As long as we are in this part of the world, might as well discuss one of the more dangerous active caldera systems in the world, the Rabaul Caldera. It lies in the NE tip of the Gazelle Peninsula in eastern New Britain.
The most recent caldera-forming eruption from Rabaul took place some 1,400 years ago. There was an earlier caldera forming eruption discovered in 1985 at Tavui some 7,100 years ago offshore to the north. This is not part of the Rabaul system. The Rabaul Caldera measures 8 x 14 km and is breached to the east, making one of the better harbors in this part of the world.
There are several relatively small active pyroclastic cones on the caldera floor near the NE and western caldera walls. Two of these, Vulcan and Tavurvur are currently active and dangerous.
Natural harbors are generally Good Things. Those created by relatively recent caldera-forming eruptions generally have some drawbacks. The West New Britain capital city of Rabaul, on the western portion of the caldera floor is one such example, as it was buried by nearly a meter of ash from a dual eruption from Vulcan and Tavurvur in 1994. While warnings were sufficient to minimize loss of life, the city was mostly moved from its original location.
This caldera is particularly dangerous as it has a pair of historically active vents, Vulcan and Tavurvur located on either side of the caldera rim that have erupted simultaneously three times since 1878. These eruptions were well into the VEI 3 – 4 range. Seismicity during these eruptions appear to be tracking fault lines marking the caldera rim.
There are at least four post-caldera pyroclastic cones. The most recent is Kalamaanagunan also known as Vulcan, formed during the 1878 eruption. Others include (in rough order of most recently active) Tavurvur, Sulphur Creek, Rabalanakaia.
A Klaus Neumann article in InsideStory.au written in 2014 makes the case that the early evacuation of Rabaul and surrounding villages took place the night before the 1994 eruption began. He writes that elders who remembered the 1937 eruption, advised evacuation following the major earthquake, 12-hour lull and subsequent increasing seismicity. Local volcanologists advised local government to declare a Stage 3 Alert at 0100. This in turn led local aviation to flush all aircraft from the Rabaul airport which sits next to Tavurvur. The noise in turn notified locals who were not already on the move to get moving fast. The eruption started less than five hours later.
The Rabaul township now hosts perhaps 15,000. 70,000 live within 15 km from the center of the caldera.
The history of Rabaul is a pretty rough one, with the city being destroyed a couple times last century by volcanic eruptions. Before the town was established, an 1878 eruption built Vulcan Island in the harbor. The German colonial period ended early during WWI when Australia defeated the German garrison stationed in Rabaul. It became the capital of the Territory of New Guinea in 1920. The 1937 eruption destroyed the town, killing over 500. Japanese occupied the town five years later and built a significant military presence. The allies chose to bypass Rabaul, cut it off from resupply, eventually rendering it useless as a military installation. The town was only retaken following the Japanese surrender in 1945. Postwar saw development of Rabaul as a shipping center due to the harbor. It was once again destroyed by dual volcanic eruptions in 1994.
This is the most recently active of seven adjoining caldera systems in northern eastern New Britain. Caldera activity has generally moved to the north over geologic time. The oldest exposed eruptives are a basaltic pre-caldera cone dated 500,000 years. Dacitic lavas exposed in the quarry walls are dated 190,000 years. They are topped by dacitic and andesitic pyroclastic flow and tephra deposits.
There are portions of at least four nested calderas at Rabaul. These are referred to as the Blanche Bay Caldera Complex. Eruptive products run the gamut from basalts and effusive eruptions, strombolian eruptions with andesites and dacites, and large rhyolite eruptions with tephra falls covered by massive ignimbrite deposits. It appears the larger eruptions start small and ramp up into catastrophic eruptions.
The Watom to Turagunam Zone (WTZ) is a line of volcanoes to the north and west of the nested caldera. These are not intra-caldera volcanoes. From west to east, these include Turagunan (482 m), Kabu (Kombiu, 664 m high), Palangiagia, Tovanumbator and Watom Island offshore to the NE. Intra caldera volcanoes include Tavurvur (232 m), Vulcan (225 m), Dawapia Rocks, Sulphur Creek and Rabalankaia.
