What happened before and after the eruption of Mt. Pinatubo in 1991?

On 2nd April the first signs of volcanic activity were seen from the volcano. There was a series if explosions that rocked the volcano and its near occupants. One Aeta said after once such eruption:
“There was a rumbling and I ran from the volcano I thought I was going to die!”

It was the first explosion recorded from Mt. Pinatubo. U.S. volcanologists began investigating the rumblings and steam eruptions 3weeks after that account. A volcanologist who wanted to remain anonymous said:
“It could be letting off steam and not going to erupt, or it could be under tectonic stress or it could be about to erupt due to a surge of magma.”

After some investigation the volcanic team agreed and declared a Level 2 and immediate evacuation began.

On 3rd June 20,000 Philippines were evacuated. Some of the people and equipment from the Clarke Airbase were flown off the island. What followed this evacuation was lots of pressure on the volcanologist team on whether they had got the decision right, if they had made the wrong call then thousands of dollars would be lost and people would have had to leave their homes disrupting their daily activities. In a documentary about the events leading up to the eruption Rick Hoblet spoke about how there had been no irregular activity from the volcano since the Level 2 had been declared.

They were discussing what to do and maybe to lower the level, as it seemed to be not as active as they thought. However once he left the meeting and looked at the seismometer the activity was huge. The activity kept rising on two of their investigative stations, UBO and PIE and it was decided that a Level 4 should be declared. This means an eruption could be probable within 24hrs therefore everyone within a 12mile radius of the volcano had to be evacuated. 120,000 people left their homes for temporary evacuation centres. General Williams Studer decided the airbase would not leave until the geologists said there was only six hours until an eruption.
On 8th June they looked at Pinatubo again. They observed a dome. One geologist, called Dave who saw it described it as follows:
“This dome is… like a slug of magma, cold, relatively cold magma, squirting out on the ground. This is a sign of a very explosive magma. The ash samples showed it was the same stuff, which had erupted out before, previously. At that time we became very concerned.”

June 10th General Studer had everyone evacuated leaving only 15,000 security officials behind. He said when questioned as to why:
“My objective has to be to get everyone to safety.”
Every hour that passed from the evacuation heightened tension on the PVO (Philippines Volcanology Organisation). They were risking a lot predicting a volcanic eruption and having so many people evacuated.

48hrs after Clarke Airbase was evacuated, Mt. Pinatubo erupted on 12th June the PVO team were ecstatic and awed by the eruption. They had got it right!
Dave said later:
“Fundamentally we knew it would explode but you start asking the ‘what ifs’. What if it takes a long time to do so.”
The city became covered in ash. Evacuations continued day and night. 48hrs after the explosions communities as far as 50miles away were showered in ash and sand. To make matters even worse a major typhoon hit the island 6hrs later.

On 15th June there were a series of minor eruptions that triggered the major huge explosion, sending ash 100ft in the air. It also exploded 20million tons of sulphur dioxide into the atmosphere. The 6am explosion was the 5th of the morning and started lightning. All personnel then abandoned the airbase. The volcanogists however stayed behind. There was no light as ash had blocked it out and it began raining rocks, pebbles, pumice, and mud.

The typhoon unfortunately spread fear and damage far into the evacuation zone. Towns 20miles away from Mt. Pinatubo were covered in ash and roofs collapsed due to the weight of rain soaked ash. Farms 50miles away from the origin of the explosion were damaged and covered in ash. 10,000 people suffered, 250,000 were displaced but less than 500 died. Weeks after 15th June it was still spewing out ash.

Mt. Pinatubo erupting on 15th June 1991.

Thanks to the prediction and investigation of the PVO many thousands of deaths were avoided.
It is now recognised as the biggest eruption of the last 80years. 5-8Km³ of ash was deposited. The eruption was 10times more powerful than St. Helens.

The eruption was later described by the Philippine Institute of Volcanology and Seismology as a Plinian eruption. A plinian eruption is an eruption of great violence recognized by large amounts of explosive ejections of pumice and pyroclastic flows. The copious extrusion of gas-rich silicious magma is commonly followed by the collapse of the top of the volcanic cone forming a caldera.