Lolobau volcano is built on the rim of a 6 km wide caldera on Lolobau Island, just N or New Britain, Papau New Guinea (PNG). The caldera was formed some 12,000 years ago. Since that eruption, subsequent volcanic activity built Lolobau, a number of smaller cones and at least three smaller volcanic islands nearby. The volcano tops out at 858 m.
We did a pair of posts about volcanic activity at either end of this island over the years – Rabaul and Langila. The island itself is 8 x 13 km and lies just off the north shore of New Britain. Granyia did a great post on neighboring Ulawun and Bamus volcanoes in 2017. Ulawun itself is monitored with a local volcano observatory in Ulamona. This monitoring should also be able to detect activity at Lolobau Island.
The area is sparsely inhabited with just over 7,200 within 10 km, nearly 10,000 within 30 km, and over 62,000 within 100 km. Its climate is tropical.
The closest village is Ulamona, near the coast with a population of 3,000 that gets evacuated from time to time when neighboring Ulawun erupts. The village is situated in a dangerous location, around 10 km from Ulawun and 15 from Lolobau Mountain. There is a small airport 7 km up the coastal road to the NE. The area is remote, though there is a road and a small airport and local palm oil plantation in operation. The Ibana Plantation is located 9 km to the SW. The local indigenous people are called Meramera. The majority still believe in their traditional religion, though the majority does attend church brought into the area by Methodist and Catholic missionaries.
New Britain was invaded and captured during World War II by the Japanese who built the Sule (Ubili) Airfield in 1943. It was used in a limited capacity primarily for refueling or emergencies. It was abandoned after the war. Today, it is lightly used by light aircraft. There are remains of Japanese WWII aircraft in the brush near the runway.
The Bismarck Sea to the N of New Britain is a popular location for divers and according to one relatively recent article, home to three of the best dive locations in all of PNG. The best known of these is Kimbe Bay just off the center of the island. Second is Witu Islands farther offshore to the W. Lolobau Island and Father’s Reefs are the third location. Father’s Reefs are named after Ulawun (Father) volcano.
Diving focuses on seamounts and coral-topped features in the Bismarck Sea. There are number of seamounts stretching perhaps 20 km to the W of Lolobau Island. These are named the Fathers Reef. One of the popular locations is named The Arch, which is a large window in the reef. The writer literally raves about the beauty and sea life found during the dives.
The immediate region is quite volcanic, with the previously discussed Ulawun, Bamus and Likuruanga located on the mainland of the island, and Lolobau and the associated volcanic islands lying offshore to the N. While neighboring Ulawun (25 km SE) and Bamus (32 km SSE) appear to be associated with the same regional fault system, they do not share the same magma system. Likewise, Likuruanga, Lolobau and the associated volcanic islands all lie on an E – W line which may represent another structural line of weakness around 60 degrees off the chain of volcanoes on the New Britain mainland. Lolobau erupts basalts, dacites and andesites and is a volcanic arc subduction volcano.
Likuruanga Volcano, North Son, is the most easterly volcano of the Bismarck Volcanic Arc. It is regarded as extinct as it has not had any historic eruptions, no hydrothermal activity, and the core of the volcano is deeply cut. It has not been described in depth in geological literature.
It is a steep-sided cone with a base elongated in a NW – SE direction. The elliptical diameter is 9 x 7 km. Flanks slope 17 – 20 degrees and are heavily forested. The crater is breached to the NW and is a horseshoe shaped amphitheater. The highest point on the crater rim is to the S at 950 m. Walls below the rim are precipitous and form a bowl-shaped depression that is drained by a system of steep gullies. There is a subsidiary conical hill on the NE flank of the volcano joined by a low saddle. This cone is either a satellite vent or eroded remnant built before the main crater.
All volcanic rocks are heavily eroded with thermal alteration. There are lava flow outcrops exposed on the flanks and crest of both volcanoes. There are scoria deposits and the remains of what may be young pyroclastic deposits on the shoreline that may have come from either neighboring Ulawun or Lolobau Island.
Two satellite cones are located NE of Likuruanga. One of these forms Cape Deschamp in the mainland. The other forms the offshore island of Kakolan. Kakolan is 1.1 km wide and rises 135 m above sea level. It is completely surrounded by coral reefs. Most exposures are lavas. Both satellite cones appear to be composite volcanoes constructed by both lava flows and ash falls. Eruptive lavas appear to have been viscous.
Lolobau Island is 20 km W of Likuruanga volcano. Lolobau is considered to be still active having erupted twice a century ago. It also has an active hydrothermal system. Coral reefs ring the coast of the island. Lake Namor, a freshwater lake is located in the southern part of the caldera. The largest post-caldera cone is Lolobau Mountain 829 m high on the western caldera rim. Its crater has the Hulu cone inside. There three smaller post-caldera cones in the center of the caldera – Tobal, Malo and Sili. Sili is the source of the 1905 lava flow. The second highest peak on Lolobau Island is Giwu, on the eastern end of the island, outside the caldera scarp. Its age is unknown as is its origin. It may represent an old vent present before the caldera forming eruption. It may have formed post-caldera. There are three volcanic islands W and S of Lolobau Island. These are Banhan, Muli and Tiwongo.
The caldera is covered by later volcanic products from Lolobau Mountain. The freshwater lake is separated from the ocean by a strip of land only a few hundred meters wide. The island is covered entirely by rain forest except for a pair of copra plantations in the E and W. A fresh lava flow in the center of the island was clear of vegetation in photos taken in 1948. The rain forest had covered it by 1969. The summit of Lolobau Mountain has stunted vegetation on the Hulu cone likely due to hydrothermal areas.
