Impact of Volcanic Hazards in Less Developed Countries

To discuss the effects of primary and secondary volcanic hazards on people and property in Less Developed Countries, we will use the case study of Mount Merapi.


Effects of Volcanic Hazards in Less Developed Countries
Example: Mount Merapi

Mount Merapi
Source: http://volcano.und.edu/vwdocs/volc_images/southeast_asia/indonesia/Merapi1.jpg

Merapi, which is a stratovolcano, is Indonesia’s most active volcano and well known for partial collapse of lava domes and the generation of pyroclastic flows. The name M erapi means ‘Mountain of Fire’. It is tall (2.8 km or 1.7 miles high) and has steep slopes. The top of the volcano sometimes has a normal crater, but usually a lava dome fills the crater. The dome plugs up the volcano, making it difficult for other lava and ash to escape. A broad gouge funnels lava and ash flows from the top of Merapi to the south slopes of the volcano. The top of the mountain has no vegetation because erupted ash often falls there. Dense vegetation covers the flanks of the volcano. Many farmers live in villages around the volcano. The volcanic ash makes rich soil for growing crops, but it is a dangerous place to live. Yogyakarta city, with a population of 3 million, is 15 miles (25 km) south of Merapi. About 50,000 people live on the southwest flank of the volcano.

Lava dome at the summit of Merapi. During 1982, the lava dome was growing about 80,000-100,000 cubic meters per month. The dome collapsed in late November, creating nuee ardentes.
Source: http://volcano.und.edu/vwdocs/volc_images/southeast_asia/indonesia/Merapi2.jpg

Like all volcanoes, Merapi has eruptions of different sizes. The volcano has had numerous moderate to large (1 million cubic m) eruptions. Small eruptions happen every 2-3 years, bigger ones every 10-15 years, and very large ones every 50-60 years. The biggest eruptions occurred in the years 1006, 1786, 1822, 1872 and 1930. The eruption of 1006 spread ash all over the central part of the island of Java. The destruction was so bad that the existing Hindu kingdom was destroyed and the island was taken over by Muslims.

Eruptions mainly begin with pyroclastic flows followed by widespread pyroclastic air fall. Plinian to subplinian eruptions are common. These older deposits indicate that Merapi is capable of eruption styles very different from those currently observed.

Pyroclastic flow deposit on Merapi.
Source: http://volcano.und.edu/vwdocs/volc_images/southeast_asia/indonesia/Merapi3.jpg

A history of violent eruptions led to Mount Merapi being designated as one of the "Decade Volcanoes." These volcanoes have been identified by the International Association of Volcanology and Chemistry of the Earth's Interior as requiring special study because of the danger they pose to populated regions. Mount Merapi's last large ash eruption occurred in 1984. A particularly devastating eruption took place in 1930, when 1,300 were killed by an eruption here. Another eruption, in 1976, killed 28 people and destroyed homes of 1,176 people. Since 1984, Mount Merapi had erupted repeatedly as glowing avalanches flowed from a growing lava dome. These glowing avalanches, or nuee ardentes, moved down a different river drainage towards the west.

Eruption on 22 November 1984

Mount Merapi erupted on November 22 at about 10:15 a.m. local time. Inconsistent news reports on the number of casualties suggest that at least 34 people were killed, several hundred were injured, and hundreds of homes were destroyed. The eruption began with steam explosions and ejection of rocks and gravel over the surface of the cone. The steam plume reached about 800 meters high. After 25 minutes of such activity, the main eruption began and sent an ash column roughly 10 kilometers high. An advisory was issued to warn aircraft from entering the ash plume and being subject to engine damage and endangering the lives of those onboard. Ash fallout was heavy as far as 45 kilometers to the northwest of Mount Merapi. On the ground, a pyroclastic flow of hot ash, gas, and other suspended particles swept 6 kilometers to the southwest down the Boyong River drainage and through Turgo Village in the Yogyakarta District. Many of the injured suffered severe burns from the hot gases. Most of the casualties appear to be in two small villages. Over 6,000 people were evacuated from the area.