Volcanic History and Eruption Processes of Blue Lake Crater, Tongariro

Blue Lake Crater (BLC) is a volcanic crater lake situated on the northern end of the Tongariro Volcanic Centre (TgVC) and is one of at least 11 vents that have been active since c. 25 ka. The stratigraphy of the proximal-to-vent deposits and associated eruption history of BLC has been poorly constrained compared to many of the other young TgVC vents. The resurgence of volcanic activity on the northern end of the TgVC in 2012 prompted research into other young northern vents to determine if any relationships exist, and whether these vents are capable of future eruptions. The stratigraphy, geomorphological and facies characteristics have been compiled from detailed field mapping and stratigraphic logs around the BLC pyroclastic rim and the Mangahouhounui Valley. BLC is surrounded by a rim of agglutinates and pyroclastic fall deposits that drape over older Tongariro landforms. The northern rim is comprised of Rotopaunga peak (~130 m above Blue Lake) which includes a series of four thick, welded agglutinate units (the Rotopaunga Agglutinates) which are considered to pre-date BLC, overlain by a succession of scoria and spatter beds (the Rotopaunga Scorias) which are sourced, at least in part, from BLC. A discontinuous exposure of agglutinate, up to 8 m thick, mantles the western and southern rim. A ~4 m thick, ~700 m long a'a lava flow derived from BLC extends over the eastern rim into the Mangahouhounui valley. The proximal-vent-forming deposits are comprised of non to intensely welded and agglutinated spatter and scoria and have all been produced by explosive Hawaiian-and Strombolian-style fire-fountaining. These deposits do not represent the original crater-forming event however, which based on geomorphology, is suggested to have been pheatomagmatic. Petrography, electron microprobe and x-ray fluorescence analyses were carried out on a selection of samples. The deposits are all plagioclase-dominated porphyritic pyroxene andesites and basaltic andesites, some of which contain rare olivine. A lack of phenocryst and whole rock geochemical trends through the stratigraphic sequence indicates that there is no progression or evolution of the magmas through time, and that multiple magma batches mingled prior to eruption. The hazards associated with BLC include: phreatomagmatic pyroclastic density currents, ballistic blocks and bombs and fire-fountaining, and lahars caused either by a future eruption or from the failure of the BLC rim. The northern vents of the TgVC (North Crater, Te Maari craters and BLC) are closely spaced and of a similar age and similar geochemical composition. BLC and Te Maari are likely to share a similar subsurface hydrothermal system and a spatial distribution of seismicity, although these alone do not imply a link in subsurface magma plumbing between the two vents