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8/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
1/12
Time-and temperature-dependent conduit wall porosity: A key control on degassingand explosivity at Tarawera volcano, New Zealand
B.M. Kennedy a,b,, A.M. Jellinek b, J.K. Russell b, A.R.L. Nichols c, N. Vigouroux d
a Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealandb Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4c Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japand Department of Earth Science, Simon Fraser University, Burnaby, British Columbia, Canada
a b s t r a c ta r t i c l e i n f o
Article history:
Received 14 February 2010
Received in revised form 18 August 2010
Accepted 23 August 2010
Available online xxxx
Editor: R.W. Carlson
Keywords:
degassing
conduit
magma
volcano
explosive
lava dome
The permeability of volcanic conduit walls and overlying plug can govern the degassing and explosivity of
eruptions. At volcanoes characterized by a protracted history of episodic volcanism, conduit walls are
commonly constructed of quenched magma. During each successive eruptive phase, reheating by ascending
magma can modify the porosity, permeability and H2O content of the conduit wall rocks and overlying plug.
We investigate whether theunusual explosivity of the 1886 basaltic eruption at Tarawera volcanois related to
the heating and degassing of the AD1314 Kaharoa rhyolitic rocks, through which it erupted. We heat cores of
perlitic Tarawera dome rhyolite to 300 C1200 C for 30 min to 3 days at atmospheric pressure. We
characterize time (t)- and temperature (T)-dependent variations in porosity, volatile content and texture
through SEM image analyses. We also directly measure pre- and post-experimental connected and isolated
porosity and water content. We identify four textural/outgassing regimes: Regime 1 ( T800 C, t2 h), with
negligible textural changes and a significant loss of meteoric water (1.40.72 wt.% H2O); Regime 2
(800T1100 C, t6 h), with cracking and vesicle growth and a 510% increase in connected porosity;
Regime 3 (800T1200 C, t30 min), with healed cracks, coalesced and collapsed vesicles, and overall
reduced porosity; and Regime 4 (T1200 C, tN30 min), with a collapse of all connected porosity. These
regimes are governed by the temperature ofthe event (T) relative to the glass transition temperature (Tg) andthe time scale of the event (t) relative to a critical relaxation time for structural failure of the melt (r). We
identify a quantitative transition from predominantly brittle behavior such as cracking, which enhances
connected porosity and permeability, to viscous processes including crack healing and vesicle collapse, which
act to reduce connected porosity. Applied to the 1886 basalt eruption at Tarawera, we show that progressive
heat transfer ultimately reduced the open porosity and permeability of the conduit walls, thereby partially
sealing the conduitand reducing volatile loss. We argue that this mechanismwas an underlyingreasonfor the
exceptional explosivity of the 1886 eruption. We further suggest that textural changes associated with
reheating could explain some of the cyclic deformation and degassing observed at many lava domes
preceding explosive eruptions.
2010 Elsevier B.V. All rights reserved.
1. Introduction
Individual volcanic eruptions can shift rapidly in style from
relatively quiescent lava dome extrusion to explosive eruptions (e.g.
Sparks, 2003; Voight et al., 1999). These shifts are generally attributed
to variations in physical properties such as gas content, viscosity,
vesicularity, wall rock permeability, and crystallinity (e.g. Gonner-
mann and Manga, 2007; Jaupart, 1998; Melnik and Sparks, 2002). To
date experimental and theoretical studies have focussed on variation
of these properties as magma rises and decompresses(e.g. Baker et al.,
2006; Gardner, 2007; Hammer and Rutherford, 2002; Larsen et al.,
2004; Proussevitch et al., 1993; Takeuchi et al., 2009; Yoshimura and
Nakamura, 2008) or is sheared (Gonnerman and Manga, 2003;
Lavallee et al., 2007, 2008; Okamura et al., 2010; Smith et al., 2009;
Tuffen et al., 2003, 2008). Natural pumice, dome rocks and
experimentally decompressed glasses show huge textural variations
in their permeablevesicleand crack networks (Jaupart, 1998; Michaut
and Sparks, 2009; Mueller et al., 2008; Rust and Cashman, 2004; Saar
and Manga, 1999; Takeuchi et al., 2008; Westrich and Eichelberger,
1994; Wright et al., 2009; Yoshimura and Nakamura, in press). Other
experimental studies investigate the effects of temperature on water
speciation, solubility, and magma viscosity (Stolper, 1989; Yamashita,
1999; Zhang et al., 2007). Rocks from conduit walls also exhibit
variation in porosity (Kennedy et al., 2005; Rust et al., 2004; Stasiuk et
Earth and Planetary Science Letters xxx (2010) xxxxxx
Corresponding author. Geological Sciences, University of Canterbury, Private Bag
4800, Christchurch, 8140, New Zealand.
E-mail address: [email protected] (B.M. Kennedy).
EPSL-10550; No of Pages 12
0012-821X/$ see front matter 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2010.08.028
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w . e l s e v i er . c o m / l o c a t e / e p s l
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.028mailto:[email protected]://dx.doi.org/10.1016/j.epsl.2010.08.028http://www.sciencedirect.com/science/journal/0012821Xhttp://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.028http://www.sciencedirect.com/science/journal/0012821Xhttp://dx.doi.org/10.1016/j.epsl.2010.08.028mailto:[email protected]://dx.doi.org/10.1016/j.epsl.2010.08.0288/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
2/12
al., 1996). Yet, no studies have addressed the isobaric time-dependent
textural changes of conduit walls in response to reheating. Here we
address two key questions: 1. How is degassing of the ascending
magma affected by changes in permeability of the conduit walls
during reheating? 2. To what extent can the release of volatiles from
the reheated wall rock contribute towards the eruption?
The vents for many explosive eruptions are often plugged by
fractured, vesicular lava domes or partially filled conduits (e.g.
Johnson and Lees, 2000; Voight et al., 1999). Surprisingly, almost noattention has been given to the effects of the hot rising magma on the
behaviour (i.e. evolution of texture and volatile content) of the lava
that plugs the volcanic conduit. This is despite observations that show
temperature rises in older lava domes prior to eruption (Wooster and
Kaneko, 1997) that correlate with eruption style (Sahetapy-Engel and
Harris, 2009). We propose that reheating can influence the degassing
of both the older plug and the rising magma. Magmatic water and
resorbed meteoricwater dissolved in glass withinthe old plug may be
available for degassing and vesiculation. This vesiculation in turn
affects the porosity and the permeability of the plug and the ability of
the rising hot magma to degas.
The 1886 Tarawera eruption (Cole, 1970; Nairn, 2002), is one of
only a few examples of basaltic plinian eruptions (Houghton et al.,
2004). At Tarawera, basalt erupts through a pre-existing dome
complex and silicic conduit system (Carey et al., 2007). This may be
a common occurrence at bimodal vents, however, descriptions of
bimodal vent exposures are absent in volcanological literature.
Detailed stratigraphic studies at Tarawera have tracked the shifting
eruption centres and fragmentation level and documented interaction
with groundwater and the pre-existing hydrothermal system (Carey
et al., 2007; Houghton et al., 2004; Sable et al., 2006, 2009). The effect
of rhyolitic conduit wall recycling during this eruption has also been
discussed (Rosseel et al., 2006).
