[edit] Global environmental effects
Space Shuttle (Mission
STS-43) photograph of the Earth over South America taken on August 8, 1991, showing double layer of Pinatubo aerosol cloud (dark streaks) above high cumulonimbus tops
The powerful eruption of such an enormous volume of lava and ash injected significant quantities of
aerosols and
dust into the
stratosphere.
Sulfur dioxide oxidised 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 eruption of
Krakatoa in 1883, with a total mass of SO
2 of about 17 million tons being injectedthe largest volume ever recorded by modern instruments (see
chart and
figure).
This very large stratospheric injection resulted in 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 0.50.6 °
C (0.91.1 °F), and a global fall of about 0.4 °C (0.7 °F). At the same time, the temperature in the stratosphere rose to several degrees higher than normal, due to absorption of radiation by the aerosols. The stratospheric cloud from the eruption persisted in the atmosphere for three years after the eruption.
Satellite measurements of ash and
aerosol emissions from Mount Pinatubo.
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 mid-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, but 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.
[edit] The area since 1991
Mount Pinatubo's caldera contains a
lake of the same name, pictured here in 2008
Following the climactic eruption of June 15, 1991, activity at the volcano continued at a much lower level, with continuous ash eruptions lasting until August 1991 and episodic eruptions continuing for another month. Activity then remained low until July 1992, when a new
lava dome began to grow in the
caldera.
This dome appeared to be composed of fresh lava from the deep
magma reservoir beneath the volcano, rather than material 'left over' in a shallow reservoir from the 1991 eruption. Thus, volcanologists suspected that further violent eruptions could be possible, and some areas were once again evacuated. However, the eruption did not become violent, perhaps due to outgassing from the deep reservoir reducing the explosivity of the lava reaching the surface. Since 1992, the volcano has not been erupting.
The Aeta people were the hardest hit by the eruption. The total destruction of many villages by pyroclasts and lahar deposits meant that many Aeta were unable to return to their former way of life. After the areas surrounding the volcano were declared safe to return to, those whose villages had not been destroyed moved back, but most people moved instead to government-organized resettlement areas. Conditions on these were poor, with each family receiving only small plots of land, which were 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.
After eruptions ended, a
crater lake,
Lake Pinatubo, formed in the 1991 caldera, with the 1992 lava dome forming an
island. At first, the lake was small, hot and highly
acidic, with a minimum
pH of 2 and a temperature of about 40 °C. Abundant rainfall cooled and diluted the lake, lowering the temperature to 26 °C and raising the pH to 5.5 by 2003.
The lake increased in depth by about 1 meter per month on average, until September 2001, when fears that the walls of the crater might be unstable prompted the Philippine government to order a controlled draining of the lake. 9,000 people were once again evacuated from surrounding areas in case a large
flood was accidentally triggered. Workers cut a 5 m notch in the crater rim, and successfully drained about a quarter of the lake's volume.
just added some tidbits of the SCIENCEsurrounding a single volcano. for the record, there are roughly 300 volcanos per year
