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Volcanic Risk Solutions, Institute of Natural Resources, Massey University, P.O. Box 11 222, Palmerston North, New Zealand (e-mail: k.nemeth{at}massey.ac.nz; s.j.cronin{at}massey.ac.nz)
Ambrym is one of the largest volcanic islands of the Vanuatu arc. It has been the focus of exclusively mafic volcanism and has a structure dominated by a central 13 km-diameter caldera. Contained within the caldera are two major cone complexes, Marum and Benbow, which have been the locus of most historic eruptions. Vents within these are constantly in a state of strong degassing, with visible lava lakes periodically being observed in several subcraters. Vulcanian and strombolian explosive eruptions occur at least yearly, along with larger subplinian events every 20–30 years. The active vent systems are enclosed by several 100 m-deep vertical-walled pit craters that expose cross-sectional views through the transition zone between the conduit and the crater. Units include coherent magma bodies with interbedded pyroclastic successions. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes, magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface and/or into the pyroclastic edifice. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic-deposit-dominated volcanic sequences of Marum plays an important role in the growth of scoria and spatter cones. Once solidified, the large intrusive bodies apparently provide important buttressing of pyroclastic cones, but during emplacement they may also cause cone collapse and lateral escape of magma to form lava flows.
