Umboi Island is the largest volcanic island between the mainland of Papua New Guinea and New Britain.  The island is separated from New Britain by the Dampier Strait, from New Guinea by the Vitiaz Strait.  The basaltic andesitic cone tops out at 1,335 m.  The island has a 13 x 15 km caldera.  Post-caldera activity built at least three stratovolcanoes in the caldera. 

The island itself is 50 km long, 28 km wide, with a total area of 930 km2.  It is heavily forested with multiple vents on the flanks of the main volcano.  It is surrounded by a zone of shallow water, less than 200 m deep.  Outside this zone, the sea floor drops steeply to around 1,000 m.  The shallow zone is 2.5 km wide off the N and NE coasts, up to 25 km off the S end of the island, and more than 7.5 km along most of the SW coast.  Much of the Umboi shoreline is fringed by reefs.

Umboi is mostly mountainous and forested.  Climate is tropical.  There are extensive flat (and in places swampy) areas in the S.  Patches of kunai (tropical grass) are found in the center of the island and on the NE coast.  There are perhaps 26 villages on the island, mainly in the center and S, but also along the NE coast.  Most of the NW part of the island is uninhabited.  There are three Lutheran Missions and two Roman Catholic Missions serving Umboi and the Siassi Islands.  Roads join a few plantations with village communities.  Total population of the islands is 5,600.  Only 83,000 live within 100 km. 

The Siassi Archipelago is off the SE coast of Umboi.  Only seven of the 18 islands are inhabited.  They are prime scuba diving locations.  The largest of these is Taum, a coral reef raised 76 m above sea level.  Malai is the second largest island, 46 m above sea level.  Numerous submerged coral reefs surround the Siassi Islands, making these islands particularly treacherous for shipping. 

Population of the Siassi Islands over the last century grew from 700 – 1,700.  It is currently around 1,600.  Japanese briefly occupied the island with 500 troops during Dec 1943.  The Siassi support themselves based on a barter system.  They are middlemen who deliver goods and pigs in their canoes by sea. 

Religious beliefs are interesting in this part of the world.  The Siassi believe in witchcraft and consider it to be the root cause of unfortunate events such as illness and crop failures.  The accused often meet unpleasant ends. 

One of the oddest beliefs on Umboi is that of the Ropen, a large, nocturnal bioluminescent winged creature, reportedly flying in the skies of these and neighboring islands.  The creature is more similar to a pterosaur than a large bat, prompting investigations by cryptozoologists and others over the years.  Five of these took place 1999 – 2004, with two sightings claimed.  Note that these islands are home to the Umboi tube-nosed fruit bat, which is mildly endangered.  The legend spawned a book by JJ Dupuis, Umboi Island:  A Creature X Mystery (A Creature X Mystery, 3), Apr 2022. 

Region

The most interesting historical event in the vicinity of Umboi and Sakar Island was the flank collapse of Ritter Island in 1888.  The collapse created a submarine debris avalanche downslope between Umboi and Sakar Islands.  It also triggered a tsunamis chain that had runups to 20 m on neighboring Umboi and Sakar Islands.  The tsunamis caused damage as far as 600 km away, killing 500 – 3,000 on neighboring islands and New Guinea itself.  Granyia posted on the Ritter Island eruption in 2015.  It was mentioned in a few comments on our previous home in 2012. 

PNG is a busy volcanic province.  There are a pair of posts of volcanoes on New Guinea proper, though they are a long way from Umboi.  Over the years, we published multiple posts on volcanoes in and around New Britain including Langila, 2019, Dakataua, 2023, Pago and the Witori Caldera, 2022, Uluwun and Bamus, 2017, Lolobau, 2021, and Rabaul, 2019.  Any of these would be great places to start looking at regional volcanic activity. 

Umboi is one of a line of volcanic islands offshore, NE of New Guinea, and W of New Britain.  We will limit the review of regional volcanoes to these.  All distances will be given from the location of Umboi Island published by the Smithsonian GVP. 

Langila

Active Langila is located on W New Britian, 60 km E from Umboi.  We discussed Langila in a 2019 post.  Neighboring Tangi is located 11 km S of Langila, 57 km E of Umboi.  It is a highly eroded 1,524 m stratovolcano topped with a 2 km diameter crater breached to the E.  There is a single flank cone on the SW flank.  The entire edifice is heavily forested.  Both Langila and Tangi make up the W end of New Britian, the Cape Gloucester Peninsula. 

