Rabaul Volcanic Caldera, PNG
Caldera Genesis and Products from the Rabaul Caldera, Rabaul, New Britain Island, Papua New Guinea.
Rabaul caldera volcano is located at 4’ latitude in the tectonically active region of New Britain, Papua New Guinea. The Quaternary volcano has been host to a number of lava and pyroclastic flows which have been produced by effusive and violent eruptions; the later producing an ocean filled caldera approximately 6000 BP. Historical eruption phases indicate that co-genetic magma mixing and sea water ingestion maybe responsible for alternating eruption styles; Vulcan and Tavurvur cones having erupted simultaneously in 1937 with the loss of 504 lives, and Tavurvur erupting periodically in 1997 and 2003.
Geographic Location and Tectonic Setting
Rabaul volcano is located at 4’ latitude at 4.271’S, 152.203’E, on the coast of the Gazelle Peninsula at the north eastern tip of New Britain Island, Papua New Guinea. The volcano is situated in a breached and collapsed caldera, which is open to the sea at Blanche Bay on the eastward side. The area is tectonically active encompassing the New Britain volcanoes. Rabaul Caldera is located proximal to a triple junction of the Pacific, South Bismarck and Solomon Sea tectonic plates. To the south of the volcano the Solomon Sea plate, which subducts the South Bismarck plate, alters direction from north to north eastward, where it begins to subduct the Pacific plate. The Benioff zone which dips at approximately 70 degrees beneath Rabaul is shallow, generally less than 300 m in depth.
Figure 1: Regional map of Papua New Guinea. Rabaul is located where the letter A is in Guinea.
Regional Geological Setting
Rabaul Caldera is part of a Quaternary shield volcano. Vulcan, formed in 1898, and Tavurvur, formed in 1937, are both active volcanic cones that lie at the caldera margins. A number of dormant or extinct satellite cones of andesitic/basaltic composition, are situated at the caldera’s northern and western walls. Sulphur Creek, which is an active fissure is located close to Palangiagia on the caldera’s eastern side. Geophysical studies by Greene in Nairn et al., 1995 verify that the majority of the caldera is flat lying or gently sloping, representing a depositional environment which is relatively undisturbed by tectonic movements. At Blanche Bay, there is a dome like bulge with adjacent slumping and sediment deformation. This is the result of emplacement of magma at a shallow depth. At Simpson Harbour north of Blanche Bay, the sediments are intruded by volcanic cones; here localised caldera wall slumping has occurred. Nairn et al.,1995 indicate that there appears to be a number of north west trending linear features. The existence of these structures within the caldera is suggested by a number of NNW, NW and E – W trending lineations between volcanic vents. Faulting is evident in the area of the dome at Blanche Bay.
Figure 2: Map indicating plate subduction direction Map based on Heming (1974) and Herzig and others (1994).
Caldera Formation, Dimension and Volumne
The Rabaul Caldera is elliptical in shape, 9 Km in width and 14 Km in length. The caldera walls are approximately 450 m in height in the south west, whilst in the north, reach heights between 100 m and 180 m. The walls have a slope of generally 70’. Blanche Bay, which is a result of breaching, occupies the south easterly wall. The volume of the caldera at an arbitrary level of 200 m abl, as defined by M’Kee et al., 1985 is 23 Km3. The caldera at its deepest transect is 295 m bsl. It is impossible to calculate the total volume of lava and pyroclastics that have been erupted from the Rabaul Caldera and it has been estimated by M’Kee et al., 1985, that the eruption of 1400 years B.P. which formed the caldera’s present dimensions, and produced approximately 3 Km3 of air fall tephra and more than 8 Km3 of ignimbrite (welded tuff), probably enlarged an already formed caldera which formed approximately 6000 B.P.
