Mount Awu from a local village. May be Tahuna. Image courtesy Volcanomapping blog
Gunung Awu is one of the most recently dangerous volcanoes in Indonesia. It is located on the northern end of Sangihe Island (Sangir Besar), the largest island of the Sangihe Islands stretching north of the NE tip of Sulawesi, Indonesia. The island is midway between Sulawesi to the south and Mindanao Island of the Philippines to the north.
There are over 100 Sangihe (Kepulauan Sangihe, Sangi or Sangir) Islands. They are all volcanic and have a total area over 1,050 km2. They stretch northward from Sulawesi 260 km and define the eastern limit of the Celebes Sea. The four main islands from north to south are Sangihe, Siau, Tahulandang and Biaro. Sangihe Island is the largest of the islands.
Regional map of Sangihe Island and surrounding islands. Sulawesi to the bottom left. Borneo to the far left. Mindanao center top. Screen capture from Google Maps
Sangihe Island itself is composed of at least four volcanic centers with the newer activity moving to the NW portion of the island over time. The newest volcanic center is the active Awu volcano which dominates the northern third of the island. Its immediate neighbor to the south is the Tahuna caldera. The extinct Tamako volcano is in the center of the island and the deeply eroded Taware volcanic center to the south. Mining geologic surveys in the southern half of the island may have identified a fifth volcanic center to the east of Tamako. The majority of the rocks in the southern half of the island are volcanic deposits that have been altered by vigorous hydrothermal activity following the end of volcanic activity. The southern half of the island is a mining district with exploratory drilling under way.
Tahuna is the largest town on the island. It is also a regional capital, 15 km to the south of the volcano, has a port and the island’s only airport. It is also situated in the amphitheater of the inactive Tahuna caldera / flank collapse volcanic system. Tahuna came under Dutch control in 1677.
Topographic relief map of Sangin Island. All four volcanic centers visible. From north to south, Awu, Tahuna caldera, Tamako volcano dominating the lower half of the island, and the deeply eroded Taware volcano. This map comes from The East Asia Minerals Corporation. Its property is bounded by the yellow line. Gold mining prospects are highlighted in red. Image courtesy East Asia Minerals Corporation Independent Technical Report, Sept. 2010
The indigenous Sangir people populate portions of northern Sulawesi, the Sangihe Islands, neighboring Taluad Islands and north into Mindanao in the Philippines. They number around 205,000 with an estimated 120,000 located on Sangihe Island itself. Most of them are practicing Christians.
Local crops include Manilla hemp, vanilla, nutmeg and cloves. Weaving hemp and spice cultivation are major industries. Fishing and shipping are also important. Tourism, primarily scuba diving is a very popular, growing industry.
Sangihe Island has a tropical climate with temperatures generally ranging 21 – 31 C. It is quite wet, with average rainfall in excess of 450 cm. There is a dry season July – Sept.
The island was originally covered by a tropical rain forest. That is mostly gone, being replaced by a variety of plantations specializing in coconut, clove and nutmeg. The island itself is home to at least two species of endangered Tarsiers and the Sangihe Islands are home to 10 species of globally threatened birds. There is an extensive power grid and mobile telephone network on the island.
Hot springs releasing CO2 and sulfuric bubbles from the top of the active Banua Wuhu volcano 45 km south of Sangihe Island. Image courtesy Top Indonesia Holidays blog, Dec. 2017
The closest neighboring volcanoes to Sangihe Island are to the south and west. Banua Wuhu is a submarine volcano 45 km south of Awu. It is part of the neighboring Mahengetamg volcano edifice. It rises 400 m from the sea floor within 5 m of the surface. Since 1835, it has built the multiple temporary islands which were eroded away back into the ocean. This has become a favorite spot for divers, who enjoy its vigorous underwater hydrothermal system with holes gushing 30 C hot water. There is a suspected undersea volcano some 130 km to the west in the Celebes Sea.
Dome built during 2004 eruption sequence. Note the remains of the crater lake to the left. Dome is still steaming and emissions from active hydrothermal system can be seen to the right of the photo. Image courtesy GuidePlanet.com
Volcano
The 1,318 m Awu is constructed in a pre-existing 4.5 km diameter caldera. It is heavily eroded, cut by deep valleys that form passageways for lahars and pyroclastic flows. The volcano itself erupts andesites, basalts, basaltic andesites and pico-basalts. It is a subduction zone volcano with ocean crust 15 km below the volcano.
1995 photo of Awu showing active crater lake and no dome. Image courtesy Kristianto, 1995, Centre of Volcanology & Geological Hazard Mitigation, Volcanological Survey of Indonesia via Smithsonian GVP
The volcano regularly builds and destroys domes. The most recent of these were 2004 and 1931. Domes are typically destroyed by subsequent eruptions and then rebuilt sometime later. Typical repose time between major eruptions is in the vicinity of 25 years.
