Private Library of Simamora, Helmut Todo Tua
Environment, Research and Development Agency
Samosir Regency Government of North Sumatera Province
INDONESIA
Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical, 2460-m-high andesitic-to-dacitic volcano is at the southern end of the four overlapping summit craters. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks of Sinabung in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.
Based on webcam data and satellite images, the Darwin VAAC reported that during 13-14 November an ash plume from Sinabung rose to an altitude of 3.7 km (12,000 ft) a.s.l. and drifted almost 150 km NW and W. According to a news article, a pyroclastic flow traveled 1.2 km down the SE flank on 14 November, prompting more evacuations from villages near the base of the volcano. The article noted that more than 7,000 people had been evacuated from 10 villages.
An explosion observed with the webcam on 18 November produced an ash plume that rose to an altitude of 7.6 km (25,000 ft) a.s.l. About 30 minutes later an ash plume also visible in satellite images rose to an altitude of 11.3 km (37,000 ft) a.s.l. and drifted 65 km W. Four hours later satellite images showed ash plumes at an altitude of 9.1 km (30,000 ft) a.s.l. to the W of Sinabung and at an altitude of 4.6 km (15,000 ft) a.s.l. over the crater. On 19 November the webcam recorded an ash plume that rose to an altitude of 4.6 km (15,000 ft) a.s.l. over the crater. A news article stated that later that night that an ash plume rose to an altitude of 10 km (32,800 ft) a.s.l.
A news article from 20 November noted that volcanologists updated the hazard map for Sinabung. The second-tier disaster-prone area, previously defined as a radius of 2-3 km from Sinabung’s crater, was expanded to 4-5 km.
The National Disaster Mitigation Agency (BNPB) spokesman Sutopo Purwo Nugroho said the intensity of eruption of Mount Sinabung was increasing.
Sutopo said Thursday’s eruption took place at 6:57 a.m. and lasted for 1,056 seconds, spewing volcanic materials up to a height of 7,000 meters. He said that based on observation conducted by the Vulcanology Center, Mount Sinabung was currently covered by thick clouds.
The pyroclastic flow of hot volcanic ash discharged from the powerful eruption of Mount Sinabung, in Karo regency, North Sumatra, on Thursday threatened the safety of nearby residents.
As a result, 10 villages located at the foot of the volcano had been evacuated as of Thursday afternoon, due to the danger from the pyroclastic flow. The vacated villages are Simacem, Bekerah, Suka Meriah, Gurukinayan, Mardinding, Sigarang-garang, Berastepu, Hutagunggung, Laukawar and Gamber.
How long is the warning between the first rumblings of a volcano and its eruption? This haunting question was raised anew with the eruption of the Indonesian volcano Mount Sinabung on August 29, 2010.
Sinabung is a small, composite, andesitic volcano in northern Sumatra. Its summit, 2,460 m (8,071 ft), rises only a thousand meters or so above the surrounding terrain, much of which is dotted with numerous small farms.
Initial reports suggested that Sinabung lay in repose for 400 years or more. An explosion may have occurred in 1881, and fumarolic (steam vent) activity persisted until 1912. But were there more recent events that might have hinted at Sinabung's unrest?
Swarms of earthquakes and tremor, followed by explosions--each progressively larger--were detected near the summit on August 29 and 31 and on September 3 and 7. The earthquakes were monitored, warnings issued, and citizens evacuated. The September eruptions were forecast by Indonesia's Center for Volcanic and Geologic Hazard Mitigation. They appear to have been mostly phreatic, that is, resulting from the interaction of ground water and hot rock.
Some news reports mentioned the glow of incandescence, visible at night, when magma reached the surface or when hot gases heated the explosions' rock debris to 450 degrees C or more. Plumes of ash during the largest explosive event on September 7 rose 5,000 m (16,000 ft or 3 mi) high. By then, as many as 20,000-30,000 people had been evacuated. Other warning signs may have preceded the September explosions; but interpreting them in what was a poorly monitored area is fraught with difficulty.
We can look at the historical record for premonitory periods of other composite volcanoes. Mount St. Helens, which had previously erupted in A.D. 1857, underwent two months of seismic unrest, extensive surface deformation, and a series of phreatic explosions preceding its collapse and plinian eruption on May 18, 1980.
The Philippines' Mount Pinatubo had a lengthy repose period--about 450 years (like Sinabung's). It sustained about two months of unrest, gas emissions, and phreatic explosions (like Mount St. Helens) before its June 1991 eruption, the second largest of the 20th century.
In May 2008, Chiaten volcano in Chile awoke from about 9,600 years of slumber--with only a few days of seismic warning and felt earthquakes--followed by one of the largest eruptions of the past two decades. As with Sinabung, Chiaten lacked a close-in network of seismometers to detect microearthquakes, quakes too small to be detected except by nearby sensitive instruments--and typically the first signs of a volcano's unrest. Volcanologists suspect that a close-in network at Chaiten might have provided weeks or months of warning.
Notably short warning marked the 1989 eruption of Mount Redoubt, Alaska, which proceeded from background seismicity to full eruption in 24 hours. Redoubt's previous eruption occurred only 21 years earlier, ending in 1968. Compared to the centuries of rest that preceded the reawakening of Pinatubo and Sinabung, its repose period was brief.
Volcanologists are aware of the potential for "open-system" (frequently active) volcanoes like Redoubt to progress quickly from unrest to eruption. In contrast, "closed-system" volcanoes (inactive for many decades or centuries, like Mount St. Helens, Pinatubo, and Sinabung) are expected to give substantial warning before eruption. At closed-system volcanoes, magma must establish a new pathway through the upper crust--a process that fractures rock, generates earthquakes, and deforms the volcanic edifice.
Perhaps Sinabung's eruption is a premonitory phase, to be followed in a few months by more substantial eruptive activity. Or maybe the explosions of September 2010 will close this episode without full-blown eruption, reminding us that this volcano, capable of renewed activity after long repose, is worthy of more detailed monitoring.
| Volcano Name | Subregion | Volcano Type | Dominant Rock Type | Pop. 30km | Has Images |
|---|---|---|---|---|---|
| Sinabung | Sumatra | Stratovolcano | Andesite / Basaltic Andesite | 163630 | Yes |
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