Conduit wall permeability is an important variable in explosive
basaltic eruptions(Houghton and Gonnerman, 2008) but has not been
investigated experimentally. We use laboratory experiments on the
Tarawera rhyolitic lava to show that during an eruption the wall rock
permeability is both time- and temperature-dependent, as is the
release of volatiles from the wall rocks into the erupting magma. Weargue that a reduction in the permeability of the conduit walls as a
result of reheating hindered outgassing and increased the explosivity
of the 1886 Tarawera eruption.
2. Methodolgy
2.1. Sampling
We collected samples that contained rhyolite and basalt from the
proximal deposits of 1886 basalticfissure eruption. The motivationfor
this sampling was to collect samples that show evidence for heat
transfer between basalt and rhyolite. Enclave samples were collected
from along the rim of thefi
ssure on the summit of Tarawera lavadome (Fig. 1). We limited our samples to the crystal rich 1314 AD lava
dome xenoliths/enclaves and excluded the older crystal-poor xeno-
liths (Carey et al., 2007). We chose a single large homogeneous glassy
and perlitic sample of the lava dome to use for our experimental
starting material.From this samplewe drilled cylindrical cores 1 cm in
diameter by 2 cm in length, which were then left in an oven overnight
at 100 C to remove surface water from the samples. The precise
dimensions, density and porosity of samples were measured prior to,
and following, heating experiments. The volume of these porous
cylindrical cores was calculated from averages of replicate (n=3)
measurements of diameter and length. This volume and the
sample mass were used to calculate the bulk density (bulk) of
the core. Skeletal (or framework) density (skeletal) is obtained by
measuring sample volume via helium pycnometry. Connected
porosity (connected) (Table 1) was calculated from skeletal and bulk
density from the relationship: connected= 1(bulk/skeletal). We
obtained values of dense rock equivalent (DRE) density for rock by
crushing three cores and performing pycnometry on the resulting
powders. All experimental cores have the same average DRE density
(2.34 g/cm30.10 g/cm3). Using this average value for powder density
we compute total and isolated porosity (Table 1) as: total=1(bulk/powder), and isolated=(bulk/skeletal)(bulk/powder) (Michol et al.,
2008). Porosity values have an uncertainty of up to 4% associated with
the largest porosities measured by pycnometer (Michol et al., 2008).
2.2. Experiments
We first heated the furnace to 3001200 C (10 C) at
atmospheric pressure. This represents the expected temperaturerange of conduit rocks in contact with the erupting basalt (Rosseel
etal.,2006). Cylindrical samples(described subsequently) in ceramic
crucibles were placed in the centre of the furnace for a specified time
Fig. 1. Tarawera volcanic edifice shown as: a) geological map illustrating sampling locations on the rim of the 1886 eruption fissure (marked by X), and b) aerial photograph of the
fissure and domes looking SW from NE of the Wahanga dome and taken by Lloyd Homer, GNS Science.
2 B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.0288/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
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(30min72 h). This timescale was constrained by (1) the 5 h eruption
duration (Keam, 1988) and (2) a total magma rise time of a ~1 day
(Houghton et al., 2004). We characterized time and temperature
dependent changes in texture and water content between 800 and
1200 C. For temperatures 300700 C, experiments were run for 2 h to
characterize temperature dependent degassing below the calculated
theoretical glass transition (Giordano et al., 2008). Immediately
following each experiment samples were removed, cooled in air,
reweighed and their porosities remeasured with a helium pycnometer.
2.3. S.E.M.
Natural samples and samples from each experiment were coated
in carbon and examined using the Philips XL30 electron microscope
(SEM) (Beam power 15 kV and setting Spot 6) at the Earth and
Ocean Sciences Department of UBC. We compare the pre- and post-
experimental textures of the samples. Whenimaging both natural and
cored samples we avoided surfaces affected by therock saw and corer.
2.4. Water content
We ground and sieved samples, including crystals, to 100 m for
bulk water analysis from experiments lasting 30 min, 2, 4, 17.5 and
36 h. These samples were analyzed at ALS Actlabs using a Leco
induction furnace combined with spectral analysis of the emitted
volatiles. Thesample (~0.3 g) is thermally decomposed in a resistance
furnace (ELTRA CW-800) in a pure nitrogen environment at 1000 C,
causing release of volatiles, including both H2O and H2O+.
Some natural and post-experimental samples were thinned and
polished on both sides to make waferssuitable for analysis of volatiles
with the FTIR. FTIR analysis was undertaken at the University of
Oregon following the technique described in Wright et al. (2007).During analysis, we made every attempt to avoid areas containing
crystals and vesicles. Total H2O contents and the speciation of H2O in
the glass were determined using the absorbances of combination
bands at 5230 cm1 (molecular H2O) and 4520 cm1 (OH). At low
total H2O contents (b0.5 wt.%; Stolper, 1982), molecular H2O is not
detectable, in which case total water content was measured using the
3570 cm1 band, representing the fundamental OH stretching
vibration. Absorbances were converted to concentrations using the
BeerLambert law and absorption coefficients at 5230 cm1 and
4520 cm1 from Zhang et al. (1997) and at 3570 cm1 from Stolper
(1982). A glass density of 2.34 g/cm3 was obtained from pycnometry.
Sample thickness was determined using a micrometer and varied
between 300 and 900 m. Results from the 3570 cm1 band and
totals from the 5230 cm1
and 4520 cm1
bands (Table 1) agree to
Table 1
Results of high temperature experiments performed on natural samples summarized as experimental conditions, and properties of starting materials and run products. The number
in brackets in the FTIR column represents the number of measurements the mean value is based upon.
Sample name Time
(t)
T/Tg Temp.
(T)
Relax.
time
t/r log
visc.
Connected
porosity
Isolated
porosity
Total
porosity
Leco
indution
H2O
FTIR (4520+
5230
peaks)
total H2O
FTIR
(3570
peak)
total H2O
FTIR
(4520
peak)
OH
FTIR
(5230
peak)
mol. H2O
FTIR
image
mol.
