Surtseyan style eruption in the Ambae (Vanuatu,New Hebrides) caldera lake in 2005 December and its implication to volcanic hazards and emergency management on an ocean island.

No abstract availabl

Phreatomagmatic volcanic hazards where rift-systems meet the sea, a study from Ambae Island, Vanuatu

Uncorrected proofAmbae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE–SW rift-controlled elongated shape. Several hundred scoria cones and fissure-fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10,000 inhabitants. Its activity remained focused at the central (crater-lake filled) vent and this is where hazard-assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred b500 and b300 yr B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-continuous cliff section exposes an up to 25 m thick succession of near-vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-grained tephra with undulatory bedding and cross-lamination as well as megaripple bedforms. These surge and fall beds are more common in the uppermost part of the succession and form a few-m-thick pile. An older tephra succession of similar character occurs below, and buried trees in growth position, as well as those flattened within base surge beds. This implies that the centre of this eruption was very near the coastline. The processes implied by these deposits are amongst the most violent forms of volcanism on this island. In addition, the lowland and coastal areas affected by these events are the most heavily populated. This circumstance is mirrored on many similar volcanic islands, including the nearby SW Pacific examples of Taveuni (Fiji), Upolu and Savai'i (Samoa), and Ambrym (Vanuatu). These locations are paradoxically often considered safe areas during summit/central-vent eruptions, simply because they are farthest from the 34 central sources of ash-fall and lahar hazard. The observations presented here necessitate a revision of this view

Syn- and post-eruptive erosion, gully formation, and morphological evolution of a tephra ring in tropical climate erupted in 1913 in West Ambrym, Vanuatu

Syn- and post-eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods. Descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare, particularly those with a well-constrained timing of events. In spite of this lack of data, cone morphologies and erosion features are commonly used for long-term erosion rate calculations and relative age determinations in volcanic fields. Here we provide new constraints on the timing and nature of erosion from a tephra ring erupted in 1913 in West Ambrym, Vanuatu and exposed along a continuous 2.5 km long coastal section. The ring surrounds an oval shaped depression filled by water. It is composed of a succession of a phreatomagmatic fall and base surge beds, interbedded with thin scoriaceous lapilli units. Toward the outer edges of the ring, base-surge beds are gradually replaced in the succession by fine-ash dominated debris-flows and hyperconcentrated-flow deposits. The inter-fingering of phreatomagmatic deposits with syn-volcanic reworked volcaniclastic sediments indicates an ongoing remobilisation of freshly deposited tephra already during the eruption. Gullies cut into the un-weathered tephra are up to 4 m deep and commonly have c. 1 m of debris-flow deposit fill in their bases. There is no indication of weathering, vegetation fragments or soil development between the gully bases and the basal debris flow fills. Gully walls are steep and superficial fans of collapsed sediment are common. Most gullies are heavily vegetated although some active (ephemeral) channels occur. Hence, we conclude that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward. After establishment of the gully network, tephra remobilisation is concentrated only within them. Therefore the shape of the erosion-modified volcanic landform is predominantly developed shortly after the eruption ceases. This observation indicates that gully erosion morphology may not necessarily relate to age of such a landform. Different intensities of erosion during eruption (related to water supply or rainfall) are probably the major influence on gully spacing, modal depth and form. Longer-term post-eruption processes that could be indicators of relative age may include internal gully deepening (below basal debris-flow fill sediments) and possibly widening and side-slope lowering due to undercutting and side-collapse

Lava lakes and shallow level magmatic feeding systems of mafic volcanoes of an ocean island: Ambrym, Vanuatu (New Hebrides), South Pacific

