Soputan Volcano

Soputan volcano is located in northern Sulawesi, Indonesia, and is the most southerly active volcano in the North Sulawesi-Sangihe Islands magmatic arc. Soputan lies near the southwestern rim of the massive Tondano Caldera, yet unlike the caldera itself and Lokon-Empung and Mahawu on the northwest rim, Soputan erupts basaltic lavas (50-51% silicate) and is considered to have a magmatic system independent of that of Tondano. Since low silicate (basaltic) lavas are usually relatively fluid and associated with primarily effusive eruptions, Soputan is unusual in that its lava is viscous, can form lava domes, and often erupts in a highly explosive manner. The high viscosity of Soputan lava is due to its high crystal content, with phenocryst and mycrolite crystals constituting about 80% of the volume of the lava. Soputan has erupted at least 33 times between 1785 and 2011, with phases of effusive activity, and recently powerful explosive activity associated with up to 13km high ash clouds and pyroclastic flows. A detailed paper on the geology and eruptive history of Soputan has been published by Kushendratno et al. (Bull. Volcanol. 74, p.1581-1609 (2012)) and is the source of much of the information on this page.

Soputan Volcano viewed through vegetation from NE View from NE to Soputan. Aesoput cone in foreground partially shrouded in cloud

Soputan viewed through vegetation from NE

View from NE to Soputan. Aesoput cone partially shrouded in cloud

Regional Geological Setting

Map of Molucca Sea Region with Collision Zone and Volcanic Arcs indicated.

Based on data from Google Maps, MacPherson et al. 2003 (Geol. Soc. London, Special Publ. 219, p.207-220), Siahaan et al. 2005 (Proc. World Geol. Congress, 5pp).

Note: Only selected extinct volcanoes have been included in the map, many others exist in the Arcs.

A detailed analysis of the tectonic and geological setting of Soputan is provided by Siahaan et al. (Proc. World Geotherm. Congr. 2005, Antalya, Turkey). The North Sulawesi-Sangihe Islands magmatic arc (hereafter Sangihe arc) is related to subduction of oceanic crust of the Molucca plate in a westward direction (thus sliding under Northern Sulawesi), whilst the nearby Halmahera arc is associated with eastward subduction of the Molucca plate. The NE-SW oriented northern tip of Sulawesi is referred to as Minahasa compartment and is made of tertiary sediments and volcanic deposits. It includes (from S to N) the active volcanoes Soputan, Lokon-Empung, Mahawu, Klabat and Dua Saudara. Soputan lies on the southwestern rim of the 15x30 km wide SW-NE oriented Tondano caldera, formed by a massive eruption 2 Million years ago and site of further large eruptions 1.3 and 0.1 Million years ago, the first of which probably forming the 5x3.5 km Pangalombian Caldera nested within Tondano, with more recent activity at various volcanic centers within Tondano (e.g. Kasuratan, Tampusu, Lengkoan) covering parts of the calderas.

Houses on stilts, Lake Tondano Houses on stilts, Lake Tondano

Houses on stilts, Lake Tondano

Houses on stilts, Lake Tondano

Houses on stilts, Lake Tondano Lake Tondano with Soputan Volcano on horizon

Restaurants on stilts, Lake Tondano

Lake Tondano with Soputan on horizon

Soputan volcano is located immediately south and adjoining to a number of older volcanic structures. Approaching the viewpoint NE of Soputan, one passes the deep Kawa Masem crater which is part of the Pre-Soputan Temboan volcanic complex. The crater contains an active Solfatara but has otherwise not been active in historical times.

Kawa Masem crater, Temboan volcanic complex near Soputan Solfatara in Kawa Masem crater- Temboan volcano near Soputan

Kawa Masem crater, Temboan volcanic complex

Solfatara in Kawa Masem crater

Why are Soputans Basaltic Lavas So Viscous ?

Soputan lava are basaltic (50-51% silicate), yet unusually viscous for such a low silicate composition. They further contain 19-20 wt % aluminium oxide and about 10% calcium oxide and 10% iron oxides. They have low levels of potassium (0.3 wt % oxide). The viscosity can be explained by the extent of crystallization of the lavas. Large (phenocryst) crystals, in particular plagioclase account for 40-50 % of the lava volume. Taking smaller crystals into account as well, one finds an overall crystallinity of about 80%. It is thought that the Soputan basalt is formed by melting of the arc mantle wedge, after which it accumulates in a deep crustal reservoir. The extensive formation of phenocrysts is proposed to occur during slow ascent of the magma or during intermediate storage in a shallow magma chamber. Microlite (tiny crystals) are additionally formed during near-surface degassing prior to eruption. Near-surface degassing is attributed to the late stage at which the basalt becomes gas-saturated as it rises up the conduit. The near-surface gas release may also explain the explosivity of the eruptions and the relatively large amounts of sulphur dioxide released compared to at other volcanoes. The petrology of Soputan is discussed in great detail in Kushendratno et al. 2012.

Eruptive History

Soputan has erupted at least 33 times since records began in 1785 and 2011. Documented activity has been at the main summit crater and from parasitic tephra cone Aesoput, 1km NE of the summit, which erupted largely effusively during the 1907-1953 period, before activity returned to the summit crater. Several lava domes have been emplaced within the summit crater, for example starting in 1966, 1991. The latter dome grew in several phases until destroyed by powerful eruptions in October 2007.