H2O
(h) (oC) (h) (Pa s) (frac) (frac) (frac) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%)
Original g 20 0.28 0.00 0.28 1.62 0.81 (4) 0.78 (2) 0.09
(4)
0.73 (4) 0.16
1.30Original h 20 0.27 0.04 0.30 1.72
Original i 20 0.21 0.08 0.29 1.61
Original j 20 0.25 0.04 0.29 1.69
mean original 20 0.25 0.04 0.29 1.660
BMK06T300 2 0.41 300 0.24 0.07 0.31 0.76
BKM06T400 2 2 0.54 400 0.23 0.05 0.28 0.43
BKM06T500 2 2 0.68 500 0.16 0.12 0.28 0.27
BKM06T600 2 2 0.81 600 0.21 0.10 0.31 0.22
BKM06T700 2 2 0.95 700 30 0.5 12.4 0.21 0.08 0.29 0.2
BK06T800 0.5 0.5 1.08 800 30 15 10.3 0.27 0.04 0.31 0.39
BK06T800 2 2 1.08 800 30 60 10.3 0.28 0.03 0.31 0.3
BK06T800 6 6 1.08 800 30 180 10.3 0.39 0.05 0.45 0.058 (1)
BK06T800 17 17 1.08 800 30 510 10.3 0 .29 0.02 0.32 0.32
BK06T800 24 24 1.08 800 30 720 10.3 0 .28 0.04 0.32 0.33
BK06T800 72 72 1.08 800 1 2160 10.3 0.32 0.03 0.34
BK06T900 0.5 0.5 1.22 900 1 476 8.8 0 .32 0.06 0.38 0.41
BK06T900 2 2 1.22 900 1 1905 8.8 0.31 0.03 0.34 0.35BK06T900 4 4 1.22 900 1 3810 8.8 0.36 0.03 0.39 0.29
BK06T900 6 6 1.22 900 1 5714 8.8 0.35 0.06 0.41
BK06T900 17 17 1.22 900 1 1.7 104 8.8 0.37 0.03 0.40 0.26
BK06T900 24 24 1.22 900 1 2.3 104 8.8 0.33 0.06 0.39 0.3
BK06T900 72 72 1.22 900 0.08 6.8 104 8.8 0.33 0.05 0.38
BK06T1000 0.5 0.5 1.35 1000 0.08 1.5 104 7.3 0.34 0.11 0.45 0.37
BK06T1000 2 2 1.35 1000 0.08 6 104 7.3 0.46 0.05 0.51 0.33
BK06T1000 4 4 1.35 1000 0.08 1.2 105 7.3 0.40 0.05 0.45 0.26
BK06T1000 6 6 1.35 1000 0.08 1.8 105 7.3 0.41 0.08 0.50 0.044 (2)
BK06T1000 17 17 1.35 1000 0.08 5.1 105 7.3 0.43 0.04 0.47 0.32
BK06T1000 24 24 1.35 1000 0 7.2 105 7.3 0.37 0.08 0.45 0.31
BK06T1100 0.5 0.5 1.49 1100 0 1.9 105 6.2 0.43 0.05 0.49 0.29
BK06T1100 2 2 1.49 1100 0 7.5 105 6.2 0.38 0.04 0.42 0.28
BK06T1100 4 4 1.49 1100 0 1.5 106 6.2 0.32 0.04 0.36 0.29
BK06T1100 6 6 1.49 1100 0 2.3 106 6.2 0.28 0.10 0.37 0.012 (4)
BK06T1100 17 17 1.49 1100 0 6.3 106 6.2 0.28 0.04 0.32 0.26
BK06T1100 24 24 1.49 1100 0 9.0 106 6.2 0.23 0.09 0.32 0.25
BK06T1200 0.5 0.5 1.62 1200 0 1.5 106 5.3 0.34 0.06 0.40 0.29BK 06T1200 24 24 1.62 1 200 1.4 106 7 .2 107 5.3
Minimum 0.5 300 0.16 0.00 0.28 0.20
Maximum 72 1200 0.46 0.12 0.51 1.72
3B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.0288/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
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within 0.004 wt.%. Multiple spots were analyzed where possible and
the number of spots per measurement is shown in Table 1 using
number in brackets after the mean measurement.
Additionally, we conducted FTIR spectroscopic imaging of a
fragment of the original dome material to investigate molecular H2O
distribution. This fragment broke naturally along perlitic cracks
during further thinning of the samples, which was carried out to
reducethe numberof crystals andvesiclesin the wafers. Images(each
350350 m) of the fragment were collected using a Varian Inc.Lancer Focal Plane Array (FPA) camera attached to a Varian FTS
Stingray 700 Micro Image Analyser spectrometer and UMA 600
microscope at the Institute for Research on Earth Evolution (IFREE),
Japan Agency for Marine Earth Science and Technology (JAMSTEC).
For more detailed discussion of FTIR spectroscopic imaging see
Wysoczanski and Tani (2006). The thickness of the fragment was
determined using reflective light spectra and the wavelength of the
resulting interference fringes following the method of Wysoczanski
and Tani (2006) and Nichols and Wysoczanski (2007). A refractive
index of 1.50 was used for rhyolite (Liu et al., 2005; Long and
Friedman, 1968). From 89 reflection spectra across the image, the
average thicknesswas 62 m (1= 1 m), and this value was used for
all calculations. The glass density used was the same as for the spot
analyses. Variations in glass density as a result of variations in H2O
concentration result in a maximum error of 0.01 wt.% on the
molecular H2O contents. Owing to the thinness of the sample, the
combination bands at 5230 and 4520 cm1 used to measure
molecular H2O and OH were below detection (see Supplementary
Figure A1). As a result, the spectroscopic image for molecular H2O
represents the absorbance of the fundamental bending of molecular
H2O at ~1630 cm1. See Supplementary Figure A1 for additional
discussion of the use of this band. Total H2O contents were obtained
from the peak at 3570 cm1 as described earlier.
3. Description of natural samples
Our 30 natural samples vary in relative proportions of rhyolite and
basalt (Fig. 2). In contrast to Rosseel et al., 2006, we have chosen to
classify samples on the basis of vesicularity rather than bomb type.The samples described subsequently are all from the AD1314 lavas or
dykes; wall rocks from older Tarawera lavas (Carey et al., 2007) were
not sampled. These young lithics can be split into four vesicularity
types. Type 1 are glassy samples of the Tarawera lava dome that we
use in the experiments (Fig. 2a). Types 24 are samples from the 1886
plinian basaltic fall deposit and contain both rhyolite from the AD
1314 Tarawera lava dome and basalt from the 1886 eruption. The
samples vary from rhyolite lithics coated in basaltic scoria to
completely remelted rhyolite enclaves entirely contained within
basaltic spatter (Rosseel et al., 2006) (Fig. 2bd).
Type 1 samples are representative of the Tarawera lava dome; these
samples were collected from outcrops withinthe 1886 craterwithin the
Tarawera lava dome (Fig. 1). Samples are variably devitrified and
texturally perlitic and show a range of vesicularities. Vesicles show arange of vesicle sizes andshapes (Fig.2a).We chosea typicallargeglassy
block without obvious devitrification but with perlitic cracks (Fig. 2a)
for our experiments. All the drilled cores are from the one sample and
show a total porosity variation of 2830 vol.%. The dome is 1535%
crystalline(Cole, 1970), and contains phenocrysts of plagioclase,quartz,
biotite and minor amphibole, orthopyroxene and FeTi oxides. SEM
images show curvilinear perlitic microcracks at 50 m intervals in the
glass (Fig. 2a(ii)) and unknown secondary minerals on glass surfaces.
Water content of the starting material measured by FTIR is 0.8 wt.%and
by Leco induction is 1.6 wt.% (see Fig. 5b).
Type 2 samples were collected from the basaltic fall deposits
draping the Tarawera lava dome, exposed on the margin of the crater.
Samples are 110 cm chunks of vesicular rhyolite (Fig. 2c), partially
coated in (b
10% by volume) basaltic scoria (b
1 cm clast diameter).
The basaltic scoria is weakly attached to the surface of the rhyolite.