Unnamed

This unnamed seamount is located some 30 km NNE from Cape Gloucester on the NW tip of New Britian, 98 km NE from Umboi.  It is likely an active submarine volcano and the source of an unconfirmed submarine eruption report during 1983.  There was an earthquake swarm during the event.  Local inhabitants (either on New Britain or Unea Island) heard sounds and saw a glow from the sea.  There is no other evidence of a submarine eruption from this location. 

Ritter Island

Ritter Island is 26 km ENE from Umboi.  We discussed Ritter in 2015.  It was previously a steep-sided, nearly circular island 780 m high between Umboi and Sakar Islands.  Several earlier historic eruptions were reported prior to a flank collapse and eruption in 1888 that destroyed the island and volcano, leaving a crescent-shaped remnant.  The collapse produced devastating tsunamis that swept the coastline of PNG and its offshore islands to the N.  Two minor explosive submarine eruptions offshore took place in 1972 and 1974.  Both of these were within the depression offshore formed by the collapse. 

Sakar

Sakar is located 29 km NE from Umboi.  It is a heavily eroded 992 m stratovolcano with a summit crater lake.  Erosion cut deep valleys on the flanks of the volcano.  The island itself is partly surrounded by coral reefs.  The 8 x 10 km island is constructed by two volcanoes.  The older one that forms much of the island is mainly basaltic.  The younger andesitic cone was constructed within the older crater, whose rims are exposed on the N and E sides.  There are no historic eruptions known from Sakar.  The island has an active hydrothermal system with warm springs found along the SW coast.  There is a pyroclastic cone on the S flank of the island that may have been active within the last 11 ka.  There is a large submarine debris avalanche deposit N of Sakar and another pair W and NW of the island.  Sakar and Umboi are usually lumped together in discussions of flank collapse tsunamis from Ritter Island in 1888. 

Smithsonian GVP carries a Sept 2009 Weekly Report that has the Darwin VAAC reporting a possible diffuse 2 km plume from Sakar on 18 Sept.  The plume drifted 165 km NW and was gone within four hours.  The plume may have been from a fire or hydrothermal activity.  Darwin VAAC later confirmed that an eruption did not occur. 

Tolokiwa

Tolokiwa is located 46 km NW from Umboi.  It is an 8 km diameter circular island with a summit crater breached to the NE.  That crater is largely filled by a more recent cone.  The volcano was built sometime in the last 11 ka.  The largest documented submarine debris avalanche deposit mapped in the Bismark Arc is N of Tolokiwa.    

Long Island

The 1,280 m Long Island is located 92 km WNW from Umboi.  VOGRIPA carries three massive eruptions in its database.  The largest of these was 19 ka, a VEI 7.4 ejecting 100 km3 of the Kiau ignimbrite.  A VEI 6.3 4.4 ka ejected the 10 km3 creating the Biliau beds along with a caldera collapse.  The most recent eruption here was in 1661, a VEI 6.5 that ejected 30 km3, the Tibito tephra.  The most recent of these eruptions deposited andesitic tephras across New Guinea highlands, prompting local legends of a “Time of Darkness.”

The island is dominated by two steep-sided stratovolcanoes on either side of the caldera, Mount Reaumur to the N and Cersis Peak to the S.  Collapse of the volcanic complex created a 10 x 12.5 km caldera now filled by Lake Wisdom.  Post caldera eruptions constructed a small cone, Motmot Island in the S central part of the lake.  Moderate explosive eruptions took place in the 20^th Century from vents at and near Motmot Island.

Smithsonian GVP carries 20 years of Bulletin Reports 1973 – 1993.  The first of these May 1973 reports a small eruption from Motmot Island within the crater lake 18 Apr.  It ejected ash and steam and produced a small lava flow.  Considerable new fumarole activity at Motmot was reported Jan 1979.  There was no new ejecta observed.  Follow-up airborne inspection in Feb confirmed increased vapor emissions only.  Another airborne inspection Sept 1990 observed vegetation spreading in some areas in the crater of the 1974 cone.

