My case study is on the 1991 eruption of Mount Pinatubo in the Philippines. I decided to choose this event to focus on because it was the second largest eruption of the 20th century and having studied hazards for the past couple of years – I had never looked into this one and I was interested to do so.
Figure 1: Pyroclastic flows from Mount Pinatubo June 15 1991 https://mb.com.ph/2021/06/15/looking-back-at-mt-pinatubos-1991-eruption/
In June 1991, Mount Pinatubo, a once dormant stratovolcano for over 5 centuries became active once again following the Luzon 7.7 magnitude earthquake a year prior. Over the course of a few days before the eruption, magma reached the surface of Mount Pinatubo by which sulphur dioxide clouds had begun to erupt and a lava dome had formed due to a loss of the once contained gas. This activity of gas-charged magma indicated a cataclysmic eruption was imminent and by June 15th this is what occurred. More than a cubic mile of material and an ash cloud of 22 miles rose into the air and continued to do so until the following morning. As a result of the eruption, a typhoon (which blew the ash in all directions – in seven days it reached the Galapagos Islands), high speed pyroclastic flows (figure 1), lahars and ash flows were created and remained a hazardous threat to the people in the region for several years. For example, a sulphuric aerosol cloud remained in the atmosphere and circled the earth for several years – up to one year after the eruption the earth was in a period of climatic cooling by 2.3°C and resulted in a counterbalance of global warming (by 1994 most aerosols had gone). Moreover, lahars remained a great threat to the areas surrounding Mount Pinatubo for decades after because the ash deposits would remobilise during monsoon and typhoon weather and result in vast downwards debris flow.
Furthermore, in terms of preparation and mitigation strategies, given that the last time Mount Pinatubo erupted was around 600 years prior – the Philippine government reacted reasonably quickly and efficiently in terms of evacuation. Figure 2 shows the mapped-out danger zones where people are the most vulnerable which was around 40km away from Mount Pinatubo and everyone in these zones (~331,000 people) was transported to evacuation camps. 657 people died in 1991 and 184 were injured – the numbers increased with the years due to remobilisation of lahars – however half of these deaths were due to collapsing roof tops from wet tephra containing many people who refused to evacuate.
Figure 2: Map of the danger zones surrounding Mount Pinatubo: https://pubs.usgs.gov/pinatubo/tayag/
In terms of recovery, the Government responded quickly with rehabilitation and reconstruction plans which included foreign aid from countries such as the UK and USA alongside support from private sectors including NGOs such as the WHO and UNICEF. This included construction of a “megadike” to control and protect remobilising lahars in future monsoon seasons. 25 years on in 2016, there was a review on the area, and it proved that the mitigation post eruption has been successful and it is evident that the preparation in terms of evacuation before the eruption was effective and helped save hundreds of thousands of lives.