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Natural Hazards: Eruption of Mount Pinatubo 1991

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The eruption of Mount Pinatubo in 1991 was the second largest eruption in the entire 20th century. Mt. Pinatubo had been inactive for 400 years.

Before the eruption, Mt. Pinatubo was considered an unobtrusive and an inconspicuous volcano which had been active in the past 10 decades. It was undoubtedly a site of failed geothermal development. It was the home to the worshippers of Apo Namalyari, their Protector and Provider. The believers thought the mountain to be the home of Apo Namalyari.

Before the eruption in 1991, the mountain was 1745 m high, and was merely 200 m higher than the other mountains in the vicinity.

Geology of Mount Pinatubo

Regional Setting

Pinatubo is stretched out at the west and underlain by the Zambales Ophiolite Complex, an east-dipping section of Eocene oceanic crust, which has been known to be uplifted in late Oligocene. The next unit contains the residues of Tarlac Formation, basically siltstone and sandstone in the older parts, whereas the conglomerates, dikes and volcanic sediments contain them in the younger parts. The Tarlac Formation can be found north, southeast and east of the mountain and has formed contemporary links with the ancient volcanic centres in the area, which include Mount Mataba. Mt. Mataba is the diorite of the Dizon mine, the sub-surface remains of an older vent. The ancient Tarlac Formation volcanoes were developed from the same east-dipping section along the trench of Manila, which is still present.

Ancient Pinatubo

The formation of Pinatubo occurred in two stages. The first ancestral Pinatubo formation dates back to 1 million years ago. An andesite was built that is dacite stratovolcano, the centre of which remains unchanged. The Ancient Pinatubo had a large number of vents that later on shaped the domes of Mataba, Mount Cuadrado, Mount Negron, the volcanic plug of Tapungho and the Bituin plug. (Green 114)

Modern Pinatubo

After a very long period of time, Modern Pinatubo originated from the most explosive eruption of Pinatubo around 35,000 years ago. This eruption was known to be 5 times larger than the one mankind faced in 1991. As a result of this massive eruption, pyroclastic material was deposited thickly on all sides of the mountain. The total amount of pyroclastic flows around the volcano covered approximately 25 km3. The removal of this large amount of magma led to the creation of Tayawan caldera. (Boer 214).

Precursors to the 1991 Eruptions

An earthquake of magnitude 7.8 struck around 60 miles northeast of Pinatubo in July, 1990. The centre of this earthquake was the island of Luzon in Philippines; the quake shook the earth’s crust under the giant volcano. However, at Mt. Pinatubo landslides, some local earthquakes occurred and there was a short-term increase in steam emanations; nevertheless, the volcano seemed to be undisturbed. Later in March and April 1991, magma (molten rock) started to rise towards the surface from about 32 kilometres beneath Pinatubo, which caused powerful steam explosions and some earth quakes. Tons of noxious sulphur dioxide gas emitted from the volcano and hundreds of minor earthquakes occurred beneath Pinatubo in April, May and June 1991.

The Eruptions

From 7th to 12th June the first currents of Mount Pinatubo magma reached the surface. Since it had already lost much of its gas while reaching the surface, magma did not cause an eruption; instead, it just oozed out to form lava. However, millions of cubic yards of gas-charged magma reached the surface on June 12th and exploded as the first spectacular eruption.

Later even more gas-charged magma reached the surface of Pinatubo on June 15th which resulted in a cataclysmic eruption that emitted 5 cubic kilometres (1 cubic mile) of material. The thick ash clouds from this eruption spread to almost 35 kilometres (22 miles) into the air. Moreover, in lower altitude regions the ash was spread in every direction due to the powerful cyclonic winds of a typhoon that occurred coincidently at the same time. The winds at higher altitude regions blew the ash in the south-western direction. A blanket of slit-size grains and sand of volcanic glass and minerals and quite larger frothy pebbles covered the entire area. Some ash residue was found as far as the Indian Ocean and the satellite confirmed the tracking of ash clouds several times around the world (Cockell 98).

Large avalanches of gas, hot ash and pyroclastic flows (pumice fragments) were blown across Mount Pinatubo filling the deep valleys with new volcanic deposits as high as 200 meters (660 feet) thick. The eruption was so massive and removed such a large amount of magma from under the volcano that the peak of the mountain collapsed and formed a caldera, which is a volcanic depression, as big as 2.5 kilometres (1.6 miles).

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Quite spectacular yet smaller eruptions of ash occurred from time to time till September 1991. Moreover, from July to October 1992, a lava dome was formed in the newly created caldera as more fresh magma rose from deep under Pinatubo.

Primary Impacts of the Eruption

Death Toll

The death toll was reported to be 847 people losing their lives in the deadly eruption. A large number of roofs collapsed due to the weight of ash accumulation. The hazard was amplified due to the simultaneous occurrence of the typhoon. The evacuation plans saved many lives; although, the spread of various diseases, the destruction of healthcare facilities and poor conditions raised the death toll in the following months of eruption.

