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Iliwerung Volcano, Lembata Island Indonesia

2 years ago 2914

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Iliwerung volcano from the east. Image courtesy VolcanoYT

The Iliwerung volcanic complex is located on the southern rim of the ancestral Ili Adawajo volcano (Lerek Caldera), forming a prominent peninsula on the southern shore of Lembata (Lomblen) Island in eastern Indonesia. The complex system has craters, lava domes, at least one flank collapse scar, a debris avalanche scar, and an active underwater vent about a kilometer to its south offshore.

Lembata Island is the largest island of the Solor Archipelago, Lesser Sunda Islands, Indonesia. It is a separate regency in the Nusa Tenggara Timur province. The volcanic island is 80 km long, 30 km wide and tops out just over 1,500 m. To the east is the Alor Strait; to the south is the Savu Sea and Timor; to the N is the western branch of the Banda Sea and other islands of SE Sulawesi.

Lembata Island with Iliwerung volcanic complex at red mark. N is up. To the S is Timor. To the W is Flores Island and the remainder of the Lesser Sunda Islands stretch E – W on either side of Lembata. Savu Sea to the S and Sumba Island to the SW. Java is off the photo to the west. Image screen capture from Google Maps

There are three active volcanoes on the island: Ililabaklekan, Iliwerung and Lewotolo.

The S coast of Lembata has one of the three remaining Indonesian whaling communities. This one is commercial, approved by the International Whaling Commission and hunts sperm whales. The natives use primitive equipment in their whaling. The island is surrounded by coral reefs with at least two rare coral reef fish species.

Connection to the outside world is generally via boat with daily connections to at least one neighboring island. There is also air service to and from the island. Total population of the island is 127,600.

Climate is tropical with two seasons – wet and dry. The area surrounding the volcano is sparsely populated, with only 4,300 within 10 km of the volcano. Population does increase farther away with nearly 80,000 within 30 km and 665,000 within 100 km of the system.

Antonio Wutun produced a rather nice 24-page pdf entitled the Lembata Regency at Glance in 2011 that is available for download. It starts out with an overall description of the Regency and ends in a 14-page list of Places of Interest describing the various towns and villages, how to get there, where to stay, where to eat, and important addresses.

Satellite view of Iliwerung. 1870 dome and crater at the center of green named mark. N is up. Dome clearly visible at water’s edge to the E. Dome on water’s edge to the S is misnamed, as Hobal is another 800 m S into the water. Two scarps are clearly visible to the N and W of Iliwerung. Image screen capture from Google Maps satellite view.

Volcano

Like many things in Indonesia, Iliwerung is a rather complex system. It is basically a subduction driven volcano erupting mainly basalts, andesites and combinations of both. The current active system consists of an active dome centered in a SE facing amphitheater. It has multiple domes and small cones on its flanks, active fumaroles, active solfataras, and at least one active vent, Hobal, 800 m offshore to the south. Recent activity appears to be slowly migrating south. The entire system has an active hydrothermal system underlying it. This hydrothermal system has contributed to at least one major landslide on the NE flanks of Iliwerung in 1979.

Today, the highest active point of the system is Iliwerung, which is a new dome centered in at least one south-facing amphitheater around 1 km in diameter. There is another ridge within a kilometer to the NW that may be the remains of an even older caldera. The remains of the inner amphitheater are named Lerek and has shown up in at least one source as the Lerek Caldera. There at least two recent domes on the coast, one directly east of the main volcano and one directly to the south. There are multiple smaller ridges which appear to be smaller domes in line between the southern coastal dome and the main crater. This volcanic system has not been well studied in what is a very busy volcanic region.

Local names are also complex. For instance, the most used name of the system is Iliwerung. It is also printed as Ili Werung. The formal name of the mountain appears to be Gunung Ili Werung. Iliwerung means “New Mountain” in the local language.

Ile Adawajo appears to be the name of the ancestral, precursor volcano that suffered the flank collapse to the S and E. The collapse is undated but precedes construction of the current iliwerung system. I have found no information as yet describing this event as either a hot or cold collapse, though as active as the system currently is, a hot collapse is certainly possible as is assistance in weakening the structure of the ancestral cone by the active hydrothermal system. The first lava dome extruded following the flank collapse, caldera creation was destroyed in a violent eruption. It was replaced by the dome extruded by the 1870 eruption that is the current Iliwerung. There are active solfataras in the crater and around the dome.

There is a N-S fissure at the base of the newly formed dome stretching toward the sea to the S. Smaller domes and cones are found along the fissure. Three largest domes on the SE flanks of the ancestral Ili Adawajo are Ili Penutuh, Ili Lusitobe and Ili Grippe. There are also remnants of a lava dome on the N slope. Three of the flank domes were constructed in 1935. The final cone, Ili Gripe was built during the 1948 eruption.

The volcano is seismically monitored. Inflation / deflation of the volcano is also monitored.

There are at least six named and active fumaroles / sulfataras on the volcano and its parasite cones and domes. Activity seems to be working its way toward the south.

The 1979 Waiteba-Lembata tsunami took place just after midnight on July 21. There were warnings in the area over the previous two years due to land movement on the NE flank of Iliwerung and lack of areas to migrate locals out of harm’s way. The tsunami run up was 50 m, height 7-9 m. Trash was caught by palm trees 7 m above the ground. The entire village of Waiteba was buried in mud by landslides. The landslide mass was 3,000 m x 300 m x 50 m thick. About a third of it went into the sea creating the three tsunamis. Four villages were buried by the landslide material. The entire event killed 539 local residents.

