PHYSICAL VOLCANOLOGIC EVOLUTION OF AN AGGLUTINATED CRATER RIM SEGMENT

Analysis of depositional characteristics of volcanic sequences allows for an understanding of eruption dynamics. The remnant crater rim feature of the Akaroa Volcanic Complex provides an opportunity to study the progression between matrix supported bombs to welded spatter within a single 26m thick cliff forming unit. Due to the height of the cliff face, paired with a reserve status, traditional observation and sampling techniques are not possible. In response, a series of non-invasive observation techniques were utilize: Angle of the cliff face (relative to verticality); rock texture categorized into: rubbly, scaly, or smooth; measurements of clasts dimensions, and clast concentration; vesicularity (clast and groundmass); and Schmidt hammer (proxy for uniaxial compressional strength of clasts and groundmass). Data was used to construct a schematic section with recorded measurements correlated to stratigraphic position to establish relationships. Within the composite section, 15 facies are identified. Recorded aspects vary throughout these facies, however distinct relationships exist within datasets and represent primary eruptive and depositional conditions. Units with a low clast abundance supported in ash, are lower in strength, and reflect pulsatory eruptions at low accumulation rates. Units rich in spatter, in which individual clasts can be differentiated and separated by groundmass, represent thermal conditions with ejecta hot enough to deform and agglutinate. Strength of groundmass and clasts relates to vesicularity, with more vesicular components having lower strengths. In increasingly welded units, spatter clast boundaries become harder to distinguish and trend away from having fluid cores and viscous/ brittle rims, an overall reduction in vesicularity. Welded units reflect increasing eruption rates and associated accumulation, with clasts flattening on impact losing their shape, compacting and welding. Direct correlations can be made between eruption processes and accumulation rates, with low eruption rates accumulating bombs with fluid cores and viscous/ brittle rims, in a fine grained matrix. Conversely, during high eruption rates accumulation rates increase, therefore timing/cooling between clasts is reduced, promoting welding or even formation of rheologic lavas.