Figure 3 (above): Map indicting caldera location, extent and volcanoes (based on Greene and others, 1986 and Heming, 1974)
Figure 4 (above right): Infrared image of Rabaul region outlined in Figure 3 (NASA SIR-C image courtesy of NASA)
Heming in Nairn et al., 1995 infer that the Rabaul Caldera had been formed by two collapse episodes, the resultant eruption producing dacitic ignimbrites, which have covered the area in thick pyroclastic deposits. Radiocarbon dating indicates that these episodes were 3500 and 1400 B.P. The vents of Vulcan and Tavurvur are parasitic cones which formed after 1400 B.P. and act as safety vents for the underlying magma chamber.
Magma Genesis and Composition
Genesis Magma genesis and composition have a direct relationship to the subduction style, steepness or angle dip of the descending plate and distance volcanoes develop from the subduction trench. Magmas alter chemical composition both along and across different parts of the subducting plate (slab), depending upon the composition of material within the subducting slab and the overlying slab; The earliest volcanoes develop closest to the trench and usually erupt tholeiitic basalts (basic/felsic) representing the oceanic portion of the slab melting as it subducts beneath the adjacent plate. Later volcanoes, which develop at greater distances from the subduction trench, typically comprise calcalkaline and alkaline magmas and lavas (acid/felsic) and produce magma chemical signatures that represent the composition of the overlying slab and the subducting slab. Superimposed on this chemical spectrum are the effects of magma evolution within the underlying magma chambers before eruption. Lower chamber mostly feed low angle shield volcanoes which erupt andesites and basalts, while higher chambers in the crust feed complex volcanoes encompassing large cones and parasitic cones which can erupt andesite sand dacites. Chambers located very high within the crust, which contain highly volatile felsic material, can produce highly explosive eruptions with extensive tephra, ignimbrites and base surges. Composition Volcanic rocks in the Rabaul area are mostly andesitic to felsic in origin. Lavas and tephras record changes in composition from basalt to andesite, with a minimum silica content of 48.4% (Wood et al., 1995). The basalts are usually porphyritic with Ca plagioclase phenocrysts. Wood et al., (1995) suggest that the high-alumina basalts are products of melting in the mantle wedge above a subducting lithosphere. Felsic compositions range from dacite (silica 69.3%) to a rare sodic rhyolite (silica 73% - 75%) (Wood et al., 1995). The rhyolite contains quartz and hornblende and is thought to be a product of co-genetic magma mixing.
Charactertistics Eruption Styles, Products and Mechansims
Volcanism at Rabaul is characterised by effusive basaltic eruptions, strombolian andesitic eruptions, and dacitic and rhyolitic explosive eruptions. The andesitic and basaltic eruptions are associated with cone building episodes, which produce effusive lava flows, lava fountains, and explosively produced, welded and non welded scoria and pumice falls. Nairn et al., 1995 claim that interbedding of lavas and scoria is common in the Rabaul cones and is typical of strombolian and violent strombolian eruption styles. Products produced from explosive eruptions are the welded and non welded ignimbrites and ground surges that are evident in the Rabaul area. These volcanic products were intermittently generated by the collapse of eruption clouds, associated with sub plinian and plinian style eruption phases. Although many of the ignimbrites were emplaced at high temperature, many appear not to have been welded. Nairn et al., 1995 suggests this was due to cooling in the high eruption column, or cooling due to ingestion of cool sea water. Nairn et al., 1995 interprets these events and the resultant stratigraphy, as evidence for an initial “wet” phreatomagmatic style eruption phase, followed by a change to a typical “dry” plinian type eruption phase; these eruptions may have occurred simultaneously, with the breaching of two different vents. Simultaneous vent eruptions of Vulcan and Tavurvur occurred in 1878 and between 1937 and 1943. Both these cones appear to act as “safety vents” (Sutherland., 1995) and indicate sequential tapping of an underlying andesitic/dacitic magma. Sea water has, and will play an important role in all eruptions from post caldera formation vents.