Ambrym is an active volcanic island with 2 major vent complexes; Marum and Benbow. These vent complexes are continuously active over at least the past two thousands of years. These active vents either produce constant degassing during quite periods or sub-Plinian to Vulcanian explosive eruptions commonly influenced by magma-water interaction triggered phreatomagmatic explosive phases. The active vents of Ambrym perfectly expos inner walls of the crater/conduit transition zone, allowing to study in cross sectional view of the interbedded coherent magmatic bodies with pyroclastic successions. In the inner crater/conduit wall of the Marum volcano, that consists of at least 3 major vents, as well as a vent that is located on its flank (Niri Taten) exposes solidified complex lava lake cross-sections, lava spatter cone feeding lava pods, shallow intrusions as well as large sills that connected through a complex network of pathways to the surface and/or into the pyroclastic edifice of the volcano. This suggests that shallow level infiltration of melt into a mafic volcano plays an important role in the edifice growth

Samoa technical report - Review of volcanic hazard maps for Savai'i and Upolu

Both main islands of Samoa, Savai'i and Upolu need to be considered as potentially volcanically active. The most recent eruptions in historic times happened on Savai'i in 1905-1911, 1902 and 1760 (estimated). Though detailed volcanic studies and dating of volcanic events are very limited there is evidence for repeated volcanic activity on both islands since the time of human occupation of the islands marked by prominent and fresh appearance of tuff cones as Tafua (= fire mountain) Savai'i, the island of Apolima, Tafua Upolu and offshore Cape Tapaga. This report examines the volcanic risks for both islands and defines for disaster management considerations potential eruption scenarios based on eyewitness accounts of previous eruptions, geological field evidence, remote sensing information and experiences from similar volcanoes. A detailed timeline of events, potential impacts and required emergency response activities are listed for the five potential eruption types (1) long-term lava field (2) short-term spatter-cone (3) explosive phreatomagmatic (4) explosive scoria-cone and (5) submarine flank collapse. Given the nature of volcanism in Samoa with hundreds of individual "one-off" volcanoes scattered along zones of structural weakness within the Savai'i - Upolu Platform - predicting the exact location of future eruption centres is impossible. At the current stage of knowledge a presentation of a volcanic hazard map is inadequate and would require additional baseline studies to statistically define recurrence intervals and areas of higher volcanic activity. Taking these limitations into account, maps showing the relative potential for new eruption vents on Upolu and Savai'i are derived from geomorphologic features. To improve our understanding and management of the volcanic risks of Samoa, suggestions for achievable future work are listed and prioritised. These recommendations include geological/volcanological baseline studies (e.g. dating/detailed analyses of past events, rock chemistry, volcano structure); installation of early warning and monitoring network (e.g. permanent GPS, seismometers); and disaster preparedness and volcanic crisis response planning

Exploding lakes in Vanuatu - "Surtseyan-style" eruptions witnessed on Ambae Island

After a long silence, Lake Vui on Ambae Island burst into spectacular life on the 28th of November 2005, disrupting the lives of 10 000 inhabitants on this sleepy tropical island in the SW Pacific. "Surtseyan- style" explosions burst through the Island's summit lake waters forming a new tuff-cone and threatening to form deadly lahars or volcanic floods. Such eruptions are rarely well observed, and these fleeting opportunities provide a chance to match volcanic processes with rock-sequences found commonly in the geologic recor

Magmaticand phreatomagmatic tephras in fine ash successions of an arc volcanic complex, the Mangatawai Tephra Formation, Tongariro Volcanic Centre, New Zealand

No abstract availabl

A large-scale, near-sea level, silicic caldera-forming eruption in Efate? An alternative model for the 1 Ma Efate Pumice Formation, Vanuatu, SW-Pacific

No abstract availabl

Modern analogues for Miocene to Pleistocene alkali basaltic phreatomagmatic fields in the Pannonian Basin: "Soft-substrate" to "combined" aquifer controlled phreatomagmatism in intraplate volcanic fields

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.Fil: Németh, Karoly. Massey University; Nueva ZelandaFil: Cronin, Shane. Massey University; Nueva ZelandaFil: Haller, Miguel Jorge F.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco"; ArgentinaFil: Brenna, Marco. Massey University; Nueva ZelandaFil: Csillag, Gábor. Geological Institute of Hungary; Hungrí

Community Emergency Management During the 2005 Ambae Eruption, Vanuatu, SW Pacific.

No abstract availabl