The most recent eruption took place Nov 1993.  First indication of new activity was a change of color of Wisdom Lake.  Source of the color change did not appear to be Motmot.  Permanent seismograph station at Karkar Island, 160 km NW recorded unusually strong, continuous tremor from Long Island starting 3 Nov.  The eruption ejected steam and ash from the eruption location under the lake. 

Direct visual observation of water and ash ejection from the lake were made on 5 Nov.  Ground-based inspections 11 Nov heard muffled thudding noises and felt earthquakes.  A seismograph set up at the time recorded volcanic tremor.  Most of the activity took place 11 – 16 Nov and started tailing off by 17 Nov.  The eruption that began under the caldera lake, N of Motmot Island ended by Dec.  Plumes of discolored water continued after ash ejection stopped.   

Crown Island

Crown Island is located some 116 km WNW from Umboi.  It is a deeply eroded, 3 km wide stratovolcano covered by rain forest.  There is no known historic eruptions or current hydrothermal activity.  There are young tephras on the island, likely from neighboring Long Island.  Possible submarine debris avalanche deposits are N and S of the island. 

Umboi

Umboi Island is dominated by several coalescing stratovolcanoes topped by a 13 x 17 km caldera.  Caldera walls rise up to 1,000 m above the caldera floor.  The caldera is breached to the sea on the NE side.  There are at least three post-caldera cones with summit crater lakes, Talo, Soal and Barik.  Talo is the largest, has an active hydrothermal system, and satellite cones on its flanks.  There are no historic eruptions.  Post-caldera activity is thought to have continued until the last few hundred years.  Umboi erupted basalts and andesites.  There are no historic or traditional reports of eruptions on Umboi. 

Umboi is constructed in two stages.  Pre-caldera activity left remnants of several coalesced stratovolcanoes.  Caldera formation created a caldera with straight-sided rims.  Its NE rim may be below sea level.  Post-caldera activity grew three younger stratovolcanoes, Talo, Soal and Barik, which coalesced within the caldera. 

Most of the geologic investigation of Umboi is quite old, first documented in a 1972 Australian Bureau of Mineral Resources, Geology and Geophysics report.  Three visits took place in 1952, 1953 and 1970.  The first of these investigated effects of earthquakes Oct – Nov 1952.  The second investigated rumors of volcanic activity and visited coastal exposures and satellite cones near Kabib and Kampalap villages on the NE coast.  In the same year, thermal areas were visited.  The final documented visit in 1970 traversed several areas and examined parts of the W Coast and post-caldera volcanoes. 

Older pre-caldera rocks cover most of the surface of Umboi,.  These are eroded and covered by paleosols.  They may be older than those of any other recently active volcanic island off the N coast of New Guinea.

The slopes of the older part of Umboi suggest subsidence at the summit of the island, which may have been up to 1,900 m high.  The summit ridge is constructed from closely spaced separate stratovolcanoes.  Their summits were lost in subsidence of the regional summit.  There are no remnants of these volcanoes in the NW part of the island.  Two of these are visible, though deeply eroded, on the SE side of the island, marking the remains of a major eruptive center.  Both of these are close to the SE rim of the caldera.  Note that the 1952 – 1970 descriptions of pre-caldera activity specifically attribute the change in topography to subsidence rather than an earlier caldera formation cycle. 

Topography differs on the older part of Umboi at the coastal areas.  In the NE, there are few flat areas at the coast.  In the SE, low, and in places swampy ground forms a coastal plain several kilometers wide.  This plain extends SE of Umboi forming the submarine shelf under the Siassi Islands and surrounding coral reefs.

Umboi Caldera

The rim of the undated caldera is defined by three generally straight scarps on its NW, SW and SE sides.  These are respectively 13, 14 and 17 km long.  A fourth scarp is thought to be preserved below sea level to the NE.  The floor of the caldera is estimated at 200 km2, almost twice the area of five other calderas in the Bismark Volcanic Arc, Karkar Island, Long Island, Dakataua, Lake Hargy and Rabaul.  The smaller calderas are more circular with no scarps below sea level.

The highest part of the caldera rim is in the SE, almost 1,000 above the floor.  In the SW, it is only 100 m.  In most places the caldera wall is less steep than the previously named calderas.  This is thought to be due to erosion, due to the age of the Umboi caldera.  Erosion also opened gaps in the caldera wall.

