The geology and geophysics of the ambrym caldera, New Hebrides

Prodigious emission rates and magma degassing budget of major, trace and radioactive volatile species from Ambrym basaltic volcano, Vanuatu island Arc

“…At~800 m above sea level, its summit part is truncated by a circular caldera 12 km in diameter ( Fig. 1b and c) and approximately 2 ka in age (MacCall et al, 1970), that probably formed during both major explosive events (Robin et al, 1993) and collapses (Nemeth et al, 2009). Ambrym volcanics are predominantly basaltic, with subordinate amounts of more evolved (andesitic to dacitic) products.…”

“…DER: D'Entrecasteaux Ridge; (b) Ambrym island with its extinct northern centers and its 12 km wide summit caldera cut by the 20°NE-SW rift zone; (c) detailed map of Ambrym caldera showing the two main active cones of Benbow and Marum, the ultimate lava flows, and the site locations of our gas measurements (stars) and rock sampling (numbers) (redrawn from Beaumais, 2013). be investigated for its structure and eruptive history (MacCall et al, 1970;Chase and Seekins, 1988;Robin et al, 1993;Nemeth et al, 2009;Nemeth and Cronin, 2011), its eruptive products (Robin et al, 1991(Robin et al, , 1993Picard et al, 1995;Peate et al, 1997;Beaumais, 2013) and, more recently, its seismic activity (Legrand et al, 2005). Paradoxically, no attention was paid to its voluminous gas emissions, despite their heavy impact upon the local environment and even the health of inhabitants in the downwind part of the island (Cronin and Sharp, 2002;Bani et al, 2009aBani et al, , 2009bAllibone et al, 2012).…”

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

“…Tephra from these frequently active centres along with periodic (including historic) intra-caldera lava flows have formed a large flat ash-plain in the caldera as well as several sediment-choked fluvial systems leading out from the caldera and down to the coast. The caldera formation is currently under debate, with a passive collapse mechanism (McCall et al, 1969) appearing more reasonable than a theory proposed later which involves a cataclysmic phreatomagmatic eruption (Robin et al, 1993). The first mechanism is supported by the typical nature of historic eruptions which have involved the passage of degassed basaltic magmas along lateral dykes down the E and W rift arms to feed flank eruptions (Wiart, 1995).…”

“…1). It is a triangular 35 × 50 km island, formed around an active E-Woriented fissure zone, which is covered by many scoria cones and fissure-fed lava flows (McCall et al, 1969;Carney et al, 1985). The northern part of the volcano is considered to be oldest and it is inferred to be part of an old shield volcano cross-cut with fissure vents (McCall et al, 1969).…”

“…At the western edge of Ambrym (Fig. 1), a phreatomagmatic volcanic field was formed in 1913 during a flank eruption (Gregory, 1917;McCall et al, 1969). This series of events produced seven tuff and scoria cones covering an area of 25 km 2 (Gregory, 1917).…”