Eruptive History
Mount Merapi is one of the most active volcanoes on earth, and has a relatively detailed eruptive history extending into the mid-1500s (Figure 9). More ancient deposits are known to have destroyed villages and temples, and some authors postulate that eruptions from Mount Merapi caused the eastward migration of the Mataram Kingdom in the 928 AD (Newhall et al., 2000).
Figure 9: Documented historic eruptions at Mt Merapi with accompanying VEI indices, from Gertisser et al. (2012).
Events occurring since the beginning of the 20th century are outlined below. These periods of activity include years with either eruptions and/or lahar-forming events. Note that not all lahars are recorded in this table; due to the high yearly rainfall, lahars are so common at Merapi that a complete record of all events is hard to come by. Nonetheless, the number of lahars outlined below (>450) gives a reasonable impression of how common these events are in the region.
Volcanic activity on Merapi usually occurs every 2 to 4 years, and primarily involves lahars as well as the growth of viscous domes that collapse to form pyroclastic flows (Surono et al. 2012). Pyroclastic flows have caused the majority of casualties in the 20th and 21st centuries. Lahars accompany eruptive phases and often occur in large numbers for up to few years after an eruption, with the highest lahar frequencies occurring during rainy seasons (Lavigne et al. 2000).
The two most significant eruptions in recent times have been the 1930-1931 and 2010- eruption sequences. The major event associated with the 2010- sequence occurred during October and November 2010 (described in following paragraphs), however, as there have been minor ash and steam ejections and continuous dome growth since, this eruptive period is considered here to still be occurring today. The 1930-1931 eruptions resulted in the greatest number of casualties in the 20th century. The climax of this eruption period occurred on December 18-19, 1930, when a series of far-traveled (up to 12km) pyroclastic flows annihilated 13 villages (partly destroying 23 more), killing 1369 people and 2100 animals.
Volcanic activity on Merapi usually occurs every 2 to 4 years, and primarily involves lahars as well as the growth of viscous domes that collapse to form pyroclastic flows (Surono et al. 2012). Pyroclastic flows have caused the majority of casualties in the 20th and 21st centuries. Lahars accompany eruptive phases and often occur in large numbers for up to few years after an eruption, with the highest lahar frequencies occurring during rainy seasons (Lavigne et al. 2000).
The two most significant eruptions in recent times have been the 1930-1931 and 2010- eruption sequences. The major event associated with the 2010- sequence occurred during October and November 2010 (described in following paragraphs), however, as there have been minor ash and steam ejections and continuous dome growth since, this eruptive period is considered here to still be occurring today. The 1930-1931 eruptions resulted in the greatest number of casualties in the 20th century. The climax of this eruption period occurred on December 18-19, 1930, when a series of far-traveled (up to 12km) pyroclastic flows annihilated 13 villages (partly destroying 23 more), killing 1369 people and 2100 animals.
Table 1: Summary of eruptive activity at Mt Merapi since 1900.
Recent eruptive activity: 2010 and 2006
2010 eruption: an example of a "100-year event"
Volcanic activity at Mount Merapi in 2010 was the largest observed since the VEI 4 eruption of 1872 (Gertisser et al. 2011). It was the worst volcanic disaster in the region in nearly 80 years and resulted in 322 deaths, and the displacement of one third of a million people (Surono et al., 2012). On October 26 Merapi began to erupt after four years quiescence. Although the sight of volcanic ash extruding from the volcano is just another day for many of the local inhabitants, the behavior of this eruption was to be dramatically different to the eruptions that had preceded it in recent times.
For nearly two months prior to the eruption, enhanced seismic activity and deformation was observed at the volcano (Gertisser et al., 2011). On October 24 several villages within 10 km of Merapi were evacuated, after a sharp increase in earthquakes and summit deformation gave a slight indication of the events that would surpass. Not all villagers evacuated, however, and on October 26 a series of pyroclastic flows enveloped the village of Kinarejo, some 5 km from the summit, resulting in the death of 34 people. For the next few days "glowing avalanches" traveled 12 km from the summit, and on November 4 a 7 km-high eruption plume transported ash 10s of kilometers from source (Gertisser et al., 2011).
