later formed on the caldera. in 1992. Thick ash and lahar deposits on the volcano and in surrounding river valleys are evident.
Explosivity of the eruption The 1991 eruption rated 6 on the
Volcanic Explosivity Index and came some 450–500 years after the volcano's last known eruptive activity. The eruption ejected over of material, making it the largest eruption of the 20th century since that of
Novarupta in 1912 and ten times larger than the
1980 eruption of Mount St. Helens. Ejected material such as tephra fallout and pyroclastic flow deposits are much less dense than magma, and the volume of ejected material was equivalent to about of unerupted material. Thermal energy released during the eruption was equal to 70 megatons of TNT. The former summit of the volcano was obliterated and replaced by a
caldera wide. The highest point on the caldera rim now stood above sea level, some lower than the pre-eruption summit.
Death toll A reported 847 people were killed by the eruption, mostly by roofs collapsing under a load of accumulated volcanic matter, 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. After the eruption, about 500,000 people continue to live within of the volcano, with population centers including the 150,000 in
Angeles City and 30,000 at
Clark Freeport Zone.
Effects on agriculture Many
reforestation projects were destroyed in the eruption, with a total area of valued at 125 million pesos destroyed.
Agriculture was heavily disrupted, with of
rice-growing farmland destroyed, and almost 800,000 head of livestock and
poultry killed, destroying the livelihoods of thousands of farmers. The cost to agriculture of eruption effects was estimated to be 1.5 billion pesos. Many farmers near Pinatubo began growing quick-ripening crops such as peanuts, cassava, and sweet potatoes, which could be harvested before the threat of lahar floods during the late summer rainy season. The eruption of Pinatubo severely hampered the
economic development of the surrounding areas. The gross regional domestic product of the Pinatubo area accounted for about 10% of the total Philippine
gross domestic product. The GRDP had been growing at 5% annually before the eruption but fell by more than 3% from 1990 to 1991. In 1991, damage to crops and property was estimated at $374 million (or $711 million today), to which continuing lahar floods added a further $69 million (or $127 million today) in 1992. In total, 42 percent of the cropland around the volcano was affected by more lahar floods, dealing a severe blow to the agricultural economy in the region. Several important river systems stem from Mount Pinatubo, the major rivers being the
Tarlac, Abacan, Pasig-Potrero, Sta. Lucia, Bucao, Santo Tomas, Maloma, Tanguay, Ashley and Kileng rivers. Before the eruption, these river systems were important
ecosystems, but the eruption filled many valleys with deep
pyroclastic deposits. Since 1991, the rivers have been clogged with
sediment, and the valleys have seen frequent lahars which continued for years after the eruption. Studies show that the river systems will take decades to recover from the June 1991 eruption. On September 3, 1995, a lahar buried
San Guillermo Parish Church in
Bacolor, Pampanga to half its height. Additional maritime evacuations began several days later with the arrival of the battle group, , and . Most personnel were initially relocated to
Guam,
Okinawa and the U.S. state of
Hawaii, although some returned to the continental United States. Clark Air Base was ultimately abandoned by the United States military because of the eruption, and Subic Bay reverted to Philippine control in November 1992 following the breakdown of lease negotiations and the expiration of the Military Bases Agreement of 1947.
Global environmental effects The powerful eruption of such an enormous volume of lava and ash injected significant quantities of
aerosols and
dust into the
stratosphere. Sulfur dioxide oxidized in the atmosphere to produce a haze of
sulfuric acid droplets, which gradually spread throughout the stratosphere over the year following the eruption. The injection of aerosols into the stratosphere is thought to have been the largest since the
1883 eruption of Krakatoa, with a total mass of of about being injected – the largest volume ever recorded by modern instruments (see
chart and
figure). This very large stratospheric injection resulted in a
volcanic winter, a reduction in the normal amount of
sunlight reaching the Earth's surface by roughly 10% (see
figure). This led to a decrease in
Northern Hemisphere average temperatures of and a global decrease of about . The 1991 eruption also caused the "Summer that Wasn't" in 1992. The extremity of this volcanic winter has been called into question by some, however, with a more conservative estimate of a 0.2 °C decrease in global temperatures for 13 months also being given. At the same time, the temperature in the stratosphere rose to several degrees higher than normal, due to the absorption of radiation by the aerosol. The stratospheric cloud from the eruption persisted in the atmosphere for three years. The eruption, while not directly responsible, may have played a part in the formation of the
1991 Perfect Storm,
1991 Halloween blizzard and
1993 Storm of the Century. The eruption had a significant effect on
ozone levels in the atmosphere, causing a large increase in the destruction rate of ozone. Ozone levels at middle latitudes reached their lowest recorded levels, while in the
Southern Hemisphere winter of 1992, the
ozone hole over Antarctica reached its largest ever size until then, with the fastest recorded ozone depletion rates. The eruption of
Mount Hudson in Chile in August 1991 also contributed to southern hemisphere ozone destruction, with measurements showing a sharp decrease in ozone levels at the
tropopause when the aerosol clouds from Pinatubo and Hudson arrived. Another noticeable effect of the dust in the atmosphere was the appearance of
lunar eclipses. Normally even at mid-eclipse, the Moon is still visible although much dimmed, whereas in the year following the Pinatubo eruption, the Moon was hardly visible at all during eclipses, due to much greater absorption of sunlight by dust in the atmosphere. It has also been suggested that excess
cloud condensation nuclei from the eruption were responsible for the "
Great Flood of 1993" in the
Midwestern United States.
Aeta people The
Aeta people were the hardest hit by the eruption. After the areas surrounding the volcano were declared safe, many Aetas returned to their old villages only to find them destroyed by pyroclastic and lahar deposits. Some were able to return to their former way of life, but most moved instead to government-organized resettlement areas. Conditions on these were poor, with each family receiving only small plots of land not ideal for growing crops. Many Aeta found casual labor working for lowland
farmers, and overall Aeta society became much more fragmented, and reliant on and integrated with lowland culture. == Humanitarian aid ==