Wherebasaltic scoria is absent, cracks arevisible on thesurfacesof the
rhyolite. The rhyolite has total porosity estimated to range between
40 and 60% (using proportion comparison charts), only one sample
was measured at 53% (Table 1). SEM images show almost no
microcracks and complex vesicle shapes with evidence of vesicle
wall retraction and vesicle collapse (Fig. 2c (ii)).
Type 3 samples were collected from the same basaltic fall deposits
as Type 2 and the talus slopes below this fall deposit. Pyroclastscontain 1090 vol.% basalt. Samples are up to 30 cm in diameter, and
visibly very vesicular N60%, one sample was measured at 73% total
porosity with individual vesicles up to 5 mm. The surfaces of these
samples are covered in basaltic scoria and spatter which is strongly
attached to thesurface of therhyolite(Fig. 2c).Adjacent to thebasaltic
spatter, the outer few mm of the rhyolite is vesicle poor and appears
locally as black obsidian (Fig. 2c). SEM images show no microcracks
and in contrast to the Type 1 dome rock, the vesicles are spherical
(Fig. 2c (ii)).
Type 4 samples were collected from spatter-rich areas in the
basaltic fall. These pyroclasts are N90% basalt (Fig. 2d). These samples
contain small (b5 cm) dense enclaves of rhyolite within basaltic
spatter. The rhyolite blebs are generally too small to measure
porosities, however, we estimate the total porosity is consistently
b25%. SEM images show a range of textures, with prominent small
spherical and irregular vesicles (Fig. 2d (ii)).
4. Experimental results
4.1. Qualitative results: SEM
We present the results of the experimental heating of 32 Type 1
lava dome cores with the aim of understanding the potential
consequences of reheating conduit filling material. SEM images
showing time- and temperature-dependent changes in the micro-
structure of the samples are shown in Figure3. Generally, the textures
progress from cracked vesicular glassy original (Type 1) textures, to
an increase in vesicles with cracked walls, to a connected network of
un-cracked vesicles, and finally to less vesicular isolated vesicles.Experimentslasting 2 h at 300700 C showno noticeabledifference
in texture from the original samples. However, the time series from
experiments at 8001200 C show remarkable differences in the
geometry and size of cracks and vesicles relative to the initial state.
At 800 C, samples heated for ~6 h are characterized by open cracks.
In addition, the widest cracks are spatially associated with dome
structures (related to vesicle growth) (Mungall et al., 1996) on the
surface of the glass which are 100 m in diameter (Fig. 3a). In samples
heated for 24 h cracks are still apparent and some cracks appear more
curved and crack edges appear slightly rounded (Fig. 3b).
At 900 C, samples heated for ~6 h (Fig. 3c) aretexturally similar to
the 800 C runs (Fig. 3a). However, at 24 h, cracks are commonly
curved. The dome structures appear to have surfaces made of a thin
film of glass and relict cracks and rafts of original surfaces exist onsome domal structures (vesicles) (Fig. 3d), suggesting that films of
melt have allowed vesicles to grow.
In experiments conducted at 1000 C, the edges of cracks are
distinctly rounded indicating (re-)melting (Fig. 3e). In the sample
heated for 24 h at 1000 C most of the glass is smooth with angular
cracks only occurring in crystals. In addition, completely smooth
domes occur with only a vague hint of relict cracks (Fig. 3f). The
domes appear as connected vesicles, however these vesicles are not
bulbous, and some appear deflated (Fig. 3f).
At 1100 C, a sample heated for 40 min shows similar textures to
the sampleheatedfor 24 h at 1000 C (Fig. 3f and g). The surface of the
sample is smooth with bulbous vesicle-like domes, and no relict
cracks (Fig. 3g). The surface of the sample heated for 6 h at 1100 C is
completely smooth and cracks are no longer visible in the glass. Most
4 B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.0288/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
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of the glass is bulbous and appears to be connected vesicles ( Fig. 3g).
The glass in the sample heated to 1100 C for 24 h also is smooth and
has no cracks. Vesicles are not as apparent on the surface of the
sample (Fig. 3h).
Finally, the sample heated for 30 min for at 1200 C (Fig. 3i) shows
similar textures to the sampleheated for 6 h at 1100 C (Fig. 3g). Glass
is smooth and unfractured and contains connected vesicles (Fig. 3i).
The sample heated at 1200 C for 24 h has smooth surfaces similar to
that heated to 1100 C (Fig. 3h), however fresh surfaces show some
isolated vesiclesexist beneath thesmooth surface (Fig. 3j). All original
textures were obliterated.
In summary, experiments at lower temperatures and/or for short
times generally produced samples with open cracks. Experiments at
higher temperatures and/or for longer times show evidence of
melting, crack annealing, the growth offilms ofmelt, and the inflation
and deflation of vesicles.
Fig. 2. (i) Photographs and (ii) corresponding scanning electron micrographs of rhyolitic samples from Tarawera illustrating 4 main types of vesicularity. (a) Type 1: (i) vesicularity in
vesicular lavadomeused as thestarting material forour experiments; (ii)S.E.M.imageshowing curvilinearmicrocracksand varietyof vesiclesizes andshapes.(b) Type2: (i)vesicularity
withina rhyolitic pyroclast partially coated in basalticscoria;(ii) S.E.M. image showingsmoothsurfaces,retracting vesiclewalls,largeconnectedvesicles andno cracks.(c) Type3: (i)high
vesicularity in a rhyolitic pyroclast coated in basaltic spatter, in some areas 13 mm thick obsidian occurs at the boundary between rhyolite and basalt; (ii) S.E.M. image showing large
connected spherical vesicles and no cracks. (d) Type 4: (i) vesicularity in basaltic spatter containing dense enclaves of rhyolite; (ii) S.E.M. image showing a dense rhyolitic enclave with
some isolated irregular vesicles towards its margin.
5B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
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Fig. 3. SEM images of experimental run products produced by heating of natural samples over fixed periods of time. a) Experimental run product for 800 C over 6 h is largely
texturally unchanged from the original. However, cracks have begun to open especially in areas around domed glass associated with subsurface vesicles. b) Run product for 800 C
over 24 h shows similar cracking associated with sub-surface vesicles;also shows convexareasand some cracksappear to be rounded. c) Runproductfor 900 C over 6 h shows open
cracks associated with sub-surface vesicles. d) Heating at 900 C for 24 h causes formation of new vesicles that feature films of melt that heal old fractures and create rafts of the old
vesicle wall on the surface of the new vesicle. e) Sample heated at 1000 C for 6 h shows angular fracture edges rounding and annealing. g) Sample heated at 1000 C for 24 h shows
almost no vestiges of the earlier cracked surfaces, however, existing dome-like structures appear deflated. g) Samples heated at 1100 C for 6 h feature connected inflated vesicles
within the original vesicle wall septa, and no evidence of the original cracked glass. h) Heating at 1100 C for 24 h produces a sample on which the surface shows no evidence of the
original bubbly and cracked surface. i)Run productfor 1200 C over30 min is similar to g), vesicle wallscontain inflated connected vesicles. j)Heating at 1200 Cfor 24 h causes the
sample to collapse and flow; material had to be chipped out of the crucible and internally it shows spherical isolated vesicles.
