Andesitic Plinian eruptions at Mt. Ruapehu (New Zealand) : from lithofacies to eruption dynamics : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, (Palmerston North, Manawatu), New Zealand
A new detailed stratigraphy was developed for a sequence of pyroclastic deposits including
the largest known eruptions associated with Mt. Ruapehu, deposited in the period ~27-10 ka
BP cal. From the largest Plinian eruption deposits in this sequence, subtle lithofacies
variations within componentry, pumice textures and sedimentary features were used to
identify a systematic change in eruptive conditions over time. Early eruptions involved steady
eruption columns, while younger eruptions involved unsteady, collapsing columns. Isopach
and Isopleth (pumice and lithic) mapping of most widespread and distinctive units show that
the largest explosive eruptions known from this volcano attained peak column heights
between 22 and 37 km, with mass discharge rates reaching 107-108 kg/s.
To characterise the conditions controlling the style of Plinian eruptions at this andesitic
volcano, and to explain the systematic variation in column stability over time, five key units
were sampled in detail, exemplifying the major contrasting lithofacies. The sampled tephras
underwent grain-size analysis, along with quantification of componentry, porosimetry and
density on particles of a range of size classes, as well as 2D and 3D microtextural analyses of
juvenile pumice clasts to define vesicularity and crystallinity. In addition, physiochemical
factors such as melt-evolution and volatile-contents were determined by analysing bulk
pumice, glass-inclusions and residual glasses with electron microprobe and FTIRspectroscopy.
Bulk compositions of these tephras vary from basaltic-andesite to andesite (56-62 wt.%,
SiO2), and had minimum pre-eruptive H2O contents of 4-5 wt.%. The evolution of eruption
behaviour over time was not correlated to any progressive change in bulk geochemical
properties, but instead resulted from variations in physical processes within the conduit.
Ascending magmas experienced heterogeneous bubble nucleation, and later-erupted units
showed increasing degrees of rheological heterogeneities developed across the conduit.
Differences between units were due to changes in the magma decompression rates, the degree
of bubble-crystal-melt interactions and bubble shearing, as well as the composition of the
residual melt. Conditions that led to the most variable physical states of the magma reaching
the fragmentation level resulted in the highest variability in pumice textures, the greatest
range in styles of fragmentation, and the most unstable eruption columns.
A new model describing the pre-eruptive magma storage region, conduit processes, magma
fragmentation, and pyroclastic dispersal during Plinian eruptions at Mt. Ruapehu is proposed.
This hypothesises that eruption column unsteadiness and collapse occurs when magma shear
reaches extreme levels along the conduit under conditions of low isolated porosity (<3
vol.%). This situation also generates the worst-case hazard scenarios expected for Ruapehu,
eruptions, where Plinian columns of over 30 km may produce widespread tephra fall, as well
as partially collapse to generate pyroclastic density currents of over 15 km runout.