The Rabaul volcano has a long and complex history of effusive and pyroclastic eruptions; the historical record dating back to 1767. Nairn et al., 1995 states that a minimum of five and a maximum of nine eruptions have occurred in last 20 Ka. Difficulties in dating and stratigraphic uncertainties, preclude a completely accurate figure for pre 1767 eruptions. Post 1943 eruptions include Vulcan and Tavurvur in September 1994, which erupted within 90 minutes of each other, producing an eruption column that ascended 20 Km into the atmosphere. During this eruption a number of additional cones were added to Vulcan cone and two small tsunamis damaged areas around Simpson Harbour and Blanche Bay (Sutherland., 1995) The latest significant eruption, belonging to a suite of periodic eruptions since 1994, occurred in January, 1997 and in October 2003. Most famous eruption; Vulcan; May 29 & Tavurvur May 30, 1937: “It was exactly like the boiling of a great cauldron on a hot fire. We saw the first signs as a bubbling and boiling froth, in an area known to be deep sea. When it was boiling exceedingly, and the sea was breaking into waves, it sprouted upwards into the sky” (developed from Johnson, 1985). This was a description told by an eyewitness of the Vulcan eruption in May 29, 1937. The explosive eruption was of sub plinian to plinian style, of a high viscosity felsic magma. The early part of the eruption was produced by a submarine and phreatomagmatic explosion, which developed into a subaerial eruption, which was the genesis for a vertical eruption plume which possibly reached a height of approximately 10 Km. Pyroclastic flows and ground surges were produced as the eruption column collapsed. The eruption column was fed by a continuous blast of gas and pumiceous tephra, whose power was undiminished for 27 hours (M’Kee et al., 1985). Large rafts of pumice covered large areas of water surrounding Vulcan Island. Tavurvur located on the other side of Blanche Bay opposite Vulcan exploded on May 30. Both eruptions caused thunder and impressive lighting displays as the volcanic dust, debris and heat conduction, produced powerful electrical discharges in the atmosphere. Ash fall out, was immense, blanketing Rabaul, reducing visibility to zero. Many man made structures collapsed due to the excess weight of the ash, whilst the heat caused trees to be defoliated. 504 people perished in the two eruptions.
The coastal township of Rabaul which supports a population of approximately 15,000, lies on the coastal strip of the exposed volcanic caldera. Approximately 70,000 people live within areas less than 15 Km from the caldera’s centre. Rabaul is vulnerable to a wide range of volcanic hazards including, but not limited to: volcanic earthquakes; pyroclastic flows; air fall tephra; poisonous gas discharges; mud rain and; mud flows. Secondary hazards as a result of volcanic eruption include: landslides, lightning strikes, tsunamis and wild fires. In the 1937 Vulcan and Tavurvur eruptions, all of the above occurred. M’Kee et al., 1985 states that air fall tephra was the most widespread hazard during the 1937 Vulcan eruption, being responsible for a significant proportion of deaths. Potential future eruption sites in the Rabaul vicinity include: Rabalanakaia, Sulphur Creek, Vulcan and Tavurvur. Wood et al.,1995 cites the progressive weakening of the faulted substructure, which maybe allowing more primitive basalts and less fractionated diatic (felsic/acid) magmas to reach the surface, as a potential danger. This may produce ignimbrites, and ultimately lead to chamber collapse.
Rabaul volcano is located at 4’ latitude in the tectonically active region of New Britain, Papua New Guinea. The Quaternary volcano has been host to a number of lava and pyroclastic flows which have been produced by effusive and violent eruptions; the later producing an ocean filled caldera approximately 6000 BP. Historical eruption phases indicate that co-genetic magma mixing and sea water ingestion maybe responsible for alternating eruption styles; Vulcan and Tavurvur cones having erupted simultaneously in 1937 with the loss of 504 lives, and Tavurvur erupting periodically in 1997 and 2003. An increased amount of seismology, the extended period of current eruptions (1997 – 2004), and the uplift of a dome on the caldera floor, indicate that the township of Rabaul and its outlying areas, with a combined population of approximately 85,000 is precarious.
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