Aesoput parasitic tephra cone, Soputan volcano Aesoput parasitic tephra cone, Soputan volcano, portrait orientation Aesoput cone with Soputan edifice behind

Aesoput tephra cone

Aesoput with Soputan behind

Aesoput with Soputan behind

The 2006-2008 eruptive period is well documented in Kushendratno et al. 2012 and illustrates the explosivity of recent eruptions at Soputan. By 2006, the lava dome was overspilling the crater rim on the southern and eastern side. On 14. December, partial dome collapse caused up to 2.4km long pyroclastic flows directed S and SE, including a SE-directed energetic surge which flattened and burnt vegetation in an area of 125000 square meters. A 12 km high ash cloud resulted from the eruption. The next major event was in October 26, 2007, when large portions of the dome collapsed down the N and W flanks, effectively opening the vent of the volcano and causing a powerful vulcanian eruption with a 13.7 km high ash column associated with localized column-collapse pyroclastic flows. The powerful explosions and tremors triggered failure of the W crater wall which formed a debris avalanche extending 2.7 km W of the summit craters with a volume of about 850000 cubic meters. In the next days, some effusive activity took place at the summit with one overflowing the crater and causing a small blocky pyroclastic flow on the N flank.

Following destruction of the 1991-2007 dome, the conduit was no longer blocked by a large mass of lava and on 6. June 2008, a powerful Vulcanian eruption creating a 13.7 km ash column occurred, again accompanied by column-collapse pyroclastic flows, extending as far as 5.5 km to the W of the summit. An estimated volume of between 6 and 17 million cubic meters of flow deposits were emplaced around Soputan. Further explosive eruptions were reported in 2008 and 2011.

Eruptions at Soputan have been seismically monitored since 2007. Data revealed that each of the larger eruptions in 2007-2008 was preceded by an increase in volcano-tectonic tremors, with between 8 and 47.5 hours between onset of the VT activity and the actual eruption. Hence, such eruptions occur with little warning and indeed the beginning of VT periods preceding eruptions may only retrospectively be recognized as such, giving even less warning time.

Soputan volcano viewed through vegetation Soputan volcano degassing at sunset

Soputan viewed through vegetation

Soputan degassing at sunset

Soputan volcano degassing, Sulawesi, Indonesia Soputan volcano rim on flank

Soputan degassing

Rim on NW flank

Soputan volcano summit region in morning light Soputan volcano dome degassing

Dome of Soputan in morning light

Degassing dome of Soputan

Soputan volcano, summit lava dome Soputan volcano, summit lava dome

Soputan lava dome

Soputan lava dome

Soputan volcano, summit lava dome degassing Soputan volcano, weak incandescence on lava dome, June 2014

Soputan lava dome

Weak incandescence on dome (July 2014)

Impact of Eruptions

The main risk posed by eruptions at Soputan is to park visitors, quarry workers on the W/SW flanks or to farmers on the S and W slopes. Fortunately, the seismic activity preceeding most eruptions by a couple of days should provide sufficient warning for the immediate vicinity of the volcano to be evacuated. Further, due to the explosivity of several recent eruptions at Soputan, the volcano was used for modelling volcanic ash dispersion to assess its potential impact on aviation (Tupper et al. 4th Intl. Workshop on Volcanoic Ash, 2007). It was suggested that due to the moist tropical environment, fine ash particles rapidly aggregated, resulting in lower proportions of ash in the umbrella cloud than compared to eruptions in a drier atmosphere. Nevertheless, ash levels may be sufficient to endanger aircraft in the area and the ash cloud tends to grow to a higher altitude and contain more ice in a tropical environment compared to with an equivalent magnitude eruption in a dry environment.

Visitor Information

Soputan volcano lies within a small National Park in northern Sulawesi. The area is best reached by road and is an about 3 hour drive from Manado, which has an International Airport. Visits to Soputan can easily be combined with a visit to Lokon-Empung volcano, which essentially lies en route, about 1 hours drive south of Manado. The main volcanic observatory post monitoring both volcanoes is located at Kakaskasen, between Lokon and Mahawu volcanoes, and it is here that data from several seismic stations at Soputan is collected.

The main trail to Soputan approaches from the NE. The trail is popular with locals and two camps are found on the way, Pine 1 and Pine 2. In total, it takes about 3 hours of gentle ascent to reach the viewpoint overlooking Soputan via the two camps. For about half an hour, between camps 1 and 2, the trail leads through the bed of Soputan river (not a dry river bed !), so water-resistant footwear is required for those not used to walking barefoot.

Sulfurous water entering Soputan river Soputan River Trail Orange coloured vegetation in Soputan River

Sulfurous water entering Soputan river

Upstream exit of trail from Soputan river

Ferrous compounds colour further downstream

Camp Pine 1, Soputan volcano trail Camp Pine 2, Soputan volcano trail

Camp "Pine" 1

Camp "Pine" 2

Soputan volcano, NE panoramic viewpoint Soputan volcano, NE panoramic viewpoint

Camping at viewpoint NE of Soputan

Camping at viewpoint NE of Soputan

Village nearSoputan volcano Geothermal well near Soputan volcanic complex and lake Tondano

Approaching volcanic complex including Soputan

Geothermal well near Soputan complex

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