katla. The ash cloud from the Eyjafjallajökull eruption resulted in the cancellation of 100,000 flights worldwide costing the airlines US$3.5 billion.
The concern is that an eruption of the more violent Katla volcano could generate extensive flooding in Iceland and produce a massive ash cloud causing much greater disruption than its neighbour.
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Some idea of the power that sits beneath Iceland is given by Professor Bill McGuire of London University who says that if the 1783 eruption of Loki, another of Iceland's volcanoes, took place today, the ash cloud would severely affect air transport in northern latitudes for six months.
This is because volcanic ash has the potential to cripple aircraft engines. What makes Iceland's sub-glacial volcanoes particularly dangerous is that during an eruption the cold water from the cap ice chills the lava causing it to fragment very quickly into particles of silica and ash.
These silica shards are hard and highly abrasive and if ingested with the ash by a jet engine, damage and stick to the compressor blades bringing a high risk of engine failure.
The most famous example was in 1982 when British Airways flight BA9 flew directly through the ash cloud produced by Indonesia's Mt Galunggung volcano.
After the flameout of all of its engines, BA9 became a giant glider and lost 7km in altitude. Incredibly, the crew eventually managed to restart the engines and the aircraft landed safely.
In Europe, the response of the authorities following the 2010 Iceland volcanic eruption was poor, infuriating the public and the airlines.
In fairness, the aftermath of the eruption presented a dynamic situation, one in which the ash cloud was launched into the jet steam some 12km above the earth's surface and transported across continental Europe at about 160kph.
As the giant ash cloud moved across Europe the affected air space had to be progressively closed.
The airspace behind the cloud was then reopened but only as long as there were no further eruptions. In 2010 any ash cloud was considered to be dangerous and the affected airspace declared by the authorities to be a no-fly zone.
This mattered to the airlines because it can take days or weeks for an ash cloud to completely disperse. Time is money to an airline and delays in waiting for complete dispersal of the ash mean huge losses for the airlines and disruption to passengers.
The European and the New Zealand Volcanic Ash Advisory Centres have instruments called ceilometers which can measure the concentration of particles in an ash cloud.
Following the 2010 eruption it was decided to define limits on particulate concentration above which a no-fly zone must be declared. If the concentration is 4 milligrammes per cubic metre or more the the airspace is closed.
This means that complete dispersal of the cloud is not required before declaring the skies safe to fly, thereby substantially reducing the disruption to airlines and passengers.
New Zealand has several active volcanoes and although they are not sub-glacial volcanoes, they have the potential to cause air traffic disruption.
This was demonstrated by the 1996 eruptions of Mount Ruapehu (2,797m) which caused eleven North Island airports, including Auckland Airport, to be temporarily closed. New Zealand has been a pioneer in this field and MetService operates a network of laser ceilometers.
As for Iceland's Katla, it rumbled again on 29 August 2016 raising concerns that something big may be on the way.
As the Prime Minister of Iceland put it, "the time for Katla to erupt is coming close...we (Iceland) have to be prepared".