DESCRIPTION:
Papua New Guinea Volcanoes and Volcanics

[Map,20K,InlineGIF]
Map, Major Volcanoes of Papua New Guinea
-- includes Bagana ... Bam ... Karkar ... Langila ... Long Island ... Manam ... Rabaul ... Ritter ... St. Andrew Strait ... Ulawun ...

South of New Britain (Papua New Guinea) lies an oceanic trench that parallels its arcuate coast. Nearing the Solomons, the trench swings southeasterly, then down along the Vanuatu chain before turning east and ending below Hunter Island. this trench system marks the subduction of oceanic crust -- the Solomon and Coral Seas -- moving north, northeast, and east under the volcanic islands formed by this process. Tectonic complications in the form of two short oceanic spreading centers affect nearby volcanoes. One extends from SE New Guinea eastward to Kavachi, and the other runs broadly east-west below the Admiralty Islands at the north end of the region.

Admiralty Islands

• St. Andrew Strait, Complex Volcano, 270 meters, Historical

Bougainville

• Bagana, Lava Cone, 1,750 meters, Historical

New Britain

• Langila, Complex Volcano, 1,330 meters, Historical
• Lolobau, Caldera, 858 meters, Historical
• Pago, Caldera, 742 meters, Historical
• Rabaul, Pyroclastic Shield, 688 meters, Historical
• Ulawun, Stratovolcano, 2,334 meters, Historical

New Guinea

• Lamington, Stratovolcano, 1,680 meters, Historical
• Victory, Stratovolcano, 1,925 meters, Historical
• Waiowa, Pyroclastic Cone, 640 meters, Historical

Offshore

• Bam, Stratovolcano, 685 meters, Historical
• Manam, Stratovolcano, 1,807 meters, Historical
• Karkar, Stratovolcano, 1,839 meters, Historical
• Long Island, Complex Volcano, 1,280 meters, Historical
• Ritter Island, Stratovolcano, 140 meters, Historical

[Map,16K,InlineGIF]
Map, Rabaul Caldera
-- Modified from: Smithsonian Institution and Almond and McKee, 1982
-- includes Kabiu ... Rabaul Caldera ... Sulphur Creek ... Turanguna ... Tavurvur ... Tovanumbatir ... Vulcan ... Vulcan Island ...

The last of two Holocene caldera-forming eruptions of Rabaul took place 1,400 years ago. Several large historical eruptions have formed intra-caldera cones. The latest significant eruptive episode, in 1994, included venting at both Tavurvur and Vulcan cones.

The low-lying Rabaul caldera forms a sheltered harbor once utilized by New Britain's largest city. The outer flanks of the asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 kilometer caldera is widely breached on the east, where its floor is flooded by Blanche Bay. Two major Holocene caldera-forming eruptions took place as recently as 3,500 and 1,400 years ago. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cones, which was formed during a large eruption in 1878, have produced major explosive activity during historical times. A powerful explosive eruption in 1994 forced abandonment of Rabaul city.

Rabaul Caldera has been the source of many voluminous explosive eruptions in the past few hundred thousand years. Reliable dates are available for two such eruptions within the past 3,500 years. A caldera-modifying eruption about 3,500 years B.P. produced primarily rhyolite but may have been triggered by an injection of basalt (Walker and others, 1981). A large eruption of dacitic magma roughly 1,400 years B.P. may have resulted in further caldera collapse, perhaps of an inner caldera inferred from bathymetry and from recent seismicity. Post caldera volcanics at Tavurvur, Sulphur Creek, and Rabalanakaia range from basalt to dacite; those at Vulcan and Davapia Rocks are dacite. The vent for the 3,500-years-B.P. eruption may have been in the north or northeast part of the present caldera (J.Mori, written commun., 1988); the vent for the 1,400-years-B.P. eruption may have been near the center of the caldera (Walker and others, 1981). Both the 3,500 and 1,400 years B.P. eruptions "began with small scale explosions ... produced small-volume pyroclastic deposits ... (and then) activity built up rather quickly to a climax" (Walker and others, 1981). "Rather quickly" in this context means only that there is no geologic evidence for a time gap between small-scale and larger scale deposits, although Walker speculates that the gaps may have been as short as a few hours. Consideration of repose periods between large explosive eruptions at Rabaul suggests that another might occur in the geologically near future (McKee and others, 1985).

The magnificent natural harbor of Rabaul, formed by a major eruption and collapse in the 6th century, was the capital of Papua New Guinea (PNG) from 1910 through 1941, and site of a 1937 eruption that killed 441 people. This event led to the founding, in the same year, of one of the world's pre-eminent volcano centers, the Rabaul Volcanological Observatory (RVO), operated by the Geological Survey of Papua New Guinea. RVO covers all the volcanoes of PNG, and its work has been particularly valuable in major eruptions such as Lamington in 1951, only a years after RVO resumed operations following WWII. ...

The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. Ulawun volcano, also known as the North Son, rises above the north coast of New Britain opposite Bamus volcano, the South Son. The upper 1,000 meters of the 2,334-meter-high volcano is unvegetated. A prominent E-W-trending escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and eastern flanks. A steep-walled valley cuts the NW side of Ulawun volcano, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

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