Explosive eruptions in the Rabaul area started earlier than 125,000 years ago. There are pyroclastic sequences dated 100,000 and 40,000 years ago. Younger eruption products are more difficult to date. There are three ignimbrites of recent caldera-forming eruptions – 7,000, 3,500 and 1,400 years ago. There are at least eight major eruptions since the last caldera-forming event. At least five and possibly nine significant ignimbrite events have occurred in Rabaul in the last 720,000 years. A 1993 field guide written in Australia suggests five of 12 identified ignimbrite eruptions took place in the last 18,000 years.
Older possible caldera collapses are dated less than 17,000 (2), 18,000 (1), and 110,000 (1) years ago.
Fine ash deposits demonstrate frequent modification of eruptions by water. Many early eruptions took place from multiple vents.
Much of the geology of Rabaul is layered by relatively thin pyroclastic deposits, mainly ignimbrites and tephras. The deposits are both welded and unwelded. There are Plinian pumice beds associated with nearly all ignimbites at Rabaul. Some of the tuffs measure up to 30 m thick.
There is a low velocity zone underlying the Tavurvur to Vulcan part of the caldera. It was analyzed in 1999 and is 3 – 4 km deep. No low velocity zone has been found under the Tavui caldera north of Rabaul.
The 1994 eruption ended up being a VEI 4 with simultaneous eruptions from Vulcan and Tavurvur. Seismicity started ramping up in August, doubling the number of earthquakes, most of them taking place 25 – 28 August. These tended to be discrete events. Most of the located earthquakes were along the ring fault near Tavurvur of offshore. The largest event was a M 2.6 on 26 Aug, but none were actually felt. The caldera network recorded what appeared to be a low frequency earthquake. Signals were complex and thought to originate near Matupit Island. Seismicity ceased after 29 Aug.
There was an M 5.1 earthquake beneath the harbor on 18 Sept. Aftershocks from this event merged into a high frequency swarm that became more intense. Peak intensity was around midnight with around two felt / measured earthquakes / hour. The swarm tapered off slightly before morning. The eruption began at 0600 19 Sept. There was only 27 hours of warning before the eruption began. Post eruption analysis of seismic logs identified several long-period events in the 12 hours before the M 5.1 earthquake.
Initial eruption came out of Tavurvur followed an hour or so later with an explosion out of Vulcan. The pattern of eruptions was similar to that of 1937. Rabaul was quickly covered with 20 – 25 cm of ash. There were thunderstorms that mixed heavy rain with the ash, creating heavy mud that damaged buildings and vegetation. Press reports of gray ash columns, ejecta as large as trucks, and “black muddy rain.” Ash fell across New Britain and neighboring New Ireland.
PIREPS reported ash cloud tops at 15 – 18 km. Plume height across central Papua New Guinea was estimated at 21 – 30 km. Tropospheric winds initially were 100 km / hr, dropping to 55 km / hr at altitude. This contributed to widespread coverage of the region by the ash cloud. Shuttle photographs of the plume estimated its height at 18 km.
Winds as the eruption began were generally NNE – NE but changed directions by the next morning, blowing generally N – NNW, swinging more westerly by afternoon. By nightfall of 20 Sept, the ash cloud arrived at Papua New Guinea.
By 21 Sept, local reports of huge mushroom clouds of dense, black smoke and debris above the volcanoes. Much of the falling ash combined with rain, destroying buildings and coconut plantations. Airport was buried; many roads were blocked; and the harbor was covered with debris and floating pumice. Of course, to use the harbor, shipping had to thread the needle between the two actively erupting volcanoes.
The eruptive column collapsed by the night of 22 Sept, generating multiple pyroclastic flows that traveled 2 – 5 km from the volcanoes. Column heights decreased to 1.5 km. Aerial inspections showed little visible change to Tavurvur, though there was a small lava flow out of Tavurvur that was active for 22 days. Vulcan’s active vent was at its breached flank crater near sea level. By this time, its eruptions were mostly Surtseyan. Seismic activity 20 – 22 Sept was a steady, small decline.
Vulcan stopped erupting by 27 Sept. Tavurvur continued to produce an ashy plume. Initial ashfall over Rabaul was estimated at a meter. This was revised downward to 75 cm. 25% of its buildings were destroyed. 50% had significant structural damage.
There was a significant SO2 emission during the eruption.
The good news out of this entire eruption was successful evacuation of 30,000 during the night of 18 – 19 Sept, the period of strong continuous seismicity. Most of the people apparently left before official evacuations began. As of 21 Sept, 43,000 were displaced. That grew to 53,000 a couple days later. 25,000 were located in Kokopo. News reports of looting by residents who did not evacuate was reported and led to deployment of military to help local law enforcement.
