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

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:

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.


  1. Just like you I had never heard of this hazard, I have also studied a decent amount of hazards between high school and college so it was interesting to learn about. Something that I am shocked about is the fact that I have never heard about the aerosol gas cloud hanging around for many years after the eruption causing global cooling. I have learned about some of the hazards that have caused global cooling and never heard of this one, so it was very interesting to learn about the effects that it had worldwide. Despite the last eruption being such a long time ago, it is good that the Philippine government was quick with its reaction to getting the people out and into safe areas. It is sad that people had to die, due to them refusing to leave despite the evacuation being in place to protect them. Overall, the government was quick with all of its reactions which is good since it resulted in fewer people dying and since the mitigations are stronger now they will be ready if it erupts again.

  2. The US had two nearby military bases, Clark Air Base and US Naval Base Subic Bay – personnel directly helped evacuate civilians from the volcano immediately prior to the major eruptions. It seems likely that monitoring of the volcano would not have been so careful if US operators were not in the vicinity. Plus, the operation had a great name – Fiery Vigil!

  3. I have always found this specific event in history to be very interesting. I actually wanted to write my case study on it, but by that time it was already taken. A couple of aspects that fascinated me were the amount of damage it cost and the response of the people. The eruption cost $700 million in damage with its Pyroclastic flows, lahars as well as the ash fall hazard. Also, the last eruption prior to this one was about 600 years ago and the eruption was very large, the second largest eruption of the 20th century, yet the government still reacted quickly and efficiently. I find that very fascinating because I did my report on the eruption of Mount Merapi, indonisia in 2010, and I found out that people responded terribly during this disaster even though the volcano has small eruptions every 2 to 3 years, and larger eruptions every 10 to 15 years.

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