Guntur is a complex of multiple overlapping stratovolcanoes 10 km NW of the city of Garut in western Java. The volcano was among the most active in western Java in the 19th Century. It has not erupted since. Its name means “thunder” in Javanese and “torrent or flood” in Sundanese. I always find it interesting that local volcano names are based on what it visits on surrounding communities.
The volcano itself erupts primarily andesites, basalts and variants of both. It is a subduction volcano driven by the subduction of the Australian Plate beneath the Indo-Eurasian Plate. The Guntur cone is the only historically active crater, with explosive eruptions observed since the mid-17th Century. It has produced ash, lahars and lava flows.
The Garut and neighboring Leles Basins are surrounded by multiple volcanoes. The region has been used for geothermal energy, tourism and agriculture. The volcanic hills are actively quarried. The bottom of the Garut Basin, the Garut Plains have been called the Switzerland of Java due to the surrounding mountains.
Garut is the name of the political subdistrict, the largest neighboring town, and the neighboring basin plains in West Java. It is the former capital of Garut Regency. Garut City is a surrounded by at least five volcanoes, Guntur, 10 km NW, Galunggung, 20 km ESE, Talagabodas, 19 km E, Cikurai, 13 km SSW, and Papandayan, 22 km SW. All of these volcanoes have active hydrothermal systems. Galunggung (1982 – 1984) and Papandayan (2002) recently erupted. Guntur last erupted in 1847 and has the occasional seismic event.
The closest habitation to Guntur is Tarogong Town (pop 25,000), 5 km SE of the crater, on the northern edge of Garut City (pop 126,000). The 1847 lava flow reached the outskirts of Tarogong. Tarogong Town is known as a freshwater fish producer for W Java. It also has a strong agricultural sector producing fruit and vegetables grown in local plantations. The population has quickly grown N toward the southern flanks of Guntur over the last decade. 3.4 million live within 30 km. 24.5 million live within 100 km.
There is a hot spring resort SW of the volcano. Guntur and neighboring volcanoes surrounding the plain of Garut are some of the oldest tourist destinations in Java. Garut City itself has at least four hot springs: Cipanas, Kawah Kamojang, Talaga Bodas and Darajat Pass. These include bathing, showers, hot water pools. The most developed of these is Darajat, some 25 km from Garut City with rides for children. It is considered to be the best family destination. Interestingly, some of the descriptions of these springs claim there is no sulfur smell from the volcanically heated waters.
Guntur is monitored by MAGMA Indonesia. There is a local volcano observatory to monitor three volcanoes: Guntur, Papandayan and Galunggung. This in turn feeds into the Gupagal Volcanic Monitoring Regional Center in W Java. Guntur has 5 telemetered seismographs; Papandayan, 1; and Galunggung, 2 stations. There is a telemetered tiltmeter on Guntur, and temperature, GPS and volcanic gas chemical monitoring done on a periodic basis. Over the years, the Observatory published a geologic and volcanic hazard map for Guntur.
For the last 150 years, local residents of Tarogong Town have not experienced eruptions from Guntur. This changed in 2002 when neighboring Papandayan to the S erupted. This was followed by the 2003 Kadungora landslide that killed 20 and injured another 74. Both events convinced local residents and governments to take the local volcanic threat much more seriously.
The Garut Basin is one of several basins in this part of Java. It is located in the Bandung Zone and the Southern Mountain Zone. The Garut basin is bounded by recent volcanic systems. Volcanism in and around the basin appear to have shifted SE over time with the older volcanic hills to the N and NW. These act as a barrier between the Garut Basin and the neighboring Bandung and Leles Basins.
There are two fault lines underneath the Leles and Garut Basins running generally NE – SW. These faults generally control the distribution of volcanoes from Cakrabuana to Papandayan. In the western part of the basin, the fault lines control distribution of old volcanoes which form the border between Bandung and Leles Basins.
Garut and Leles Basins are ellipsoidal, with the long axis generally parallel with the two fault lines. The faults are strike slip in action with some E-W extension. Over time, collapse of a large anticline formed basins among the mountains as old faults reactivated. These fault structures allowed pathways for magma to reach the surface. The region is also an active earthquake zone with a M 6.1 Richter quake 101 km SW of Garut City April 2016.
Gunung Guntur is on the boundary between Garut and Leles Basins. As it was recently active, it becomes an increasing threat to the surrounding community that is experiencing rapid population growth around its S and E slope as a local tourist area. Floods often occur in the Garut basin. There was a large flash flood in Sept 2016 with numerous casualties.
The Guntur complex is a volcanic chain distributed along a NW – SE trending belt. One of the centers of magmatism north of the chain, Kamojang Crater is the first geothermal field developed in Indonesia. It is currently producing electricity, located 7 km WNW of Guntur. Gunung Guntur is the youngest of 4 cones of the complex. The younger portion is on the SW portion along a NW-SE trend. The older, more eroded complex lies to the NW. There are relatively recent lava flows on the lower flanks of Guntur. The volcanic chain is aligned NW – SE and includes Gunung Masigit and Guntur. The most recent eruption in 1847 produced a tholeiitic basaltic lava flow that reached neighboring Tarogong Town, 5 km SE of the crater.
There are forested hills surrounding Guntur that are the remains of a debris avalanche from an undated prehistoric flank collapse of Guntur. Volcanic threats from Guntur include pyroclastic flows, ash falls, effusive lava flows and flank collapses. A low velocity zone has been imaged 6-8 km beneath Guntur crater. Volcanic – Tectonic earthquakes mostly concentrated less than 5 km beneath western flank of Guntur crater. There are relatively complicated tectonic structures beneath Guntur volcano complex.