6 B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
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4.2. Porosity
Time series of fractional porosity evolution during the experiments
are given in Table 1 and illustrated in Figure 4. Both total and
connected porosity initially increase with time at all experimental
temperatures. The time taken to reach a maximum in total porosity
decreases as the experimental temperature increases. This increase in
porosity is then followed by a reductionin porosity,wherethe specific
total vs. connected porosity path is dependent upon temperature.Isolated porosity is illustrated in Figure 4c by the vertical distance
between any point and the dashed, fully connected porosity line. At
b700 C, overall porosity and volume were indistinguishable within
error from the unheated samples (Tables 1 and Supplementary Table
A2).
At 800 C theconnected porosity reaches a maximum of 0.39 in the
experiment conducted for 6 h (Fig. 4a). Experimental times greater
than 6 h correspond with lower connected and total porosities.
Isolated porosity remains less than 0.05 in all experiments at 800 C.
The time series at 900 C also shows an initial increase in
connected and overall porosity, connected porosity then fluctuates
and reaches a maximum at 17.5 h of 0.37. Thereafter the connected
porosity drops monotonically to 0.33 at 36 h (Fig. 4b). Again, the
isolated porosity remains similar during this time series (Table 1).
The experiments at 1000 C, have a maximum total and connected
porosity of 0.46 at 2 h. Longer experiments correspond with smaller
porosities (Fig. 4c). This time series shows higher isolated porosities
(up to 0.11) compared to 800and 900 C,particularly theexperiments
for 2, 6 and 24 h.
At 1100 C a maximum porosity of 0.43 occurs at 30 min.
Thereafter, the connected porosity drops sharply to 0.23 by 24 h.
Despite the consistent decrease in connected and overall porosity,
isolated porosity varies considerably and shows a substantial peak
(~0.1) at 6 h (Fig. 4d).
Samples heated at 1200 C collapsed and lost their cylindrical form
(textural collapse), the sample heated for 30 min remained generally
cylindrical whilst the sample heated for 24 h collapsed completely.
In summary, at all experimental temperaturesan initial increase in
total porosity is followed by a decrease.
4.3. Water content
The water content of the glass in the original sample (starting
material) varies between 1.6 and 0.8 wt.% as measured by Leco
induction furnace and by FTIR spot analyses (N=4), respectively
(Fig. 5b, Table 1). These differences in measured water contents
between Leco Induction furnace and FTIR are absent at higher
temperatures and lower water contents. The FTIR spectroscopic
image of the absorbance for the molecular H2O band (Fig. 5a)
supports this, with absorbance increasing, by up to a factor of 7
compared to the interior of the fragment, at the perlitic margins.
Notwithstanding the qualifying statements of Zhang et al. (1997)(described in the figure caption of the Supplementary Fig. A1) this
represents an increase in molecular H2O content from approximately
0.16 to 1.30 wt.%. All time series show that N75% of bulk water
measured by Leco induction was lost within 0.5 h. Over 2 h at 500 C
the bulk water content shows a systematic decrease as temperature
increases from 1.6 wt.% to around 0.22 wt.%, which is close to the
detection limit of this instrument (Fig. 5b). Within the uncertainty of
the measurements, repeated experiments at temperatures above
500 C showed no change in bulk water content. FTIR spot measure-
ments using the 3550 cm1 band support the same general pattern of
rapid water loss and show a mean initial water content of 0.78 wt.%
was reduced to 0.06% after 6 h at 800 C (Fig. 5b). Additionally, these
FTIR spot analysis support continued degassing to 0.01 wt.% after 6 h
at 1100 C.
4.4. Results summary
Our observations and measurements of the rhyolite lava prior to
experimentation illustrate that the lava has an intricate network of
perlitic cracks (Fig. 2a) and connected vesicles (connected porosity
0.26 and isolated porosity 0.04). In addition, a significant portion of
the water contents of the original dome glass are not accounted for by
the water species measured by either spot or FTIR mapping. Water
contents that are effectively mapped by FTIR are shown to be stronglyenriched along the margins of the perlitic cracks (Fig. 5a). Our
experimental results show that our samples, where heated above
500 C lose most of their water within 2 h. However, changes in total
porosity do not occur until 800 C. All time series above 800 C
showed a small but continued degassing and an initial increase in
connected porosity associated with cracking and vesicle growth,
followed by a decrease in porosity associated with vesicle collapse.
Generally, the hotter the experiment the earlier in the time series the
porosity decrease occurred. Textural changes are summarized in
Fig. 6, experiments are subdivided into samples with (1) no textural
changes, (2) cracking/ inflation,(3) vesicle collapse/deflation,and (4)
textural obliteration. This subdivision is used to develop degassing
and deformation regimes which are discussed below.
5. Discussion
5.1. Comparison between natural and experimental samples
By comparing the textures of the naturally heated samples and the
experimentally heated samples we constrain the porosity and, by
inference, thepermeability history of thedome and conduit wall rocks
in response to reheating. In general, there is a striking similarity
between the textures of type 24 samples and the highertemperature
and long timescale experiments we performed (Figs. 2 and 3). The
textural similarities support our estimates of the timescales and
temperatures of reheating that were derived from the eruption
timescale (Keam, 1988). However, we do not imply that the thermally
driven textural changes in the natural samples occurred in-situ at theconduit margin; most of these erupted as bombs and probably
followed the thermal history described in Rosseel et al. (2006). The
open cracks and cracked vesicle walls of lower temperature and
shorter timescale experiments were not seen in our suite of naturally
heated samples, although this may be due to sampling bias.
The type 2 samples (Fig. 2b) have textures and porosity similar to
experiments for 24 h at 1000 C (Fig. 3f). In both these rocks, glass
surfaces are smooth, crackshave healed but vesicles remain small and
only partially inflated.
The type 3 samples (Fig. 2c), appear similar to experiments above
1100 C (Fig. 3g and h) and show no evidence of the original textures
of the type 1 rock (Fig. 2a). However, the experiments do not exhibit
the large vesicle sizes and high porosities of the natural type 3
samples (Fig. 2d). In our experiments above 1100 C, at timescaleslonger than 40 min, vesicles coalesced, connected with the ambient
pressure and collapsed. The large size of the natural samples allowed
more vesicle growth and coalescence, and the developments of a
larger isolated porosity before depressurizing to ambient pressure.
Such an internal pressure could prevent collapse of the sample and
loss of porosity (Yoshimura and Nakamura, 2008). Alternatively,
vesiculation of these samples could have been aided by decompres-
sion as they erupted.
The type 4 samples are similar to experiments at 1200 C for
timescales greater than 30 min, no original vesicle textures resem-
bling the type 1 sampleremain. Theamoeboid outer shapes and dense
textures of the type 4 samples imply that they fully melted. Vesicles
are generally small, implying that larger vesicles coalesced, collapsed
or escaped by migration through the melt.