The original volcano was a stratovolcano destroyed by the caldera-forming eruption some 12,000 years ago. Lava flows are exposed intermittently on the coastline and are interbedded with a variety of pyroclastic materials, scoria, lapilli and ash. Activity continued following caldera formation and covered much of the island with pumice. How much of this was deposited by the caldera eruption and how much post-caldera is unknown.
The largest post-caldera cone is Lolobau Mountain on the western part of the caldera. It mantles the western and northern caldera rim. The summit crater is breached on the NE side. Crater walls are steep and heavily vegetated. There are no rock outcrops. Exposure is also limited on the Hulu cone in the middle of the crater topping Lolobau Mountain. It is 30 m high with a small depression at its top around 300 m across. Visitors in 1937, 1942 and 1950 documented three areas of thermal activity – steaming fumaroles, sulfur deposits. Observed activity decreased with each visit.
There are three minor cones, Tobal, Malo and Sili similar in size to Hulu present in the caldera floor E of Lolobau Mountain. Malo and Sili are the source of lava flows that flowed to the S. The most recent of these breached the SE side of the Sili cone and made it all the way to the present-day N shore of Lake Namor. Its edges are 10 – 15 m tall. It is similar to “slab pahoehoe” described in Hawaiian basalts. There are thick deposits of coarse ash fall pyroclastics close to the three vents. Much of it is lapilli. This material came from all three vents and perhaps even from Hulu.
Giwu is the largest volcanic cone on the east of Lolobau Island. Its age is unknown, though the caldera escarpment cuts through the western flank of Giwu and dikes are exposed by gullies on the S side of Giwu.
There are two smaller cones on Lolobau Island. One is a subsidiary hill that forms the western tip of the island. There are lavas overlain by what appear to be water-lain tuffs. On the other end of the island, there is a cliff that exposes 30 m of bedded lapillis and ash layers. The deposits appear to form a low angle cone destroyed by marine erosion.
The three neighboring small volcanic islands are all fringed by reefs. Banban Island is 7.5 km to the W, is dome shaped, around 800 m wide, and 201 m tall. It appears to be mostly constructed from lavas. Muli is 5 km W, flat topped, and appears to be well-bedded layers of lapilli and lava blocks. It is either an uplifted block covered by volcanic sediments or a submarine extrusive dome. The final island, Tiwongo is around 700 m off the SE Lolobau island coast. It is heavily forested and seems to consist of lavas.
VOGRIPA lists the caldera forming eruption some 12,000 years ago as a VEI 6.7 that ejected some 50 km3 of material.
The 1100 AD eruption is listed as a VEI 4.4 ejecting a little more than 0.2 km3 from the Hulu vent in the central crater of Lolobau.
The first written account I can find of Lolobau was by D’Entrecasteaux in 1793. There was no reported activity. What is now named Lolobau Island was named Du Portail after another expedition member. None of the peaks and islands were recognized as being volcanoes at the time. A subsequent expedition by Powell in 1878 depicted Lolobau Mountain with a plume in a sketch. Neighboring Ulawun, Bamus and Likuruanga were identified as volcanoes and named Father, South Son and North Son. Both Ulawun and Bamus were depicted with volcanic plumes in another sketch.
The 1904 – 05 eruption was not quite as vigorous, ejecting around 0.1 km3 of material in a VEI 4. The 1911 eruption was similar in size to the 1904 eruption. Both took place on the E flank from Sili / Malo vents. Hulu may have been involved in the 1904 – 05 eruption. There may have been an unconfirmed eruption in 1908.
The most recent activity from Lolobau took place with a pair of VEI 4 eruption sequences 1904 – 1905 and 1911 – 1912. Both of these took place from the east flank of the volcano from the Sili vent. The previous eruption was a VEI 4 that took place around 1100 from the top of the volcano constructing the Hulu dome topping the volcano. The 1904 – 05 eruption produced ash and a dacitic lava flow.
There was an increase in solfatara activity 1937 – 1950 that stopped entirely by 1969.
As we’ve seen with previous posts (linked earlier) about New Britain volcanoes, this is a complex tectonic region, with multiple small plates / platelets / micro-plates jostling one another as the main collision between the Indo-Australian, Eurasian and Pacific Plates progresses. For its part, New Britain is located on the southward moving South Bismarck Plate. The North Bismarck Plate lies to the N and is moving generally to the W. SE of New Britain is the Solomon Sea Plate which is also moving generally to the W and subducting under the South Bismarck Plate. It is this subduction that provides an active supply of melt to power activity on the main volcanic arc on the island. The Solomon Sea Plate dips steeply beneath the South Bismarck Plate and is visible via earthquakes to more than 500 km below the surface.
Holocene volcanoes in New Britain are volcanic arc are fueled by melt from the ongoing subduction. Crust beneath New Britain is 20 – 40 km thick. Volcanic rocks range from basalts to rhyolites. Volcanic activity has been taking place since the Eocene and concentrated in at least two episodes. Four of these volcanoes have had eruptions VEI 5 or larger. Initial activity was generally andesitic and ended around the same time the Ontong-Java Plateau is thought to have arrived to the N at the Kilinailau Trench. It resumed in the Pliocene with basalts, andesites and dacites. The period between volcanic episodes saw subsidence, extension and hydrothermal activity.
While inactive for at least a century, Lolobau represents a continuing active volcanic system. The post-caldera growth of multiple cones, neighboring volcanic islands and activity of neighboring Ulawun and Bamus demonstrate that there is a vigorous magma supply in the region. This is a volcanic system that should be treated with a great deal of respect and deference.