The volcano held a crater lake that like the domes, also comes and goes, changes temperatures and pH in the leadup to eruptions. It also has an active and vigorous hydrothermal system. There is at least one publication that described it similar to Kelud in activity and eruption style.
The following video was taken during the 2004 eruption sequence.
Eruptions
Powerful eruptions in 1711, 1812, 1856, 1892 and 1966 produced pyroclastic flows and lahars that cause over 8,000 fatalities. There were at least 19 eruptions, mostly explosive since 1640. Eruption intensity of these range from VEI 2 – 4 with a lot of VEI 3s. VEI 4 eruptions took place in 1966 and 1812. Most of the 19th Century and earlier eruptions are classified as VEI 2 – 3. My guess is that a lot of the smaller eruptions and phreatic blasts were unreported.
The Dec 11 – 16, 1711 eruption was described as turning the entire volcano into a field a fire with enormous explosions. These were followed by streams of burning water (hot mud flows) which may have been lahars or pyroclastic flows. The crater lake was reported to have been thrown out creating the mudflows. This was followed by what was described as ash and stone eruptions. At least 2,030 were killed by the eruption, over 400 of them by pyroclastic flows. Villages at the foot of the mountain were destroyed.
June 10, 2004 eruption from Awu. This was the largest of the 2004 sequence. Image taken early morning at the beginning of the eruption. Image courtesy Wittiri, VSI, Directorate of Volcanology and Geological Hazard Mitigation via Smithsonian GVP
The 1856 VEI 3 eruption began in March with loud explosions. It produced what is described as lava flows that reached the sea, stones and ashes (all likely pyroclastic flows). The eruption killed 2,806 people.
The 1892 VEI 3 eruption was reported in Nature Magazine as the entire island being destroyed by a volcanic eruption, killing all 12,000 inhabitants. The report was filed by a passing ship via telegraph. The captain said the ship passed through miles of volcanic debris near Sangir Island.
The August 1966 VEI 4 eruption produced pyroclastic flows that killed 39 and forced evacuation of 11,000 within the danger zone near the mountain. This eruption ejected a crater lake that had been present since at least 1922.
Pyroclastic flow down the western flank of Awu above the city and harbor of Tahuna. Eruption took place Aug. 12, 1966 (date of the photo). Average travel of pyroclastic flows was 5 km from the crater. This eruption killed 39 and injured over 1,000. Image courtesy J. Matahelumual, Volcanological Survey of Indonesia via Smithsonian GVP
There was an earthquake and tsunami in August 1968 that killed many people on an island in the Northern Celebes. At least 10 locally felt earthquakes around this time caused avalanches from the inner crater wall of Awu into the lake. Smithsonian GVP describes this as an uncertain eruption with a VEI 2 magnitude.
A VEI 1 eruption took place in 1992. There were no casualties to this eruption. pH in the lake dropped precipitously in February and 80% of its volume disappeared. A new crater in the SE part of the crater was discovered, surrounded by ejecta. It is thought to be caused by a phreatic eruption, which in turn ejected the lake water from the crater. There were active solfataras along the S and E walls of the crater.
Newly emplaced dome from the 2004 eruption sequence. Image courtesy A. Solihin, VSI Directorate of Volcanology and Geological Hazard Mitigation via OSU VocanoWorld
The buildup to the 2004 eruption began in the last half of 2000 with increased seismicity, deep volcanic earthquakes. Deformation was also measured. Tremor was recorded in October 2002. The tremor decreased and volcanic earthquakes started to be recorded and continued to be recoded through March 2003.
The 2004 eruption began showing signs in May with earthquakes, gas plumes, and recorded tremor. Dome building began with the first photos of an emerging dome taken June 2. The first explosion took place June 6 with volcanic earthquakes continuing. The second blast on June 7 put a plume 1 km above the summit. Ash clouds continued as did tremor and volcanic earthquakes. June 7 – 8 saw almost continuous explosions, five of them major. Ash started dusting Tahuna, closing the airport.
Newly emplaced dome photographed June 2, 2004. Major eruption from this string still 8 days away. Image courtesy Penduduk via Smithsonian GVP
The sustained eruption of June 10 was the largest, 34 minutes long. It put a plume at least 3 km into the air and dusted the ocean N and NE of the crater. There were several smaller eruptions and aftershocks following the blast. The activity forced evacuation of 18,000 of 28,000 residents within the danger zone south to temporary shelters in Tahuna, 15 km away. This eruptive sequence built a dome. Subsequent steam and ash plumes were observed into 2005.
The most recent unrest took place in Nov. 2015, when shallow volcanic earthquakes were detected. The volcano was raised to a Level 2 Alert and a 3 km exclusion zone around the volcano. There were also volcanic earthquakes May – Nov. 2016. The Darwin VAAC reported an ash plume from Awu August 4, 2019, though this may be misreported as activity from Dukono or Sangeang Api.