Some unknown amount of subsidence of the island or part of the island allowed the sea to flood the caldera.  The entire island may have subsided, though it is unclear if the subsidence lowered the entire island or tilted it a bit.  Tilting would explain the uplifting of shelf under the Siassi Islands, coastal plains on the S and the lack of a coastal plain on the N of the island.  

Post – Caldera Activity

The three post-caldera volcanoes have numerous satellite cones on the flanks of these.  One of these is separated from the stratovolcanoes in the relatively flat S corner of the caldera.  None of the stratovolcanoes are eroded and their activity is thought to continue until the last few hundred years.  Talo is considered active based on several hydrothermal areas on its flanks.  Post caldera lava flows are stacked against the caldera rim in places but did not overtop the rim.  There is a crater lake with no known outlet whose level fluctuates.  Most, if not all of the post-caldera vents have lakes in their craters. 

Talo (Tolo) is the highest and largest of the post-caldera stratovolcanoes at 1,494 m.  It measures 12 x 10 km at its base.  Its lava flows are confined to the caldera.  The summit has two well-preserved craters, 550 and 900m in diameter.  Both are 50 – 70 m deep with small crater lakes.  The craters are heavily vegetated.  Rocks at the summit are hydrothermally altered.  Talo has several satellite cones.  The two largest are located some 3 km from the summit.  Neither of these have a summit crater. 

Soal is a well-preserved cone on the NW coast of Umboi.  It measures 8 x 6.5 km at its base and is about 1,000 m high.  It is topped with a well preserved 120 m deep summit crater with a lake.  It is likewise vegetated.  There are three satellite cones on the SW flank, one on the NE flank.  Barik is NW of Soal.  It measures 5 km wide, is 825 m high and has no summit crater.  It is asymmetric and may be the youngest volcano on Umboi.  There are several lava flows exposed in the stream valley the cuts the joint flanks of Soal and Barik.  Source of these flows is unknown. 

There are two prominent satellite cones on the NE coast outside the caldera.  The first has a 450 m crater with a lake.  There are bedded deposits on the coast exposed in 12 m cliffs.  Several explosions built the cone.  The second satellite center is a grass-covered dome-shaped hill.  Native legends suggest that at least one of these were in eruption during the last couple hundred years before neighboring Ritter built its cone above sea level.  There are two final areas.  One of these may be an eroded satellite vent.  The other is close to the S corner of the caldera, along a line joining the two pre-caldera crater areas.

Flank Collapse

Flank collapse and debris flows are common in volcanic islands in this part of PNG, with Umboi, Sakar and Ritter all showing evidence of past flank collapse events.  Sakar has a pair of overlapping landslide deposits W – NW of the island.  They are similar sized at 15 – 26 km3, originate from roughly the same location, and are separated vertically by sediment.  The lower landslide was a chaotic landslide with a well-defined front, that deformed the seafloor as it flowed across it.  The younger deposit was simpler, flowing across the seafloor, though still with blocks.  The older landslide was smaller in total volume and had a longer runout. 

The location of the younger of these two avalanche deposits make it possible for its source to be either Ritter, Umboi or Sakar.  The shape tends to support Sakar as its source.  It is possible that both NW Sakar debris avalanche deposits are part of one multistage sector collapse.    

There is also a field of hummocks N of Sakar, a third debris avalanche, covering perhaps 30 km2, traveling over 10 km from the island coast.  There are no collapse scars on Sakar for any of these debris avalanches, though there are several irregular valleys on the flanks that may be potential source of the landslides.

The submarine deposits from the 1888 sector collapse of Ritter traveled between Umboi and Sakar.  It appears that the flank collapse triggered a submarine volcanic eruption.  Total volume of the collapse is up to 15 km3, several times that of the initial sector collapse volume.  The deposits stretch 40 – 85 km from Ritter.  There is some erosion of the seafloor by the avalanche.  The collapse created a single tsunami wave train, meaning that the collapse itself was the primary tsunami creating mechanism rather than the subsequent volcanic eruption. 

Hydrothermal

There were at least six thermal areas on the flanks of described in 1953.  None of these were observed active by 1970, though there was a slight sulphur smell at the summit.  No other thermal areas are known on the island. 