The initial caldera forming eruption is referred to as Tavui and took place 7,100 years ago. Sits north of the Rabaul caldera. First discovered in 1985. Measures 10 x 12 km. No longer has a low velocity zone beneath it.
A caldera-modifying eruption took place 3,500 years ago. It produced primarily rhyolite but may have been triggered by an injection of basalt. The eruptive center is thought to be in the N or NE part of the caldera.
There is historical evidence in Europe for a large eruption in 535 AD, with Rabaul suspected of being one of several possible sources.
The most recent caldera-forming eruption took place 1,400 years ago (683 AD). It produced dacite magmas. The initial three post-caldera cones all produced dacite products. This eruption appeared to be centered in the middle of the caldera floor. Progression of the 3,500 and 1,400 year old eruptions appear to be relatively quick, with small explosive eruptions and pyroclastic flows quickly progressing into large, climactic eruptions. Given experience with recent eruptions, that time lag may have been as short as hours. Over 11 km3 of ignimbrite was erupted in this eruption.
The first known historic eruption took place in 1767. Actual cone was not identified.
May 1791 Tavurvur emitted columns of black smoke.
Indigenous reports of activity from Sulfur Creek 1845 – 1850 had deaths due to falling pumice blocks.
A submarine eruption in 1878 formed Vulcan Island which merged with the mainland. It produced a large amount of pumice. Tavurvur also erupted at the same time. This is the first of three simultaneous known eruptions along the rim of the caldera.
Prior to 1994, the last major eruption took place in late May – early June 1937. It caused over 500 fatalities. Primary eruption was from Vulcan, with Tavurvur active for less than a day. This pair of eruptions put around 5 cm of ash on Rabaul. This eruption built Vulcan to 243 m. It also triggered a tsunami. This eruption was a VEI 4.
Intermittent phreatic and explosive eruptions took place 1940 – 1943.
There was a period of unrest starting in 1971 lasting through mid-1984. The intensity of seismic swarms generally increased over time. Another observation from that period is that its magma reservoir seems to be responsive to relatively minor external disturbances – tectonic shaking and tides. Low tides seem to trigger volcanic earthquakes (decreased load on the system).
Seismicity and deformation was significant Sept 1983 – July 1985 without an eruption. There were 1,717 earthquakes recorded in a single day. The increased seismicity suggested an eruption was imminent. 7,000 residents left town. Seismicity returned to normal levels 13 months after it began.
1994 – 1995 were the last eruptions of Vulcan.
7 Oct 2006 eruption of Tavurvur broke windows up to 12 km and sent an ash plume 18 km. It was measured as a VEI 4 eruption.
July 2010 Tavurvur erupted 23 – 26 July. Increased seismicity began the day before.
29 Aug 2014 Tavurvur started a VEI 3 eruption. Plumes were measured to 15 km.
Tavurvur was filmed in August 29, 2014 with a spectacular detonation and shock wave. It continues to be a most dangerous volcano. This video is an example of the sort of volcanic activity on New Britain. While located on the other end of the island, the explosion along with the associated shock wave is too good to pass up.
As I described in the Langila post, 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.
Crust under New Britain is over 25 km thick.
The areas 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.
There are frequent significant earthquakes in and around New Britain measuring well into the M 8.0 range. They take place often and are usually followed with a Tsunami.
The Gazelle Peninsula portion of New Britain is heavily fractured, with multiple fault lines south of Rabaul generally oriented N – S. There is a lineament separating Rabaul and Tavui. Several unrelated volcanoes sit along this line. There may be an associated fault line.
As with any active, dangerous volcano, people want very hard to predict the next large eruption and earthquake. There are those that point out a 30-year periodicity in its activity. While such a periodicity may indeed exist, I haven’t found anything that goes beyond the “this might be the case” point of the discussion.
Rabaul is an active, dangerous volcanic system. With 70,000 living in the vicinity of the caldera rim, though no longer as close to Tavurvur or Vulcan as they were before 1994, significant loss of life during a future eruption is always a possibility. This system has been active and forming calderas for over half a million years in this part, erupting ignimbrite sheets, and injecting Plinian ash plumes 20 km into the sky on a relatively frequent basis. It is one of the most dangerous volcanic systems I have written about. It can go from relatively quiet to a full VEI 4+ eruption in a relatively short time.