North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre
and represents one of at least eleven vents which have been active on Tongariro since
the last glacial maximum. The most recent cone-forming activity at North Crater is
thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped
andesite cone. This project focused mainly on the cone-building eruptions at
North Crater, including stratigraphic correlations with distal tephra, interpreting
eruptive processes, and establishing the sequence of events during cone construction.
Detailed field work identified key stratigraphic sections and facies in the proximal,
medial and distal environments. These sections allowed stratigraphic correlations to
be made between proximal cone-building facies and distal sheet-forming facies at
North Crater, and established a complete North Crater eruption stratigraphy. In the
proximal environment, welded and non to poorly welded facies formed from fallout
of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the
medial and distal environment, the lithofacies consist of fallout from a convecting
plume and minor pyroclastic flow. Convective fall and non to poorly welded
pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting
explosive eruptions involving a gas-rich magma. A change to welded deposits
produced from lava-fountaining occurs later in the cone-building sequence and
suggest a change to lower explosively and eruption of gas-poor magma.
Grain size, componentry data, density, petrography and SEM analysis were carried
out on representative samples to characterise the different facies, and reveal
information about eruption processes. The non to poorly welded deposits are typically
made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and
lapilli. The welded facies are relatively dense and clast outlines are often difficult to
distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt
orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and
indicate crustal assimilation. Mingled clasts of light and dark glass were found to
have microlites present in the dark glass, but were absent in the light glass. Electron
microprobe analyses found that the dark and light glass components in a single clast
had similar compositions, showing that the contrasting physical appearance of the
glass is not due to a different chemical composition.
Forty three whole rock XRF analyses showed that the magmas ranged from basaltic
andesite to andesite, and Harker variation plots display linear trends typical of magma
mixing. Magma mixing as the most important magmatic process is supported by
disequilibrium of phenocryst compositions and phenocryst textures. Magma
viscosity, bulk density and temperature was determined using MAGMA (Kware), and
indicate that they fall into the range of typical andesites.
Eruptive activity involved vigorous lava-fountaining, minor convecting eruption
plumes and dominant collapsing eruption plumes. This activity has produced welded
and non-welded pyroclastic flow and fall deposits to form the large cone seen today.
There are significant volcanic hazards associated with this style of activity at North
Crater, characterised by lava-fountaining, eruption plume fallout, and widespread
pyroclastic flows and lahars extending beyond the ring plain. These could all be
potentially devastating to the central North Island of New Zealand.
