Granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow (<20 mm) felsic dykes, in Fiordland, New Zealand, forming~10-80 mm wide garnet reaction zones. The metamorphic reaction changed the abundance of minerals, and their shape and grain size distribution. The extent of reaction and annealing (temperature-related coarsening and nucleation) is greatest close to the dykes, whereas further away the reaction is incomplete. As a consequence, grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres. The aspect ratios of clusters of S1 pyroxene and pargasite in the orthogneiss, which are variably reacted to post-S1 garnet, decrease from high in the host, to near equidimensional close to the dyke. Post-reaction deformation localized in the fine-grained partially reacted areas. This produced a pattern of 'paired' shear zones located at the outer parts of the garnet reaction zone. Our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction. The weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage.

On the early morning of 13 March 1888, roughly five cubic kilometers of the Ritter Island volcano fell violently into the sea northeast of Papua New Guinea (Figure 1). This event, the largest lateral collapse of a volcanic island in historical time, flung devastating tsunamis tens of meters high onto adjacent shores [Cooke, 1981]. Calamitous as they might be, natural disasters like these must be viewed in perspective. One should ask, “Were the events of that March day unique, or are they common geological occurrences?” Partly to address this question, a seafloor imaging and sampling program was conducted around Ritter Island and elsewhere along the Bismarck volcanic arc during November–December 2004 on the research vessel Kilo Moana.