Whilst prediction is arguably one of the most important methods of reducing death tolls from eruptions, they must be communicated to the people who can act on them in a timely way. This will hopefully include hazard maps drawn up in advance of an eruption – a sort of risk assessment to inform and alert people of the need to evacuate should an eruption occur.
Effective
Hazard maps identify areas at risk from different volcanic hazards eg Mount Rainier in Washington state, USA is an andesitic stratovolcano. It is part of the Cascadian subduction zone (along with Mount St Helens) which hasn’t erupted since the mid 19th Century. It is capped by 25 glaciers. The hazard map is separated into:
1. A central high risk zone 20km from summit at risk from pyroclastic flows and lava flows
2. Outer lahar-risk zone within populated river valleys extending to 100km of the summit. 80000 people live in this zone. The western side of the volcano is deemed to be most at risk eg Nisqually River Valley
This hazard mapping has fed into:
1. Mount Rainier Volcanic Hazards Response Plan -operates at national, state and local levels
2. Published volcanic hazard evacuation routes which are signed on roads leading to high ground
3. Educated local communities and schools on lahar risks
4. Automated lahar warning system which detects ground movement from a passing lahar- which is crucial in this area as a lahar could reach the major town of Orting just 40minutes after it is triggered.
5. Warning sirens tested twice a year and schools have evacuation drills. After sirens, warnings are transmitted to radio and phones and evacuation is key to protect life if not possessions. Residents are advised to evacuate on foot to avoid bottle necks and take a grab bag with first aid and water – community preparedness is key here.
Hazard mapping without doubt is one of the most important aspects of reducing death tolls from eruptions but they must be communicated to the people who can act on them in a timely way.
· Taal, Philippines 2020 – following successful monitoring and warnings, a 14km exclusion zone meant no deaths
· Mount Merapi, 2020 – following successful monitoring by Yogyakarta’s Volcanology and Geological Hazard Mitigation Centre,, evacuation routes planned in a 6 mile zone and lights erected along routes and at evacuation centres and livestock sheds to ensure people could evacuate at night. 1300 people evacuated
Limitations
· Less effective for hazards like pyroclastic flows – they may travel short OR long distances and are not necessarily linked to height of volcano
· Eruptions can also take an unexpected route which contradicts hazard map– the plinian style eruption at Mt St Helen’s in 1980 saw a side vent blast laterally extending 8km and lengthening the impacts of the pyroclastic flow beyond the exclusion zone that had been mapped and killing 57 people.
· Moreover, whilst may lives were saved in this HIC with a volcano that was monitored 24 hrs per day as one of the most closely observed volcanoes in the world, the impacts on the economy were considerable despite lives saved - International Trade Commission estimated the damages to agriculture, timber and civil losses to be $11billion. 200 homes destroyed and thousands of animals killed
· Some hazards eg lahars and ash travel a long distance so are more difficult to map as are affected by global circulation
· Ash - Accounts for less than 5% of deaths but even a light covering of ash can contain toxic chemicals which contaminate farmland and water supplies.
· Mt Pinatubo 1991 – livelihood of 5000000 farmers affected as ash fell 30km away
· Even at boundaries triggering low VEI eruptions, impacts maybe amplified under certain conditions – eg Icelandic eruptions may combine with meltwaters to produce ash with considerable, far reaching and prolonged impacts
· Relatively small eruption in Iceland in 2010, no direct deaths but in the wrong place and demonstrates the far-recaching impacts ash may have as a hazard which cannot be hazard mapped
· Began as a normal effusive, basaltic lava volcano typical of divergent plate boundary - low in Si so effusive rather than explosive and should not have been that hazardous.
· But volcano occurred under the glacier which generated ash which became a major hazard with global impacts.
· As the ice started to melt, glacial water began flooding into the volcano where it met the bubbling magma at the centre of the eruptions. This rapid cooling caused the magma to shear into fine, jagged ash particles.
· Large plumes of volcanic ash quickly spread above the volcano, moving eastwards with the jet-stream towards the Faroe Islands, Norway, and northern Scotland.
· By April, much of Europe affected – more than 100,000 flights affected as aircraft grounded
· Wind direction and velocity influenced the impacts
· Iceland responded by declaring a state of emergency and European airspace was closed as a safety precaution. It is estimated that airlines lost an estimated £130m every day that airspace remained closed, while millions of passengers were left stranded.
· With airfreight impacted , world Bank estimated that African countries may have lost $65 million due to impacts of shutting down airspace and transport of perishable goods like fruit and flowers from countries like Kenya, Zambia and Ghana
· Ash Can have local to global impacts – 100-10,000km away and even affect weather patterns and these impacts can have very much longer term impacts even years after
· Topography eg valleys and steepness of slopes will affect some hazards eg lava flows and lahars– their speed of onset and how far they run out
· Higher volcanoes will affect how far volcanic landslides travel
Gas is notoriously difficult/impossible to hazard map:
Most deaths associated with CO2 – gases are colourless, odourless, heavier than air so dense and accumulates in valleys undetected by people so less or no warning and may not be detected prior to eruption so impossible to hazard map unless the hazard is known about.
Moreover, gas may have regional or even global impacts making hazard mapping less relevant and specific– 10-100km and even global impacts when greenhouse gases enhancing global warming and climate change
· EG In 1986 - CO2 emitted from Lake Nyos in Cameroon killed 1700 people (Limnic eruption)
· EG Sudden eruption of Mount Ontake Japan , 2014, killed 60 – phreatic steam eruption without warning. Despite small scale of this volcano, it was the deadliest in Japan for 90 years. Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic-flow deposits). The intense heat of
such material (as high as 1,170 ° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and bombs
Overall, the success of hazard mapping depends on action :
· To save lives vulnerable people must act on hazard mapping advice and prepare for an eruption– Mount Vesuvius, Italy – Naples lies only 15km away and 600000 people live in the 18 towns in the so-called ‘red zone’ at greatest risk. Estimates suggest it would take 3 days to evacuate people along narrow and congested roads. No evacuation drill has ever been carried out and plans assume precursors would provide 4 weeks warning time.
· Currently, no evacuation plan as assumed it will only be a VEI4 eruption and that prevailing winds will carry most debris to SE away from Naples. However, there is an argument that it could be a VEI6 eruption which could send pyroclastic flows 20-30km from Vesuvius
Overall, the success of these will depend, however, on type of volcanic hazard, local geography and degree of development and governance. Even with HICs, there are limitations to the success of hazard mapping and it is severely impacted by perception of risk. Moreover, whilst hazard mapping may save lives, it can do little to save property and assets especially once settlements are built