The Eruption of Mount Pinatubo: A case study
Introduction
What is a volcano?
A volcano is a hole in the Earth’s crust which magma comes out of. Then the lava builds up and forms a volcano.
What processes cause there to be an eruption of a volcano?
The pressure of gasses in the magma chamber. If the pressure of the gasses is high the eruption will be a powerful one. High gas pressures result in explosive volcanoes.
If the lava is viscous, thick and treacly, it does not flow easily out of the vent and tends to solidify here forming a volcanic plug. Then when the gas pressure builds up, eventually the plug will be blown off by an explosion, thus forming a crater.
Mount Pinatubo
Location: Manila, Philippines
Latitude: 15.13 North
Longitude: 120.35 East
Height: 1745metres before the eruption
Type: Stratovolcano
Mount Pinatubo is one of a chain of composite volcanoes that constitute the Luzon arc. The arc parallels the West Coast of Luzon and reflects eastward-dipping subduction along the Manila trench to the west. Mount Pinatubo has one of the highest peaks in west central Luzon. The volcano’s former summit, at 1745 metres, may have been the crest of a lava dome that formed about 500 years ago during the most recent previous major eruptive episode. The volcano’s lower flanks, dissected and densely sheathed in tropical vegetation before the 1991 eruptions, were composed of largely pyroclastic deposits from voluminous, explosive prehistoric eruptions.
Before the eruption, more than 30 000 people lived in the small villages around the volcano. Clark Air Base is situated to the east of the volcano, within 25 kilometres of the summit. Subic Bay Naval Stations are situated to the south-west of the volcano.
The volcano before its eruption consisted of rounded, steep-sided, domelike mass which stood 700 metres above a broad, gently sloping, deeply dissected apron of pyroclastic and epiclastic materials.
If you compare the volcano to the well- known stratocones, such as Mayon or Fuji, it was small and inferior, but its extensive pyroclastic apron showed that there had been explosive eruptions of the volcano before many years ago.
The Filipinos call the volcano "Appo Malyari". They think of the volcano as being controlled by the gods. When the volcano lets off steam the local people think of it as a good sign from the gods. Even when the volcano was giving out gas the Filipino people still thought that it was safe to go up the mountain to farm due to the gods controlling it.
Date |
Report |
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2.4.91 |
People in village of Patal Pinto, on the Philippine island of Luzon, saw small explosions followed by steaming and the smell of rotten eggs coming from the upper slopes of Mount Pinatubo. This volcano was thought of as dormant because its last known eruption was 600 years ago. | |
23.4.91 |
The Philippine Institute of Volcanology and Seismology [PHIVOLICS] immediately installed portable seismometers near the volcano. They recorded several hundred earthquakes a day. U.S Geological Survey personnel arrived at the request of PHIVOLICS, made through the USAID office in Manila. | |
23.4.91- 7.5.91 |
The team of Philippine and American volcanaologists installed a radio-telemetred seismic network capable of locating the increasing number of earthquakes. They began tape measurements across fractures, which were opened during the steam explosions. They later installed tiltmeters to detect new ground movement. Now with the help of the U.S Air Force measurements of SO2, the gas in the steam plunes were begun. The gas was clearly visible at Pinatubo. | |
13.5.91-28.5.91 |
A 10-fold rise in SO2 content was
measured. The signals indicated that magma was rising within the volcano. At the same time Philippine and American geologists made a geological survey of the volcano and made a set of alerts ranging from 1( Low-level unrest) to 5 ( eruption underway). |
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13.5.91 |
Alert system complete. The level was set at 2 meaning that magma was probably involved in the trouble. The geological survey showed that the lava products were dacite domes near to the summit and valley-filling pyroclastic flows extending from the summit to a distance of 10 to 15 kilometres. Small villages on the northwest slope and part of Clark Air Base lay within the range of the pyroclastic flows. Many communities, including Angeles, a city many miles away from the volcano with a population of 300 000 lay within the range of debris flows extending far beyond the volcano. Some of the debris even hit towns and cities 50 miles from the volcano. | |
23.5.91 |
A hazard map was distributed to local officials showing virtually all of the hazards subsequently encountered when Pinatubo erupted. | |
5.6.91 |
PHIVOLICS raise the alert to 3 which means an eruption is possible within the next two weeks. | |
7.6.91 |
A small dome forced out on the north flank along with thousands of small earthquakes triggering a level 4 alert. This meaning that an explosive eruptions possible within the next 24 hours. | |
9.6.91 |
PHIVOLICS raised the alert level to 5 saying that an eruption had begun because of continuous ash emissions. | |
10.6.91 |
Clark Air Base evacuated. | |
12.6.91 |
