Mount Merapi is located in Central Java, some 60 km from the coast. It is one of the subduction zone volcanoes within the Pacific Ring of Fire. Due to its highly destructive potential and proximity to the metropolis of Yogyakarta, it is classified as a high-risk volcano and is continuously monitored by volcanologists.
Typical of subduction zone volcanoes such as Mount Merapi, is the flow of highly viscous magma, which does not emerge in classic lava flows or lava fountains, but is dammed in the form of a dome at the summit of the volcano, until the edges of the lava domes are so steep they slip off, or until the dome breaks down in itself, thus creating the dreaded pyroclastic flows.
In historical times larger eruption on Mount Merapi occurred every 10 - 15 years. The largest took place in the years 1006, 1786, 1822, 1872 and 1930.
The recent eruptions on the MerapiIn 1994, a pyroclastic flow travelled 12 kilometres and reached the foothills of the village of Kaliurang. At that time 66 people died. During the eruption phase in 2006, the PFs covered a maximum distance of 6 km. Two men died in a shelter near Kaliadem; they had helped evacuate the village and at the last second sought shelter in the emergency bunker on the outskirts of the village, which was buried by a pyroclastic flow. Rescue workers immediately set to work and measured temperatures of 300 degrees. Initially there was still contact via mobile phone, which broke off when the men ran out of air in the bunker.
The last eruption phase began on 26 October 2010. Numerous explosions caused the cathedral to collapse and generated pyroclastic currents. These destroyed villages on the volcano flank. During an eruption on 5 November 2010, pyroclastic flows destroyed villages 15 km away. More than 350 people died. In Yogjakarta, 40 km away, volcanic ash fell and air traffic was obstructed. Some volcanologists estimate that the eruption is more explosive than the 1872 eruption.
After a pause of several years, the Merapi returned in May 2018 with phreatic eruptions. In August of the same year, a small lava dome began to grow in the crater. This could develop the potential for a larger eruption with pyroclastic flows.
Subduction and earthquakes at MerapiTectonic earthquakes are often associated with subduction zones, as is volcanism. At subduction zones, the oceanic crust submerges below the continents and is partially melted in the depths of the upper mantle. Part of the magma thus formed rises again behind the subduction zone and emerges at the volcanoes. The high water content of the oceanic crust also increases the gas concentration and the explosive power of these volcanoes, making them particularly dangerous.
The earthquake of 27.05.2006 was related to the forces of plate tectonics, which are responsible for the formation of subduction zones. The epicenter was located in the coastal area of Java, only 50 km from Merapi. The 6.2 magnitude shock not only destroyed numerous towns and villages and cost 5800 lives, but also triggered a large pyroclastic flow.
One day before the recent eruption, a severe earthquake also shook Indonesia. Before Sumatra an earthquake occurred with the strength 7.7.
Scientists from the Geoforschungszentrum Potsdam used seismometers to analyse the behaviour of the earthquake waves under the Merapi. Different densities of the material ensure different propagation velocities of the seismic waves. The researchers discovered a huge, sponge-shaped structure under the Merapi that appears to be filled with a fluid. Speculations about a well-filled magma chamber and an imminent mega eruption are ghosts through the press. The researchers, however, hold back with unambiguous comments; although there is a possibility that the fluid is magma, it is not certain. If it is magma, then there is about three times as much magma slumbering under the Merapi as was produced during the eruption of Tambora in 1815.