The volcano is divided into three units: the crater, the body and the foot. The crater is the current (in)active vent. The body is dominated by pyroclastic flow deposits. The foot of the volcano is dominated by lava flows. Volcanic deposits covering the volcano are generally pyroclastics and lava flow deposits. The pyroclastics cover more than half of the volcano and are divided into fall and flow deposits. Lava flow deposits are andesites, basalts and trachyandesites with the trachyandesites being the oldest deposits.
Satellite based Synthetic Aperture Radar (SAR) was used to image the Guntur complex and identify various volcanic products, generally lava flows. There were 9 groups of volcanic rocks that formed the main body of the volcano (khuluk in Indonesian volcanology). There were 15 groups of volcanic products from an eruption center or single parasitic eruption (gumuk in Indonesian volcanology). The khuluk’s can be made of multiple gumuks but never vice versa. The seven most recent eruptive units date 330,000 – 50,000 years ago. There are two older units that are undated. Chemistry over the years has changed a bit, becoming less acidic. Early eruptions with the exception of a dacitie eruption 330,000 years ago were all andesites. The two most recent imaged units were basalts. Lava from the 1840 eruption covers the eastern part of the volcano.
While a relatively young volcano, some parts of original lava flows have been altered by the geothermal system beneath Guntur. The altered rock is visible by SAR. There is no cap rock on top of the hydrothermal system, which has allowed fluids to leak to the surface. This means that geothermal potential for Guntur is quite low like Mount Spurr in Alaska, it is not pressurized. An alternative to geothermal exploration would be tourism as there are multiple hot springs at the foot of the volcano. There are fumaroles around the active crater and hot springs below the volcanic complex.
Historic activity since 1690 includes lava flows from the active Guntur crater and ash falls up to 240 km from the crater, mainly to the NW and W of the crater. Observed eruptions produced felt earthquakes, ash eruptions, gas emissions and pyroclastic flows. Eruption sounds were very loud. Pyroclastic flows reached both Tarogong 5 km SE and Garut City 9 km SE. There is geological evidence of flank collapses / debris avalanches. Eruptions took place 1 – 38 years apart 1690 – 1847. The volcano has been quiet since then. For a time, it was the third most active volcano in Java after Merapi and Semeru.
There were possible eruptions in 1885 and 1887. In the 19th Century, there were at least 24 eruptions, most of them in the first half of the century. Most of these were VEI 2 with a single VEI3 in 1843. Three additional recorded eruptions took place in 1780, VEI 2, 1777, VEI 2, and 1690, VEI 3.
The most recent activity from Guntur were several episodes of volcanic earthquakes, the most recent of which took place late Aug – early Sept 2013. Seismicity was a number of deep and shallow volcanic earthquakes starting mid-month in Aug. Continuous tremor was recorded Aug 30. Alert level was raised to 2 and residents and visitors were warned away from a 2 km radius from the active crater. The episode settled back to normal levels of seismicity within a few days.
Another episode took place Dec 2002 – May 2003. This one also included deep and shallow earthquakes. There were also tectonic earthquakes and tremor was recorded. There was a white ash plume above two of the craters that reached 3 km. Temperature in the Guntur crater increased a bit and 11 cm of inflation was measured in the crater. Activity subsided by mid-May 2003. Another earthquake swarm took place in May 1999. There was an extended period of unrest Oct 1997 – Apr 1999.
Volcano-tectonic earthquakes 1991 – 2005 around Guntur were analyzed. During the period, nearly 5,900 events were recorded, 32 / month on average. Monthly average before May 1997 was 21. After May 1997, they had increased to 42. Monthly numbers were greater than 100 in Oct 1997, May 1999, Nov 2002 and June 2005. There were two felt earthquakes May 1999. Both were followed by 60 aftershocks. The May 1997 increase in earthquakes was accompanied by upward tilt indicating minor inflation. This repeated itself in 1999, 2000 and 2002. This is interpreted as magma stopped rising some 2 km below the summit area of the Gandapura crater of the Guntur volcanic complex (Masigit to Guntur peaks). Earthquakes in the Kamojang geothermal area W of the summit were located 2 – 8 km below the summit.
Tectonics of western Java are driven by the subduction of the Indo-Australian Plate beneath the Eurasian (Sunda) Plate. As such, there is a significant amount of melt available for the volcanic arc on Java proper. Subduction takes place into the Java Trench offshore to the south. The Benioff Zone is around 120 km beneath Guntur. The collision velocity is quite high at 6-7 cm/yr.
A tectonically active region has multiple subduction earthquakes. Indonesia has produced 18 earthquakes larger than M 8.0 in the last 200 years, four since 2000. An earthquake larger than M 7.5 takes place every 2-3 years. An earthquake causing loss of life takes place every 1-2 years. The worst recorded earthquake, a M 9.3 – 9.1 took place in 2004 off the northern end of Sumatra. The 2006 M 6.3 Yogayakarta earthquake was relatively shallow and the second most damaging earthquake in Indonesian history, because it impacted densely populated cities killing over 2,500. A 2009 M 7.0 in-slab event killed 70+ and severely damaged property in western Java.
While Java has escaped major earthquakes in recent years, this may be more an artifact of relatively short period of written history rather than actual lack of earthquake hazards to the island. The subduction continues unabated making future megathrust earthquakes inevitable.
Gunung Guntur is an example of one of hundreds of currently calm volcanic structures in Java. Scientists have imaged magma movements via earthquake swarms, identified low velocity zones beneath the volcano, and observed the active hydrothermal system associated with the volcano and surrounding volcanoes. Sadly, more people seem to choose to live closer to the volcano every year increasing the possibility of major impact to neighboring towns and cities when the volcano becomes active once again.