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The similarity of textures shared by the experimental and natural
rocks implies that reheating drove vesiculation and vesicle collapse in
the natural samples. Natural type 2 samples found in the basaltic
scoria reached temperatures of 1000 C for at least 24 h, whereas type
3 and 4 samples found in spatter imply higher temperatures and/or
longer hot residence times (Carey et al., 2008; Rosseel et al., 2006;
Sable et al., 2009). In summary, the comparison of Figures 2 and 3
show that the temperatures and timescales of reheating during our
experiments were appropriate for the eruption, and that largevariations in conduit wall porosity occurred during the eruption. An
ongoing study of the scoria and spatter filled basaltic dyke margins
exposed in the base of the fissure support this porosity variation.
Additionally, this fieldwork constrains the thermal impact of the
basalt to be between a few millimetres to tens of centimetres, in some
areas we identified a distinct 5 cm thick welded portion; and these
detailed field descriptions will be the subject of a future publication.
5.2. Degassing, and deformation regimes
Degassing and deformation proceeded differently in our experi-
ments relative to other experimental studies involving hydration of
melts (Baker et al., 2006; Gardner, 2007; Larsen et al., 2004; Takeuchi
et al., 2009; Yoshimura and Nakamura, 2008). Our samples were
composed predominantly of glass naturally hydrated by water along
perlitic cracks at temperatures well below Tg (Denton et al., 2009). As
a result this water can be outgassed at temperatures well below Tg(Tuffen et al., 2010). Isotopic studies indicate that this water is likely
to be meteoric in origin (DeGroat-Nelson et al., 2001; Friedman and
Smith, 1958; Friedman et al., 1966; Shane and Ingram, 2002). An
additional contrast to previous experiments is that our experimental
sample already had a high proportion of connected porosity through
cracks and vesicles. This distinction is important because therelationship between permeability and porosity is different during
vesicle growth and vesicle collapse (Michaut and Sparks, 2009; Rust
and Cashman, 2004). A sample that has undergone vesicle collapse
will have a higher ratio of permeability to porosity than a sample of
similar porosity that has not undergone vesicle collapse (Michaut and
Sparks, 2009; Mueller et al., 2008). The complexities of this
relationship are most apparent when illustrated by up to 6-fold
increase in permeability over the porosity range of 3040% (Wright
et al., 2009). The permeability/porosity relationship is further
complicated by the presence of cracks with the ability to open and
to heal (Yoshimura and Nakamura, in press). However, all our
experiments had the same starting texture and we are confident that
incremental changes in open porosity correlate with changes in
permeability.
Fig. 4. Experimental results summarized as total porosity vs. connected porosity and labelled by experimental times. Open headed arrows follow a temporal evolution of increasing
experimental time. A dotted 1:1 line indicating equality between connected and total porosity is shown on all plots; the vertical distance between each data point and this line
measures theisolated porosity withineach sample. a) Time seriesat 800 C;insetof solid headedarrows shows howporosityis affected by various relevant processes.b) Time series
at 900 C. c) Time series at 1000 C. d) Time series at 1100 C.
8 B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
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On reheating, our experiments imply that the porosity and
permeability structure of lava domes or plugs will pass through a
series of regimes, the nature of which depend on 1) the temperature
of the event; 2) the time spent at that temperature (Fig. 6a); and 3)
the volatile content and original vesicularity of the rock being heated.
Physically, the character of the change in porosity and, by inference
the permeability, depends on whether dome rocks respond in a
viscous or brittle way to heat transfer from the newly erupting
magma. Thus, the key parameter is the glass transition, which
depends on the temperature of the event relative to the glass
transition temperature Tg (Knoche et al., 1994) and the time scale ofthe event relative to a critical relaxation time for structural failure of
the melt. Consistent with experimental results, we take this critical
time to be proportional to the viscous relaxation time r=/G (Webb
and Dingwell, 1990) (Table 1). Here, G =1010 Pa is the elastic
modulus, is the melt viscosity at a given temperature and water
content and C is a constant to be determined from our experiments.
We take Tg to be the temperature corresponding to a viscosity of
1011.4 Pa s which correlates well with the calorimetric Tg (Giordano
et al., 2008). The value ofTg is calculated to be 740 C for this rhyolite
glass composition (Nairn et al., 2004) at 0.1 wt.% H2O using the
viscosity model of Giordano et al. (2008) (see Supplementary Table
A2). Thetransition from brittle to viscous behaviour(Regimes1 to 3 in
Fig. 6) depends on the temperature (T) relative to Tg and the time
scale for the experiment (t) (or eruptive event).
To gain additional insight we replot the data in Figure 6a in terms
ofTnormalized to Tgand tnormalized to r(Fig. 6b). Thedata collapse
to a power law defining the transition from brittle to viscous
behaviour of the form T/ Tg=C(t/r)0.046, where C ~ 0.86. This result
shows that whether or not brittle processes govern the final porosity
of the sample depends on the response time of the melt, which is
governed by its strongly temperature-dependent viscosity, relative to
the time scale of the thermal forcing applied in the experiment.
We use our experimental degassing and deformation data to
propose four degassing and deformation regimes (Fig. 6). In Regime 1
(T800, t2 h; T/TgNCt/r0.046) degassing occurs without significantdeformation (Fig. 6). The rock maintains its vesicle structure
connected by microcracks and ruptured vesicle walls (Fig. 2a, b)
and consequently maintains a high proportion of connected to total
porosity (Fig. 4). No visible changes to this structure could be seen
with the SEM. Between 0 and 700 C we attribute this initial H2O
degassing to (1) release of water captured in micropores and/or low
temperature alteration hydrous minerals (Denton et al., 2009), and
(2) diffusion of resorbed meteoric water out of the perlitic margins
(Fig. 5a) of the glass (Tuffen et al., 2010). 0.200.05 wt.% magmatic
water remains dissolved in the glass interior (Fig. 5). This initial
period of degassing had no impact on the textures of the rock as the
rock was not sufficiently above its calculated glass transition
temperature (Giordano et al., 2008) and ductile deformation did not
occur.
Fig. 5. a) FTIR spectroscopic images of a fragment of unheated original sample that broke along perlitic cracks: (i) photomicrograph of fragment; (ii) planar spectroscopic map of
absorbance of the molecular H2O band at 1630 cm1 (see Supplementary Fig. A1) across the fragment; the fragment is resting on a H2O-free IR-invisible KBr disk (absorbance=0);
(iii) three-dimensional view of same image indicating that there is some absorbance in the interior of the fragment. Note that one of the edges of the fragment is not enriched in
molecular H2O, suggesting that this edge did not form along a perlitic crack. Colour scale applies to both images. Molecular H2O contents are estimated using a constant molar
absorption coefficient and an average thickness and density for the whole fragment (also refer to Supplementary Fig. A1). b) Weight percent water plotted against temperature for
experiments lasting 2 h. Black squares show water contents of glass measured with FTIR spectroscopy and black circles show bulk water content measured by Leco induction.
Vertical bars show the range in water content from multiple analyses and are due to variation within samples.
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Regime 2 (800T1100 C, t6 h; T/TgC t/r0.046) is a transi-
tional regime showing both brittle and viscous deformation (Fig. 6).