Map of Molucca Sea region. B) Detailed map of Sangihe Arc. Triangles are volcanoes. Open triangles are active volcanoes. C) Cross section of collision zone. Image courtesy Jaffe, et al, 2004
Tectonics
The Molucca Sea region is located to the east of the Sangihe Island volcanic arc. The underlying Molucca Sea Plate is being subducted beneath Eurasian Plate to the west and the Philippine Sea Plate to the east. This collision is unusual in that it is the only place on earth that has two converging volcanic arcs. Seismic data tracks plates down as much as 600 km below the Sangihe Arc and 250 beneath the younger Halmahera Arc. Dubduction angles are 40 – 45 degrees. Convergence rate on the Sangihe side is 4 cm/yr and 3 km/yr on the Halmahera side.
The Molucca Sea Plate is thought to have been created by the collision of the Philippine Sea Plate with the Australian Plate some 25 Ma. This trapped a fragment of Indian Ocean lithosphere which became attached to the Philippine Sea Plate. Subduction on the western side forming the Sangihe Arc started soon after the collision. Eastward subduction began about 15 Ma ago.
Detailed tectonic map of Molucca Sea area. Triangles are active and quaternary volcanoes. Isopatch lines are the Benioff contours of descending slabs. Image courtesy Clor, et al, 2002
The Sangihe Arc is the oldest subduction zone in the Philippines – Indonesia region, with subduction related rocks 25 Ma in the NE arm of Sulawesi. The southern portion of the arc ends in northern Sulawesi in the Sorong Fault. The northern portion is quite messy with back-arc thrusting, and remnants on Mindanao. Post collision volcanic activity continues in central Mindanao. Active sections of both arcs are several hundred km apart. The collision between the two arcs is propagating southward and will continue until the Halmahera Arc accretes onto the Eurasian margin.
The Sangihe arc is the western arc of the collision. It has over 25 quaternary volcanic centers, eight of them currently active in the southern half of the arc. The active volcanic front is 100 – 110 km above the western dipping Benioff zone. Volcanoes extend 70 km behind the front. Earthquakes due to the subduction are 100 – 180 km deep.
Tectonic setting in the Halmahera region 0.5 Ma ago. Collision between Halmahera and Sangihe volcanic arcs clearly depicted. Philippine Trench and collision zone between two arcs propagating south. Screen capture from Robert Hall, 1987
The active Sangihe Arc is divided into two sections. The first are four main volcanic islands north of Sulawesi (Sangihe Islands) and eight volcanoes in NE Sulawesi. Active volcanoes in the northern section include Awu, Banua Wahu, Karangetang, and Ruang). Active onshore volcanoes include Tongkoko, Mahawu, Lokong-Enoybg and Soputan. There are four other young hydrothermally active volcanoes along the southern portion of the arc. These are Duasudara, Klabat, Manado Tua and Ambang. The active Sangihe Arc is bounded by Awu to the north and Ambang to the south.
There are chemical variations in erupted magmas with northernmost volcanoes (Awu and Karangetang) having higher He and CO2 isotopic ratios. Slab contributions of these are thought to be due to greater volumes of sediment subduction in the northern arc or enhanced slab-derived fluid/melt production as the collision progresses southward.
Crater rim of Awu. Image courtesy Hattarua Guesthouse, Oct. 2015
Conclusions
Gunnung Awu is and should continue to be one of the most dangerous volcanoes in Indonesia. It is part of an active, subduction driven volcanic arc. Its history is one of large, explosive eruptions, dome building, dome destruction by subsequent eruptions, lahars, pyroclastic flows, and ashfall. Given the number of people who live near it and the violence of its eruptions, this volcano should be treated with a great deal of respect. One of the fortunate things about Awu is that its buildup to eruptions appear to be visible and easily identifiable. It is also well monitored.
Looking south from the rim of the Awu crater. Tahuna Bay is the indention in the shoreline. Image courtesy Hattarua Guesthouse, Oct. 2015
Update – 10/19/20 –
Removed extreme diving photo previously attributed to Buana Wuhu but actually of the Silfra Fissure in Iceland. Updated the number of threatened bird species from 2 to 10 in the Sangihe Islands. Thanks to correspondent Michael Hattarua for the correction.
Additional information
https://volcano.si.edu/volcano.cfm?vn=267040
http://volcano.oregonstate.edu/awu
https://reliefweb.int/report/indonesia/major-eruption-indonesias-mount-awu-volcano
https://reliefweb.int/report/indonesia/indonesia-mount-awu-volcano-subsides-after-major-eruption
https://geographic.org/photos/volcanoes/volcano_photos_107.html
https://www.sciencedirect.com/science/article/abs/pii/0377027383901294
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2003GC000660
http://earth.leeds.ac.uk/esc_wg/Oldwork/Indonesia/workshop2013_files/fieldguide.pdf
https://www.blue-bay-divers.de/en/liveaboard/tours/sangihe-islands-liveaboard
http://www.volcanolive.com/awu.html
https://www.nature.com/articles/046287a0
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