November 5 marked the climax of the eruption. On this day, an unusually large pyroclastic flow traveled 15 km south of Merapi along Gendol Valley. Villages just outside the valley were enveloped by ash-cloud surges. The villages were almost completely destroyed, and hundreds of fatalities resulted. Video footage of a 2010 pyroclastic flow can seen in video 1 (cf. "video" section).
In summary, this eruption is considered a "100-year" event and a worst case scenario. The eruption that preceded the 2010 event (described below) is more common.
Figure 10: Images of the 2010 eruption of Mt Merapi. The top two images are from Google Earth with dates of the photos in the top left of each image. These images reveal the before and after of a ~1km-wide block-ash-flow that flowed down Gendol valley and completely inundated a village. The bottom images show the devastation in the aftermath viewed from the ground (image references from left to right are 3, 4, and 5).
Figure 11: Extent of the pyroclastic flow deposits of the 2010 eruption at Mt Merapi from Gertisser et al. (2011). Note the greater area covered by the 2010 deposits compared to recent eruptions and the extraordinary distance flows traveled to the south, resulting in hundreds of fatalities. The 1930 deposit (light green) also represents far traveled flows, which resulted in 1369 casualties.
2006 eruption: just another day at Mt Merapi
Although the eruption occurred in 2006, activity began in June 2005, when an increased number of volcanic earthquakes and deformation on the summit area was observed (Charbonnier and Gertisser, 2008). Extrusion of a new lava dome began in March 2006. Dome growth increased from April onwards and was accompanied by systematic rock fall and pyroclastic flows during May and June. In the earliest stages (May 5-27), rock fall and pyroclastic flows were concentrated within an area ~5km SW of the summit. During this time, the alert level at the volcano was raised to 4 and up to 22,000 residents in the hazardous zone 7km from the volcano were evacuated.
On May 27, a devastating magnitude 6.3 earthquake struck the region, resulting in 5000 deaths. To make matters worse, lava extrusion rates on Merapi increased and the dome reached a height 116m above the summit by June 4. Pyroclastic flows became more voluminous and began to flow south of the volcano (refer to Figures 11 and 12). On June 14, the longest traveled flows reached distances of 5-7km from the volcano down the Gendol valley, resulting in 2 fatalities.
Although the eruption occurred in 2006, activity began in June 2005, when an increased number of volcanic earthquakes and deformation on the summit area was observed (Charbonnier and Gertisser, 2008). Extrusion of a new lava dome began in March 2006. Dome growth increased from April onwards and was accompanied by systematic rock fall and pyroclastic flows during May and June. In the earliest stages (May 5-27), rock fall and pyroclastic flows were concentrated within an area ~5km SW of the summit. During this time, the alert level at the volcano was raised to 4 and up to 22,000 residents in the hazardous zone 7km from the volcano were evacuated.
On May 27, a devastating magnitude 6.3 earthquake struck the region, resulting in 5000 deaths. To make matters worse, lava extrusion rates on Merapi increased and the dome reached a height 116m above the summit by June 4. Pyroclastic flows became more voluminous and began to flow south of the volcano (refer to Figures 11 and 12). On June 14, the longest traveled flows reached distances of 5-7km from the volcano down the Gendol valley, resulting in 2 fatalities.
Figure 12: Images of the 2006 eruption at Mt Merapi. Top two images reveal the before and after (dates of images in the top left). Note the change in the dominant direction of the pyroclastic flows from WSW to S, exemplified by the far-travelled flow down the Gendol valley, which resulted in 2 fatalities. Remarkably, some of these southerly traveling flows were caught in the act in satellite imagery (image A: from Thouret et al. (2010)). These flows were also photographed from the ground (images B and E: image references 6 and 7, respectively). Although there were less casualties than the 2010 event, the eruption destroyed many villages (image D: from Thouret et al. (2010)). The top right photo (image C: reference 8) is of hot lava oozing from the summit dome.