Degassing of magmatic water (b0.1 wt.%) continues (Fig. 5) and
vesicle and crack expansion cause 520%increasesin overall porosity(Fig. 4). Observations show the porosity changes are due to local
viscous vesicle growth and coalescence with concurrent brittle
cracking in areasof high strain (Fig. 3ac). Our experimental samples
have initial porosities of 0.290.31, and connected porosity of 0.27;
the growth of isolated vesicles frequently impinge on other vesicles
(Fig. 3ac), resulting in coalescence and increasing connected
porosity with relatively small increases in isolated porosity
(Fig. 4a). In addition, the opening of cracks (Fig. 3b, c) connects
isolated vesicles, increasing the connected porosity and reducing
isolated porosity (Fig. 4a).
In Regime 3 (800T1200 C, t30 min; T/TgbCt/r0.046) porosity
loss occurs due to vesicle deflation (Fig. 6). This regime is
characterized by crack sealing and vesicle collapse leading to a
transient but generally decreasing overall porosity (Fig. 4). Both
processes involve viscous relaxation and appear to occur at similar
temperatures and timescales. Vesicle collapse is driven as the internal
pressure of the vesicle can no longer balance the surface tension and
gas leaks out of the vesicles through the connected porosity. Crack
healing occurs as melt connects opposite sides of a crack (Fig. 3e,f).
Healing of cracks reduces connected porosity and increases isolated
porosity (Fig. 4). However, generally, in our experiments observations
of crack healing correlate with decreases in overall porosity which can
only be explained by vesicle collapse (Figs. 3 and 4). Occasional smallincreases in porosity are seen in this regime and we interpret these to
be due to continued degassing and vesicle growth, as larger coalesced
vesicles collapse.
In Regime 4 (T1200 C, tN30 min; T/Tg NNCt/r0.046) textural
collapse occurs due to viscous flow of the melt in response to gravity
(Fig. 6). Although we could not measure the residual porosity of these
run-products, SEM image analysis shows a significantly reduced
porosity comprising isolated pores (Fig. 3k) implying a much reduced
permeabilty.
The temperature/time space of each of these regimes is ultimately
controlled by the viscosity of the melt/glass framework in the dome
lava which is itself strongly influenced by the dissolved water content
(inset Fig. 6a). The calculated dry value for Tg of this melt (see
Supplementary Table A2) is 770 C. The equilibrium volatile content
of the glass will be controlled by pressure and therefore the
openness and porosity of the volcanic system. An open, degassed
system such as the system reproduced by our experiments has a high
Tg of 740 C (~0.1 wt.% H2O) and is relatively difficult to remelt and
initiate Regimes 2, 3 and 4. However, a closed system with a higher
ambient pressure and equilibrium volatile content will have a
depressed Tg. For example, this melt with 1 wt.% H2O has a calculated
Tg of 606 C (see Supplementary Table A2, Giordano et al., 2008).
Therefore, if such a system is reheated it may achieve Regimes 2, 3,
and 4 at lower temperatures.
Thestyle of hydration, the original texture, andthe amount of time
a reheated rock/magma spendsin Regime 1 will strongly influence the
relationship between water content and its calorimetric Tg, and ability
to continue to other regimes. Similarly the amount of time a rock
spends in permeable Regimes 1 and 2 will influence the ability of anymagma beneath it to degas. For these reasons heating rates and
partially closed systems become important considerations and
avenues for future research.
6. Conclusions and implications for Tarawera
The frozen magma forming the conduit walls will be reheated
during the ascent and eruption of fresh magma. The time and
temperature dependent textural changes in the plug or walls have
implications for the monitoring of degassing and deformation of
active volcanoes, and on the resultant style and magnitude of an
eruption. In particular, a key issue is whether reheating leads to the
production or destruction of permeability in the conduit walls. For
example, enhanced permeability facilitates outgassing, which canreduce the overpressure in the intruding magma, and favour effusive
volcanism (e.g., Quane et al., 2009). In contrast, reduced permeability
can inhibit outgassing, leading to greater overpressure in the new
magma and an increased likelihood for explosive volcanism.
From our experiments, the evolution of wall rock texture during an
eruption depends on the temperature and duration of the event (Fig. 6).
In Regime 1 (T/TgNCt/r0.046), there is degassing of rehydrated meteoric
water from the rhyolite. For Tarawera dome rock, up to 1.4 wt.% water
could be released by conduit wall and old dome rocks into the erupting
magma during reheating. In Regime2 (T/TgCt/r0.046) both ductile and
brittle deformation cause increases in connected porosity and inflation.
This increase in connected porosity should correlate with an increase in
permeability and hence aid the degassing of the fresh magma below.
Regime 3 (T/Tgb
Ct/r0.046
) is a result of vesicle collapse and porosity
Fig. 6. a) Sample deformation style separated into Regime 1 no deformation, Regime 2
cracking/ inflation, Regime 3 vesicle collapse/ deflation, and Regime 4 textural
obliteration plotted in time and temperature space.The dashedlines markapproximate
boundaries between regimes. The arrow suggests a heating trajectory based on the
thickness of the remelted dyke margin measured in the field. The curved solid line
represents the calorimetric Tg which increases as water is lost; below this line
conditions are equivalent to Regime 1. The line is calculated approximating time to a
decreasing H2O content (measured by FTIR spectroscopy, Fig 5b) and from the
calorimetric Tg calculated using Giordano et al. (2008; see inset and Supplementary
Table A2). b) Tnormalized to Tg and tnormalized to a critical strain rate for structural
failure of the melt (r). This plot shows that transitional Regime 2 plots at T/Tg~t/r0.046,
below this critical power law degassing occurs through pre-existing structures and
above power law viscous behaviour allows vesicle collapse.
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reduction. Although a connected pore structure still remains, perme-
ability will be reduced progressively closing the degassing system
(Westrich and Eichelberger, 1994; Yoshimura and Nakamura, 2008;
Yoshimura and Nakamura, in press). Regime 4 produces a generally
isolated pore structure and permeability would be significantly
decreased causing pressure build up, that could give rise to explosive
eruptions.
The magnitude of thermally-induced permeability change is
expected to depend on the length scale L = 2
ffiffiffiffiffi
t
p
, where the thermaldiffusivity k of potentially foamed rhyolitic dome rocks is order
107 m2 s1 (Bagdassarov and Dingwell, 1994). For Tarawera, over the
proposed 524 hour magma rise and eruption time (Houghton et al.,
2004; Keam, 1988) L is likely to be approximately 825 cm (Fig. 6a).
Indeed, on-going field studies of the margins of basalt dykes in the
lava domes reveala 5 cm thick remelted layer of rhyolite with reduced
porosity. This 5 cm thick remelted layer is remarkably consistent with
the eruption timescale and thermal diffusion length scale (Fig. 6a).
However, detailed descriptions of these dyke margins are not yet
complete and beyond the scope of this paper.
For the 1886 eruption, we propose a reheating trajectory of 5 h
(Fig. 6a) which is consistent with (1) the eruption timescale
(Houghton et al., 2004; Keam, 1988), (2) the presence of all three
naturally heated sample types, and (3) the thicknesses of remelted
dyke margins.
