Have you ever heard of the second largest classic explosive eruption of the twentieth century? If not, let’s examine the cataclysmic eruption of Mount Pinatubo in June 1991. This volcano is located on the island of Luzon in the Philippines, a mere 90 kilometers (55 miles) northwest of Manila, the capital city (Rosenbu, 2010) (see Figure 1). In this report, we will provide a vivid description and information about the volcano. Additionally, we will discuss various factors related to its eruption such as its consequences and aftermath.
This article will discuss the impact of Mount Pinatubo’s eruption on the climate and hydrological cycle of Earth. Mount Pinatubo is part of the Luzon volcanic arc, which consists of a chain of composite volcanoes (refer to Figure 2) (Wolfe, E. & Hoblitt, R., 1999). Composite or Strato-Volcanoes are formed by alternating layers of lava and rock fragments (Oracle Thinkquest, 2011). The arc runs parallel to Luzon’s west coast and reflects subduction along the Manila trench to the west. Mount Pinatubo is one of the highest peaks in west-central Luzon.
Mount Pinatubo’s former summit, which was at an elevation of 1,745 meters, may have been the crest of a lava dome that formed about 500 years ago during the most recent major eruptive episode. The volcano’s lower flanks were composed largely of pyroclastic deposits from voluminous and explosive prehistoric eruptions. These flanks were intricately dissected and densely sheathed in tropical vegetation prior to the 1991 eruptions (Wolfe, E. & Hoblitt, R., 1999).
The first recognized signs that Pinatubo was reawakening after a 500-year slumber were a series of small steam-blast explosions in early April 1991 (Hendley II, J., Newhall, C., & Stauffer, P., 2005). Scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) intensified their monitoring of the volcano. They were soon joined by scientists from the U.S. Geological Survey (USGS) and other organizations.
While evaluating the threat from the volcano, these scientists discovered that countless giant mudflows of volcanic debris known as lahars had created most of Mount Pinatubo’s gentle slopes surrounding it due to powerful ancient eruptions that built it up (Hendley II,J., Newhall,C.& Stauffer,P.,2005).
On June 15th in 1991 morning Mount Pinatubo exploded in what would be largest volcanic eruption on earth for more than three-quarters century.The most powerful phase lasted over ten hours creating an enormous cloud ash rising as high as twenty-two miles into air growing more than three hundred miles across turning day into night over central Luzon(Hendley II,J.Newhall,C.& Stauffer,P.,2005).
According to an article in the Manila Bulletin, a four-year-old girl was interviewed and expressed her fear during the eruption of a volcano. She said, I thought it was the end of the world. All day long, it was dark like night” (Villasana, B., 2012). The eruption caused falling ash that covered thousands of square miles and pyroclastic flows, also known as nuee ardente, which deposited ash up to 600 feet thick in deep valleys (see Figure 3).
Falling ash from the eruption of Mount Pinatubo in 1991 caused numerous problems, including collapsed roofs, clogged stream channels, and impassable roads. The cleanup effort was massive (Newhall, C., Hendley II, J., & Stauffer, P., 2005). The lahars resulting from the eruption deposited over 0.7 cubic miles (3 cubic kilometers) of debris on the surrounding lowlands and buried hundreds of square miles of land (see Figure 4). These lahars quickly filled small stream channels (Newhalll, C., Hendley II. J & Stauffer, P., 2005). Stream channels are long trough-like depressions where natural streams flow or may flow; they are also known as streamways (Answers, n.d.).
The lahars destroyed more than 100,000 homes and caused significant damage to crops and infrastructure. In total, the damage amounted to at least 10. billion pesos ($US374 million) in 1991 and an additional 1.9 billion pesos ($US69 million) in 1992. Business losses were estimated at around 454 million pesos ($US17 million) in 1991 and an additional $37 million pesos ($US1.4 million) in business losses occurred in the following year.
From June of ’91 until November ’92 livelihoods were lost or impacted by partial or complete loss of housing across 364 barangays (villages), affecting approximately one-third of Central Luzon’s population – about 329k families or roughly 2.1m people who lived there during that time period according to census data from a year earlier(see Figure5).