The first of a few major explosions took place, sending ash airborne to the west and pyroclastic flows down the northwest slope of the volcano. Areas that are potentially in range of pyroclastic flows had been evacuated in response to the level 3 alert. The city of Angeles put on evacuation alert. | |
15.6.91 |
A climatic eruption took place during the passing of a typhoon, which had already resulted in the closure of the International Airport at Manila. The typhoon resulted in mudflows in addition to the ash fall and disrupted the evacuation efforts immensely. | |
16.6.91 |
The top of the volcano had gone, replaced by a 2-kilometre-wide caldera, and pyroclastic flow deposits had largely filled the valleys on all sectors of the volcano existing before the eruption. Ash had fallen over a vast area beyond the volcano. The heavy ash combined with the continued earthquakes resulted in the collapse of many buildings in Philippine cities and villages as well as on Clark Air Base and the more distant Subic Bay Naval Base. Continuous ash emissions at a lower level still continue today | |
| How did Mount Pinatubo erupt? The Philippines lies at the edge of a tectonic plate that is rigid and rocky that makes up the outer crust of the earth. At its western edge the outer plate is sinking down into the Earth beneath the Eurasian Plate which is next to it. As this happens the plate takes down with it water from the Philippines Sea which has collected in the rock. At a depth of 100 km the Earth is extremely hot and some of the rock becomes molten due to this. The molten rock whose name is magma rises through weaknesses in the overlying rocks to the surface forming volcanoes. The magma contains a lot of water, as steam and gas under pressure. This pressure builds up over many years until finally there is an explosion, and rocks, dust and ash are thrown violently into the air. At Pinatubo these explosions continued for several days |
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Contents Page
Title of Page Number of Page
Introduction 1-2
Report 3-4
A diary of the events of Mount Pinatubo 5-6
What were the short and long term affects 7
of the eruption?
What did the scientists do? 8
Volcanic Hazards 9-10
A diary of the events of Mount Pinatubo
Day One, Sunday, 9 June 1991
Eight hours of ash-laden steam clouds ejection lead to one of the world’s most violent and destructive eruptions of the century, beginning at around 6 AM. This was followed by pyroclastic flows which down Pinatubo’s gullies into Maraunot and Moraza rivers. The pyroclastic flows reached 4-5 kilometres from the centre of the volcanic activity. Phivolics issued a Alert Level 5 at about 3.15 in the afternoon. Irregular occurrences of small pyroclastic flows went on throughout the morning.
Day Four, Wednesday, 12 June 1991 At about 8.51 on the morning of Independence Day, viscous seismic activity was followed by three major explosions. The most powerful of these explosions ejecting a huge, grey, mushroom-shaped cloud that reached 20 kilometres above the vent. Ash, pumice and other larger volcanic fragments were also ejected by Mount Pinatubo. Major rivers and tributaries were overflowing due to the pyroclastic flows from the volcano. Just before midnight, there was another series of explosions that threw clouds of ash and other pyroclastic materials 25 kilometres in the air.
Day Five, Thursday, 13 June 1991
24 hours after the first major explosions, another violent eruption occurred resulting in heavy ash falls that blanketed most of Zambales, Tarlac and Pampanga. Prevailing winds spread the volcanic ash hundreds of kilometres away in all directions.
Day Six, Friday, 14 June 1991
The fourth major eruption occurred at 1.09 in the afternoon. Pinatubo ejected another vertical ash column that reached a height of 25 kilometres above the vent. This was followed by smaller irregular eruptions. At 3.30 in the afternoon, a much bigger eruption occurred which produced a cauliflower-shaped column that threw out volcanic debris to a maximum height of
30 kilometres. Pyroclastic flows flowed down the Maraunot River towards Sitio Ogik reaching 15 kilometres from the source. Three smaller eruptions took place at 6.53 pm, 10.18 pm and 11.21 pm.
Day Seven, Saturday, 15 June 1991
Two explosions occurred at dawn with pyroclastic flows that moved at between 70 kph and 80 kph. This was followed by seven more eruptions in midmorning, thus producing ash columns from 15-18 kilometres wide at the base and got up to heights of 25 kilometres above sea level. At 10.27 am there was a violent eruption which threw out a 40 kilometres high eruption column. This was followed by five explosions that were of a similar strength from 11 am to 1.42 pm. The explosions were in close succession. Phivolics recorded a total of 19 eruptions this day. The eruptions that occurred after this date were not recorded by and organisations.
After June 16
Pinatubo’s activity was less frequent and of a lesser power than of previous eruptions. Only minor eruptions were recorded.
What were the short and long term effects of the eruption?
Short term effects
Tremors were felt in the town of Manila from the eruption that was 80 kilometres away.
Rocks the size of tennis balls were hurled up to 50 kilometres from the volcano.
The cloud of ash.
Lava flows. Molten rock was flowing away from the volcano slowly.