However, 500,000 locals still live in the valleys under the giant Pinatubo within 40 km of the mountain. (Fiocco (Ed) 24)

Agricultural Impacts

An area of 37,000 acres was destroyed due to the deadly eruption. Many reforestation projects were wiped out and overall 200,000 acres of farmlands that used to cultivate rice  were affected. 800,000 domestic animals died, thus affecting the livelihood of a large number of farmers. The agricultural losses due to the eruption estimated to 1.5 billion pesos.

Secondary Impacts of the Eruption

Local Economic and Social Impacts

Totally, 2.1 million locals and 364 communities were directly affected by the catastrophe. 73,000 houses were damaged and 8,000 were destroyed completely. Moreover, apart from the grave damage caused to these citizens, roads and communication structures were affected badly too. Aggregate losses in 1991 were estimated to 10.6 billion pesos including the damages to the public infrastructure which was estimated to 3.8 billion pesos. It is important to note that children were also affected as their education was disrupted due to the ruining of school buildings.

The financial growth of this region was seriously affected by the eruption. 10% of the total Philippine gross domestic product was accounted because of the gross regional domestic product of Pinatubo area. The GRDP before the eruption was growing by the rate of 5% annually but fell to 3% between 1990 and 1991. Property and crops damages constituted $374 million. The continuous flowing of Lahars added to the losses to further $69 million in 1992. The agricultural economy of the region was severely affected since nearly 42% of the agricultural land was affected by the mudflows. (Rantucci 137)

Mudflows and Lahars

After the eruption, every heavy rain caused lahars (mudflow) from the mountains, thus affecting many people, buildings, crops and communicational infrastructure. A great provision of funds was used to construct dams and dikes in order to control the consequences.

There are many river streams that flow out of Pinatubo: Tarlac, Sta, Abacan, Bucao, Tanguay, Kileng, Maloma, Santo Tomas, Lucia and Ashley rivers. These rivers were a perfect ecosystem before the 1991 eruption but the cataclysm destroyed them and filled a lot of valleys with the pyroclastic deposits. Since then, these currents have been blocked by the sediments and the natural resources have been suffering from lahars which used to emerge for many years after the tragedy. Experts say that the river eco-system will take a long period of time to fully recover from the impacts.

Environmental Impacts

The stratosphere was injected by enormous amounts of dust and aerosols as a result of the massive eruption. Moreover, the droplets of sulphuric acid were produced as a result of the oxidizing of sulphur dioxide, which then expanded across the stratosphere over the year after eruption.

This injection in the stratosphere resulted in a 10% decrease of the ordinary quantity of sunlight getting to the surface of the Earth in that location. As a result, the temperatures in the northern hemisphere decreased by 0.5-0.6 °C whereas the global temperatures fell by 0.4 °C. The temperature of the stratosphere increased by a couple degrees because of the absorption of the aerosol. The stratospheric cloud caused by the eruption remained in the atmosphere for 3 years (Boer 25).

The ejection had serious effects on the ozone levels, causing amplification in the rates of the ozone layer destruction.

Pinatubo Eruption 1991 vs. Mount Saint Helens Eruption 1980

The eruption of Mount Pinatubo in 1991 was 10 times bigger and more devastating than the Mount St. Helens eruption in 1980. The death toll in Pinatubo eruption was far more than during the St. Helens eruption. If compared, the VEI (Volcanic Explosivity Index) of Pinatubo was 6 out of 8 whereas that of St. Helens was 5.

Pinatubo Today

Since 1991, the activity of the volcano persisted at a less significant level with ash eruptions until August 1991. Later in 1992, another lava dome began to form in the caldera; the volcanologists were seriously concerned about that issue. However, there was just a minor eruption in 1992; since then up to present the volcano has remained silent and dormant.

Recently, in 2011, another earthquake of magnitude 5.9 struck Pinatubo; however, no reports were received of any damages or deaths.

Measures to Prevent Damages

After the initial eruptions and earthquake, US Geological Survey and the Philippine Institute of Volcanology and Seismology (PHIVOLCS) worked hand in hand to persuade the locals to evacuate the area due to the severity of the threat. The volcanologists were under immense pressure to evacuate the area on time before the devastating eruption began. Due to the timely evacuation plans, the death toll was not as large as compared to the severity of the hazard. (Fisher 68-70)

After the eruption, rescue operations began to search for any survivors. Most of the survivors were seriously injured and vulnerable to various skin diseases, like measles. Due to the destruction of hospitals and healthcare facilities, the survivors were not treated on time; as a result, the diseases spread fast and wide. The situation would have been better if the survivors had been treated soon in the nearby safer areas.

The government of Philippines also organized the re-settlement areas for those who lost their homes and farms in the deadly eruption. However, these small pieces of land distributed by the government as compensation were not enough for those poor Aeta people who could hardly grow any crops there. Farming was the major occupation at Pinatubo; hence, the government of Philippines should have granted them separate lands for their homes and cultivation in order to make them able to earn their living.

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