Landslide scarp from the Waiteba – Lembata tsunami-causing landslide on the NE flank of Iliwerung. Landslide was not connected to volcanic activity, though continuous action by the hydrothermal system did weaken the material of the cone. Image courtesy Signo Temporum Indonesian blog

There was a dispute in Indonesian writings that that these tsunamis were connected to an eruption of Hobal, which erupted offshore in 1976 and produced small local tsunamis. No casualties from these, however. Timing of the tsunamis and landslide has yet to be completely resolved, though it appears that the landslide took place first and was unconnected to any local volcanic activity. A 2015 paper by Yudhicara, et al makes the case that the Geothermal System as the Cause of the 1979 Landslide Tsumami.

Iliwerung dome with crater on top. Image taken from the rim of ancestral Ili Adawajo volcano looking S. Image courtesy Signo Temporum Indonesian blog

Eruptions

Iliwerung is an active volcano, with at least 17 eruptions since the first observed dome-building eruption in 1870. The largest was the 1870 eruption at VEI 3 that eventually built the summit dome. Most of the rest have been VEI 2 at the largest. Recent eruptions have been somewhat smaller.

The most recent took place Aug 19-20, 2013 at Mount Hobal, an underwater satellite vent on the south flank of Iliwerung. Hobal is 800 m off the coast. Precursors to the eruption were 181 earthquakes on Aug 19. The eruption began with significant changes in the color of water from blue to yellow, closely followed by intense bubbling and steam plumes. At 0714 L steam plumes were rising 1 – 2 km above the ocean. A half hour later, a volcanic glow was observed. The eruption was not sufficiently vigorous to show up in satellite photos or IR observations, though tiny spots thought to be floating pumice did show up in a few images. VSI recommended a 2 km radius exclusion zone from the volcano. There was also heightened concern about possible tsunami risk from the eruption.

Erik Klemetti also reported on the Aug 2013 eruption in his Rocky Planet blog. His report was similar to the previously linked report with the following additional details. Apparently Mount Hobal does not exist in the Smithsonian GVP database. But it does exist as part of the larger Iliwerung complex. He noted that the complex was producing diffuse fumarolic activity since the first of the month before the eruption.

Previous activity took place Oct 31 – Nov 6, 2001. This was an increase in tectonic earthquakes Oct 17 – 26. VSI raised its alert level from 1 to 2. 2 – 11 earthquakes were felt per day. Monitoring did not show a significant change in volcanic activity.

Most recent activity from Iliwerung is from the Hobal vent. Mount Hobal previously erupted in Dec 1973, Mar 1976, Sept 1993 and May 1999. All these eruptions were relatively minor. The 1973 – 1974 eruptions created three ephemeral islands. The Mar 1976 eruption created a small cone above sea level that lasted two days before it disappeared back beneath the waves. The 1993 eruption produced a water column 100 m high 1 km off the coast over the Hobalt vent.

The Ili Gripe lava dome was extruded on the E flank during the May 1948 eruption. Initial eruptions built a new lava dome. This dome was partly destroyed in an eruption May 7 that produced a pyroclastic flow that reached the east coast, killing 300 head of cattle and destroying farming lands. A subsequent eruption two days later that produced a 4 km plume and opened a new 120 m wide crater. The new dome filled that crater, reaching 50 m high by 11 May.

Tectonics

As we saw with our Paluweh post, tectonics of the Lesser Sunda Islands are driven by the collision of the northward moving Indo-Australian Plate into the Eurasian Plate. North of the main collision, the Pacific Plate is also colliding generally traveling to the west. This complex collision zone created the curved Banda Arc, a set of inner and outer arcs. The Lesser Sunda Islands are located on the Inner Banda Arc. The inner arc is generally back arc volcanic islands separated from the older accretionary outer arc by a spreading forearc basin. The arc is over 1,000 km long. The Indo-Australian Plate is subducting under the Eurasian Plate along this arc.

The Lesser Sunda Islands are bounded by two basins – the South Banda Basin to the NE and the Savu Basin to the S. Sumba Island is located S of the western portion of the string and Timor is located S of Lembata Island. There is speculation that Sumba is a sliver of microplate while Timor is the result of accretionary thrusting between the Savu Basin and the Timor Trough to its south.

Linement extraction of Flores Island to the left center and other Lesser Sunda Islands to the right. N is up. Geothermal prospects are highlighted in yellow. Recent volcanoes are highlighted as red triangles. Image courtesy Purwandono, et al, June 2019

This portion of the Banda Arc had two tectonic phases. The first was an extensional phase 20 – 4 Ma. The S-migrating Sumba microcontinent opened the Savu basin. The incoming Indo-Australian Plate started a collisional phase around 8 Ma. Eastern Indonesia underwent a counterclockwise rotation with increased velocity between Bali and the E end of the Timor Trough. Flores Island is the transition zone from subduction to collision, likely explaining its vigorous volcanic activity.

The crust underneath the volcanic arc thins eastward from Java, where it is 35 km to 20 km beneath Alor Island. The subducted plate is cold Indo-Australian oceanic crusts with ages varying from W – E 80 – 150 Ma. The subducted plate along the Timor Trough are a buoyant Australian continental plate around 40 km thick.

Conclusions

Iliwerung is a vigorous, active volcano. Eruptive style is currently dome building, dome destruction, the occasional pyroclastic flow, and dome building. There is an active and vigorous hydrothermal system supplied with plenty of water from rainfall due to its location in the tropics. The volcano has suffered at least one flank collapse, possibly a hot one and a recent landslide (cold) from its flanks. It is close enough to the ocean that these landslides create deadly tsunamis. Finally, the currently active vent is offshore, increasing the possibility for future tsunamis caused by eruptions from that vent.