People had to leave their homes.
People got ash all over their bodies.
The death toll was 500.
Houses, bridges and walls collapsed.
Half a million pounds worth of crops were thought to have been lost due to the ash covering them.
There was a an arrival of rain from the typhoon Didi. This resulted in the tonnes of ash being transformed into mudflows going down the mountain at up to 90 km per hour.
The mud swelled the Maraunot River, doubling its width and destroying bridges and property.
The sun was completely blocked out by the cloud. The people lived in darkness for 24 hours at one time.
Long term effects
Farmland was buried and animals died.
There was to be a change in the temperature of 1oC for the next five years because there was rock produced by the eruptions in the atmosphere.
Valleys were filled with pyroclastic flow deposits.
For the next five years in the rainy season there would be more lahars from Mount Pinatubo.
The Americans were never able to return to Clark Air Base as the damage was so great that it would not be worth repairing the base. This would result in a loss of revenue for the government and the people of the Philippines.
650 000 people lost their jobs as a result of the eruptions.
Crop damage in the estimated 1 500 hectares of land in ash may run up to over �10 million.
What did the scientists do?
What did they monitor?
How many earthquakes took place each day around the volcano and whether the numbers were increasing
How much gas was being given off by the volcano each day and whether the levels were rising.
The temperature of the magma in the volcano and whether it was rising.
Whether the magma was rising.
Whether sea levels are rising or not which is caused by global warming . If the sea levels had risen this could trigger volcanic eruptions of hundreds of new or dormant volcanoes. As the water rises it begins to erode the lava, with the water lubricating landslides and weakening the rock. If this is so the mountain becomes unable to withstand the internal pressure of the molten rock and explodes.
Why is it so important to predict eruptions accurately?
To be able to evacuate people from towns.
To divert lava flows and lahar flows to the sea.
To know what is going to happen at a particular time so that people can react appropriately and not be surprised by the eruption and there resulting a panic by the people. Everything could be controlled and planned properly and organised well with precautionary measures.
Volcanic Hazards
Pyroclastic Flow
There are hot rocks and gasses in what people describe as a cloud that burns everything in its path.
Ballistic Projectiles
It sends out rock sideways from the volcano rather than upwards. This kills people.
Lahars
When it rains on ash, there is an ash flow, which goes into rivers. This damages dams and bridges. It has a much longer range than other hazards.
Ash fall
The ash falls on the tops of houses and other building causing them to be set on fire and much damage is caused to the buildings.
Debris flow
Material from the volcano side collapses and causes land slip and the material to flow in a surge down the mountain which results in damage to buildings and terror to the people.
Lava
The lava flows slowly through towns and wrecks them but it travels too slowly to kill people. The people can see it coming and the towns can be evacuated.
Gasses
Sulphur and carbon dioxide are the gasses given off when an eruption takes place. Both of these gasses are bad for your health.
How did people deal with these volcanic hazards?
Pyroclastic flows
Wear gas masks. This protected people from the gasses but not from the hot rocks. Children in a school near the volcano wore hard hats that were supposed to protect them from the rocks. It is extremely unlikely that the hats would protect the children from the rocks. This is because the hats would not be able to withstand the hard, heavy rocks that were very hot. If the rocks hit the children wearing the hats it is very likely that they would die instantaneously. The hats would not do any harm but they would not prevent the hot rocks injuring them. This technique was not successful. You could also move house away from the range of the rocks.
Ballistic projectiles
People could not deal with these at all, all they could do was to get out of the way of them. The areas that they would pass could be evacuated.
Lahars
The lahars could be channelled away to a safe place by throwing massive rocks in its path and also spraying water by powerful hoses at the mud flows pointing the mud flows towards the sea and this directs it to the sea successfully. You could also move house away from the area within range of the lahars.
Ash fall
Put roofs over graveyards, which would prevent the graveyard from being burnt. This would be successful. You could wear hard hats, which would prevent the ash from touching your body. This may be successful in not being burnt but the ash may burn the hat and therefore you would be burned. You could shower the ash in water which would prevent the ash burning everything but would result in lahars, so this would prevent the ash burning things but would result in damage to dams and bridges. Therefore this is not a good idea. You could also move house away from the area effected by the ash fall.
Debris flow
The debris could be directed away from towns by showering the debris with water and you could direct it to the sea. This would be successful. There is very little you can do to prevent debris flow.
Lava
You could cool the lava down by showering it with water. This would prevent it from burning anything. Massive rocks could be thrown in front of the flow and this would block the lava flow. This has been proved successful by an American geologist and his team on another volcano.
The lava could be diverted towards the sea by showering the lava with water towards the sea.
Gasses
People could wear gas masks that would prevent gasses from entering their body. This is a successful precaution.