In 1991, a total of 4,979 houses were completely destroyed while 70,257 houses were partially damaged. The number decreased in the following year when only 3,281 houses were completely destroyed and another 3,137 units were partially damaged (Lacsamana, J., Mercado, R., & Pineda G., n.d.) (see Figure 6). When the mountain with a height of 5,770 feet or approximately1760 meters erupted and released sulfur dioxide into the atmosphere that reached up to a distance of twenty-five miles or forty kilometers away. The resulting cloud mixed with water vapor and circled around the globe for twenty-one days. This event temporarily offsetted the effects of global warming.
Satellite images taken of the area after the eruption showed a dust-like smudge in the stratosphere (Gale: Environmental Encyclopedia, 2011) (see Figure 7). The stratosphere is the layer of atmosphere that extends from about 10 km to 50 km (6-30 miles) in altitude (Przyborski, P., 2012) (see Figure 8). The sulfur dioxide cloud deflected two percent of Earth’s incoming sunlight and lowered temperatures worldwide on average.
This gas cloud was chemically converted into a sulfuric acid aerosol, which was predicted to cause a 0.5°C global temperature decrease (Sprod, I. E., 1994). Pinatubo ejected about 15 million tons of sulfur dioxide into the stratosphere, where it reacted with water to form a hazy layer of aerosol particles composed primarily of sulfuric acid droplets (Przyborski, P., 2012). An aerosol can be defined as a system of solid or liquid particles suspended in air or another gaseous environment (Aerosol Basics, n.d.).
A photograph taken on August 8, 1991, less than two months after the Pinatubo eruption, shows a similar type of image as the one captured on August 30, 1984. Two distinct dark layers of aerosols are visible in the atmosphere. The estimated altitude of these aerosol layers is between 20 to 25 km which is consistent with measurements made by other space-based instruments (Earth Observatory, n.d.) (see Figure 9). Unlike the troposphere that extends from the surface up to approximately 10km and has rain clouds that quickly wash out pollutants, the stratosphere does not have such mechanisms.
Therefore, heavy influxes of aerosol pollutants, such as the plume from Mount Pinatubo, can remain in the stratosphere for years until chemical reactions and atmospheric circulation filter them out (Przyborski, P., 2012). The hydrological cycle is the continuous process by which water circulates throughout the earth and its atmosphere; it is also known as the water cycle (Your Dictionary Science, n.d.). Large volcanic eruptions are a good analogue for albedo enhancement experiments due to an enhanced loading of stratospheric aerosols (Esch, M. Graf, H., Hagemann, S., Jungclaus, J., Launderer F., Schmidt H. & Timmreck C., 2008). Albedo refers to the ratio of light reflected by an object to incident light; it measures an object’s reflectivity or intrinsic brightness (Albedo Arts, n.d.). The effect of Mount Pinatubo’s eruption in June 1991 on the hydrological cycle was remarkable: precipitation over land decreased significantly in 1992 along with associated runoff and river discharge into the ocean (see Figure 10).
Top-of-atmosphere (TOA) radiation measurements, such as those from the Earth Radiation Budget Satellite (ERBS), show how a veil of debris formed in or injected into the stratosphere, blocking out the sun and resulting in a significant decrease in absorbed solar radiation within the Earth-atmosphere system (Trenberth & Dai, 2007). ERBS was designed to investigate how energy from the sun is absorbed and re-radiated by the earth (National Aeronautics and Space Administration, 1984). This decrease was caused by an increase in albedo of up to 0.07 due to reflection of up to an additional 2.5 W m?2 solar radiation over two years following the eruption (Trenberth & Dai, 2007).
June 15, 1991 was an unforgettable day for many Filipinos living near Mount Pinatubo. The cataclysmic eruption of this composite volcano has become one of the most important phenomena of the twentieth century due to its significant influence on various factors including its consequences and aftermath.
On the island of Luzon, Mount Pinatubo turned day into night. The climate was also impacted by the massive eruption. The volcano released a significant amount of sulfur dioxide into the air, causing temperatures around the world to drop by 0.5 degrees Celsius for almost two years. Additionally, there was a decrease in precipitation over land and associated runoff and river discharge into the ocean as the water cycle of Earth was greatly affected by the eruption.
Although Mount Pinatubo is currently inactive, we never know when it will awaken again and unleash another colossal eruption.
