Mount Merapi, Central Java
| Mount Merapi | |
|---|---|
 Merapi |
|
| Elevation: | 2,911 metres (9,548 feet) |
| Location: | Central Java (Indonesia) |
| Range: | |
| Coordinates: | 7°5′S, 110°4′E |
| Type: | Stratovolcano |
| Age of rock: | 400,000 years |
| Last eruption: | 2002 |
Mount Merapi is a mountain in Central Java, Indonesia. It is the most active volcano in Indonesia and has erupted 68 times since 1548. Its name means Mountain of Fire. It is very close to the city of Yogyakarta, and thousands of people live on the flanks of the volcano, with villages as high as 1700m above sea level. In light of the hazards it poses to populated areas, it has been designated a Decade Volcano.
There is another Mount Merapi, also volcanic, in Sumatra (also known as Marapi).
Geological history
Merapi is the youngest in a group of volcanoes in southern Java. It is situated at a subduction zone, where the Indo-Australian Plate is sliding beneath the Eurasian Plate. Eruptions in the Merapi area began about 400,000 years ago, and from then until about 10,000 years ago, eruptions were typically effusive, and the lava emitted was basaltic. Since then, eruptions have become more explosive, with viscous andesitic lavas often generating lava domes. Dome collapse has often generated pyroclastic flows, and larger explosions which have resulted in eruption columns have also generated pyroclastic flows through column collapse.
Typically, small eruptions occur every 2-3 years, and larger ones every 10-15 years or so. Notable eruptions, often causing many deaths, have occurred in 1006, 1786, 1822, 1872 (the most violent eruption in recent history) and 1930 (13 villages destroyed and 1400 people killed by pyroclastic flows).
A very large eruption in 1006 covered all of central Java with ash. The volcanic devastation is believed to have led to the collapse of the Hindu Kingdom of Mataram civilization, and allowed Muslims to become the rulers of Java.
Recent eruptions
The volcano's most recent eruption began in 1992 and continued for ten years. During this time, a lava dome was extruded, growing by up to half a metre per day. In 1994, the dome reached the edge of the crater, and from then on, rockfall from the dome produced frequent pyroclastic flows. In late 1994 almost the entire dome collapsed, generating very large pyroclastic flows, which travelled several kilometres from the summit and killed 43 people.
Following the large eruption of November 1994, a new dome formed in the crater, and small explosive eruptions continued for several years, generating scores of lava avalanches and pyroclastic flows every day. Eruptions ended in late 2002.
Monitoring
Merapi is the site of a very active volcano monitoring program. Seismic monitoring began in 1924, and the eruption of 1930 was found to have been preceded by a large earthquake swarm. There is currently a network of 8 seismographs around the mountain, allowing volcanologists to accurately pinpoint the hypocentres of tremors and quakes. A zone in which no quakes originate is found about 1.5km below the summit, and is thought to be the location of the magma reservoir which feeds the eruptions.
Other measurements taken on the volcano include magnetic measurements and tilt measurements. Small changes in the local magnetic field have been found to coincide with eruptions, and tilt measurements reveal the inflation of the volcano caused when the magma chamber beneath it is filling up.
Lahars are an important hazard on the mountain, and are caused by rain remobilizing pyroclastic flow deposits. Lahars can be detected seismically, as they cause a high- frequency seismic signal. Observations have found that about 50mm of rain per hour is the threshold above which lahars are often generated.