 Smithsonian GVP carries a single bulletin report dated Sept 1985.  The report describes damaging earthquakes and changes at a thermal area Aug 1985.  These reports triggered a short visit of the island by a geologist out of the Australian Bureau of Mineral Resources, Canberra 5 Sept.  He reported no signs of imminent eruption and recommended further inspection by personnel from the Rabaul Volcano Observatory. 

Their visit took place 12 Sept, inspecting the thermal area on the W flank of Talo volcano (W side of Umboi Island).  There was no increase in temperature.  However, there was subsidence, likely due to the strong earthquakes on 19 Aug.  Nine local earthquakes were recorded 11 – 14 Sept. Two of these were felt.  The quakes may or may not have been related to volcanic activity.  Quakes continued at a low level in mid-Sept.

Note the discrepancy between inactive thermal areas described in 1970 and the visit of those same areas in 1985 that describes unchanged activity.  It is unclear what the later visit means by what they call active hydrothermal vents. 

Tectonics

Given that Umboi is 57 km W from Langila, I will reprise the description of regional tectonics from that 2019 post below. 

At its most basic, New Britain is a subduction-driven region.  There are numerous microplates jostling between the ongoing collision between the Indo – Australian and Pacific Plates.  New Britain resides on the South Bismarck Plate.  To the north, is the North Bismarck Plate, which moves generally to the west.  The north boundary of the North Bismarck Plate is generally subduction driven with the Caroline and Pacific Plates subducting under it.

The western portion of the north boundary of the South Bismarck Plate is generally moving in a strike – slip fashion (similar to the motion along the San Andreas Fault system).  The eastern portion is a series of spreading centers.

The southern boundary of the South Bismarck Plate is subduction, with the Solomon Sea Plate / Woodlark Plate subducting under it.  It is this subduction that drives volcanic activity in New Britain.  There is some dispute about the extent of the Woodlake Plate.  Some depictions of it show its northern portion (currently rifting) as the Solomon Sea Plate.

The area is tectonically complex, with recent formation and movement of subduction zones, rifts, spreading centers.  Due to the forces of large plate impact (Indo – Australian and Pacific), activity along the boundary of microplates defining the impact zone have changed significantly in a very short period of geologic time.

As with any tectonically complex volcanic region, this region is also subject to large earthquakes in the M 7 – 8 range.

Conclusions

Umboi is a currently inactive volcanic island located just to the W from New Britian.  Its close neighbors are volcanically active with multiple sector collapse debris avalanche deposits.  While none of these appear to come from Umboi, this does not preclude older flank collapse events from Umboi.  The island appears to be differentially subsiding, with the N part sinking while the S part rises.  The post-caldera activity is currently quiet, even including the hydrothermal system.  The problem is that in this part of the world, with chaotic tectonics, volcanically active neighbors and a robust regional magma supply, a currently inactive volcanic system does not necessarily stay that way. 

Additional information

Notes on Ritter, Sakar, Umboi and Long Island volcanoes, GAM Taylor, 1953

Seismic reconstruction of seafloor sediment deformation during volcanic debris avalanche emplacement offshore Sakar, Papua New Guinea, Kuhn, et al, Sept 2021

Umboi Island, CreationWiki

Umboi Island, en-academis.com 

An 1888 volcanic collapse becomes a benchmark for tsunami models, Micallef, et al, Oct 2017

1888 Ritter Island Tsunami, Scholarly Community Encyclopedia

From catastrophic collapse to multi-phase deposition, Watt, et al, 2019

Problems of volcanic hazards in Papua New Guinea, PL Lowenstein, Report 82/7, Mar 1982

Geology and petrology of quaternary volcanic islands off the north coast of New Guinea, Johnson, et al, Record 1972/21, Bureau of Mineral Resources, Geology and Geophysics, 1972

Annual Report 2002, Centre for Ore Deposit Research, University of Tasmania

An 1888 volcanic collapse becomes a benchmark for tsunami models, Micallef, et al, Oct 2017

Seismic reconstruction of seafloor sediment deformation during volcanic debris avalanche emplacement offshore Sakar, Papua New Guinea, Kuhn, et al, 2021