50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

with 63.8%, and then [3] with 49.9%. The
permeability of the Burrell Lapilli samples is
generally up to 10 times higher at given porosities
(e.g. 5.3x10 -11 m 2
at �=75.1%) compared to
literature data for dacitic and rhyolitic pumice
compositions. In cores drilled on three mutually
perpendicular directions, there was no correlation
between porosity and permeability. However, in
four specimens with �>60%, the highest
permeability occurred with the oriented core of
intermediate porosity value. This result suggests
that vesicle structure is anisotropic in respect to
flow path.
Long cores cut into two were analysed for
reproducibility and homogeneity. The data clearly
show that the four pumice types are internally
homogenous with minor within-category
differences in �of 0.1 – 4.0%. However, single
larger pumice clasts are heterogeneous and their
bulk vesicularity can vary up to 19.9% (n=6).
The bulk rock compositions of all pumice types is
nearly identical, implying the difference among
pumice types is solely based on physical properties.
Vesicle textures reveal that type [1] pumices have
thinner bubble walls and less microlites as well as
heterogeneous bubble size distributions compared
to type [2]. Vesicles in [2] are often deformed,
presumably causing higher porosities and
permeabilities due to the creation of additional
apertures between neighbouring vesicles.
The development of different coloured pumice
types is the result of slightly differing physical
magma properties, such as temperature and
viscosity, resulting in changes in vesicle texture and
glass composition. This is directly controlled by
syn-eruptive decompression and fragmentation
processes rather than the pre-eruptive magma
evolution.
Type [1] pumice represents the initial
heterogeneous magma foam just prior to eruption
followed by type [2] which was able to crystallise
microlites due to sudden decompression and/or
slower ascent rate. During magma ascent these two
magma foams mingle in the conduit imperfectly
due to viscosity variations, as observed in [4].
Deformation in type [2] pumices is related to higher
eruption temperature, and hence lower viscosity,
than type [1] pumice.
POSTER
EXPLOSIVE VS. EFFUSIVE – THE JEKYLL
AND HYDE NATURE OF TARANAKI
ANDESITES
T.Platz 1 ,S.JCronin 1 ,R.B.Stewart 1 ,
I. E. M. Smith 2 &S.F.Foley 3
1 Institute of Natural Resources, Massey University,
Palmerston North, New Zealand
2 Department of Geological Sciences, University of
Auckland, Auckland, New Zealand
3 Institut für Geowissenschaften, Universität Mainz,
Mainz, Germany
(t.platz*massey.ac.nz)
The volcanic activity at Mt. Taranaki, New Zealand
is principally characterised by three end member
type eruptions: [1] lava flow eruptions (effusive),
[2] lava dome forming eruptions
(effusive/explosive), and [3] sub-Plinian eruptions
(explosive). The geological record of volcanic
events at Mt. Taranaki is predominantly marked by
type [1] and [2]; in the past 800 years
predominantly by [2]. The transition from effusive
to explosive eruption style within one eruptive
event and within long term eruptive periods is still
poorly known. To understand the different eruptive
behaviour, petrographical and geochemical studies
have been undertaken to approach this problem.
The Taranaki andesite rocks can broadly be
subdivided into three categories: virtually
hornblende (hbl) free rocks [1], hbl containing
rocks with different degrees of hbl alteration [2],
and unaltered hbl containing rocks [3]. The
presence or absence of hbl in volcanic rocks is
crucial in terms of water contents of the magma,
which strongly influences the eruptive style.
Petrographical data, e.g. degree of alteration of hbl
and groundmass glass textures, as well as
geochemical data based on e.g. melt inclusions in
clinopyroxene and hornblende, give rise to the
conclusion that there are two end member magma
compositions. We propose the existence of a dry
and a wet andesitic magma. The terms dry and wet
are used to illustrate whether the magma contains
hbl (wet) or not (dry).
We conclude that both magmas are derived from
the same initial parental melt at the base of the
continental crust which then evolves differently
during ascent within the crust. The two magma
compositions erupt either as lava flow (dry) or
pumice fall (wet), representing the effusive and
explosive end members as observed on the earth’s
surface. Lava dome forming eruptions producing
predominantly block-and-ash flow deposits
represent hybrid magmas caused by
mixing/mingling of dry and wet magma.
ORAL
50 th Kaikoura05 -69- Kaikoura 2005