Mount Pinatubo is an active stratovolcano in the Cabusilan Mountains on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. Before the volcanic activities of 1991, its eruptive history was unknown to most people. It was heavily eroded, inconspicuous and obscured from view. It was covered with dense forest which supported a population of several thousand indigenous people, the Aetas, who fled to the mountains during the Spanish conquest of the Philippines.
The volcano's Plinian / Ultra-Plinian eruption on 15 June 1991 produced the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in the Alaska Peninsula. Complicating the eruption was the arrival of Typhoon Yunya (Diding), bringing a lethal mix of ash and rain to areas surrounding the volcano. Successful predictions at the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but the surrounding areas were severely damaged by pyroclastic flows, ash deposits, and subsequently, by the lahars caused by rainwaters re-mobilizing earlier volcanic deposits causing extensive destruction to infrastructure and changing the river systems months to years after the eruption.
The effects of the eruption were felt worldwide. It ejected roughly 10,000,000,000 tonnes (1.1×1010 short tons) or 10 km3(2.4 cu mi) of magma, and 20,000,000 tonnes (22,000,000 short tons) SO
2, bringing vast quantities of minerals and metals to the surface environment. It injected more particulate into the stratosphere than any eruption since Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F) in the years 1991-93 and ozone depletion temporarily increased substantially.
A reported 847 people were killed by the eruption, mostly by roofs collapsing under the weight of accumulated wet ash, a hazard amplified by the simultaneous arrival of Typhoon Yunya. The evacuation in the days before the eruption certainly saved tens of thousands of lives, and has been hailed as a great success for volcanology and eruption prediction. However, damage to healthcare facilities and the spread of illnesses in relocation facilities due to poor sanitation led to rising death tolls in the months after the eruption.
After the eruption, about 500,000 people continue to live within 40 km (25 mi) of the mountain, with population centers including the 150,000 in Angeles City and 30,000 at Clark Freeport Zone.
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