The volume of AD 1314 Tarawera rhyolite affected is difficult to
estimate dueto uncertainties in thesubsurfacegeometryof these and
older rhyolite lavas and feeder dykes, and their relationship to the
basalt dyke (e.g. Fig. 4 in Carey et al., 2007). We take the basalticfissure to be 8 km long (Fig. 1) in map view and 0.28 km deep (the
lower bound assumes that the basalt interacts only with the shallow
dome; the upper bound implies interaction with a conduit over the
full depth to the 8 km rhyolitic magma source (Shane et al., 2008)).
Over the 5 houreruption duration the intruding magma will heatand
degas the surrounding rhyolitic wall rocks to a distance of 8 cm
(Fig. 6a). From these dimensions and assuming that the wall rocks
contain 1.4 wt.% H2O, reheating would release around 6.4106
2.6108 kg of H2O. From our experiments, the majority of this
volume would be released within the first 2 h of reheating and maysignificantly contribute to precursory signs of eruption, although it
may not be significant in terms of the total volatile budget of the
eruption.
The similarity between the textures in the rhyolite erupted from
Tarawera volcano and the experiments defining Regime 4 imply that
conduit-wall permeability may have been catastrophically reduced
during the early stages of the Tarawera eruption. We propose that
such a sealing of the system may have produced a closed system and
aided the unusually explosive basaltic eruption of Tarawera in 1886.
This offers an additional and complimentary explanation to the
driving mechanisms suggested by previous authors e.g., phreato-
magmatic interactions and bubble/microlite coupling (Carey et al.,
2007; Sable et al., 2009).
Finally, our work also has implications for eruptions related to re-intrusion of similar temperature magma. A closed pressurized lava
dome and conduit with 13 wt.% H2O (e.g. Burgisser et al., 2010)
could potentially depress Tg (Fig. 6a inset) and hence allow melting
associated with the re-intrusion of magmas of similar composition
(Giordano et al., 2008), i.e. a 700 C rhyolite could re-intrude and melt
a water rich rhyolite with a Tg depressed to 600 C. Our work predicts
that as an old lava dome or plug heats up it will initially inflate,
partially degas, then deflate, and seal up leading to continued
pressurization and possibly eruption (Fig. 6). This sequence may be
followed by another reheating event and lead to the cyclic
deformation observed at many lava domes ( Johnson et al., 2008;
Matthews et al., 1997; Voight et al., 1999).
Supplementary materialsrelated to this article canbe found online
at doi:10.1016/j.epsl.2010.08.028.
Acknowledgements
Funding for MJ was provided by NSERC, Canadian Institute for
advanced Research, and Marsden (UOC0508). Funding for BK was
provided by Marsden Fast start (09-UO-017C). Help with measuring
water contents was provided by Paul Wallace, and John Stix.
Additional help with field access was provided by Ken Ruaeti and
the Ruawahia 2b Trust and Judy Collins of Mt Tarawera tours, Paul
Ashwell, Felix VonAulock, and FabianWadsworth. We would also liketo thank reviewers Dr. Heather Wright and Dr. Hugh Tuffen for their
detailed and insightful reviews that significantly improved the
manuscript.
References
Bagdassarov, N., Dingwell, D., 1994. Thermal properties of vesicular rhyolite. J. Volcanol.Geoth. Res. 60, 179191.
Baker, D.R., Lang, P.G., Robert, G., Bergevin, J.F., Allard, E., 2006. Bubble growth inslightly supersaturatedalbite meltat constant pressure. Geochim. Cosmochim.Acta70, 18211838.
Brooker, R.A., Kohn, S.C., Holloway, J.R., McMillan, P.F., 2001. Structural controls on the
solubility of CO2 in silicate melts: Part II: IR characteristics of carbonate groups insilicate melts. Chem. Geol. 174, 241254.Burgisser, A., Poussineau, S., Arbaret, L., Druitt, T.H., Giachetti, T., Bourdier, J.-L., 2010.
Pre-explosive conduit conditions of the 1997 Vulcanian explosions at SoufrireHills Volcano, Montserrat: I. Pressure and vesicularity distributions. J. Volcanol.Geoth. Res. 194, 2741.
Carey, R.J., Houghton, B.F., Sable, J.E., Wilson, C.J.N., 2007. Contrasting grain size andcomponentry in complex proximal deposits of the 1886 Tarawera basaltic Plinianeruption. Bull. Volc. 69, 903926.
Carey, R.J., Houghton, B.F., Thordarson, T., 2008. Contrasting styles of welding observedin the proximal Askja 1875 eruption deposits II; local welding. J. Volcanol. Geoth.Res. 171, 2044.
Cole, J.W., 1970. Structure and eruptive history of the Tarawera complex. NZ J. Geol.Geophys. 13, 879902.
DeGroat-Nelson, P.J., Cameron, B.I., Fink, J.H., Holloway, J.R., 2001. Hydrogen isotopeanalysis of rehydrated silicic lavas: implications for eruption mechanisms. EarthPlanet. Sci. Lett. 185, 331341.
Denton, J.S., Tuffen, H., Gilbert, J.S., Odling, N., 2009. The hydration and alteration ofperlite and rhyolite. J. Geol. Soc. 166, 895904.
Friedman, I., Smith, R., 1958. The deuterium content of water in some volcanic glasses.Geochem. Cosmochem. Acta 15, 218228.
Friedman, I., Smith, R.L., Long, W.D., 1966. Hydration of natural glass and formation ofperlite. Bull. Geol. Soc. Am. 77, 323328.
Gardner, J.R., 2007. Bubble coalescence in rhyolitic melts during decompression fromhigh pressure. J. Volcanol. Geoth. Res. 166, 161176.
Giordano, D., Russell, J.K., Dingwell, D.B., 2008. Viscosity of magmatic liquids. EarthPlanet. Sci. Lett. 271, 123134.
Gonnerman, H.M., Manga, M., 2003. Explosive volcanism may not be an inevitableconsequence of magma fragmentation. Nature 426, 432435.
Gonnermann, H.M., Manga, M., 2007. The fluid mechanics inside a volcano. Ann. Rev.Fluid Mechs. 39, 321356.
Hammer, J.E., Rutherford, M.J., 2002. An experimental study of the kinetics ofdecompression-induced crystallisation in silicic melt. J. Geophys. Res. 107, 124.
Houghton, B.F., Gonnerman, H.M., 2008. Basaltic explosive volcanism: constraints fromdeposits and models. Chem. Erde 68, 117140.
Houghton, B.F., Wilson, C.J.N., Del Carlo, P., Coltelli, M., Sable, J.E., Carey, R., 2004. Theinfluence of conduit processes on changes in style of basaltic Plinian eruptions:Tarawera 1886 and Etna 122 BC. J. Volcanol. Geoth. Res. 137, 1 14.
Jaupart, C., 1998. Gas loss from magmas through conduit walls during eruption. In:Gilbert, J.S., Sparks, R.S.J. (Eds.), The Physics of Explosive Volcanic Eruptions: Geol.Soc. Sp. Pubs, 145, pp. 7390.
Johnson, J.B., Lees, J.M., 2000. Plugs and chugs seismic and acoustic observations ofdegassing explosions at Karymsky, Russia and Sangay, Ecuador. J. Volcanol. Geoth.Res. 101, 6782.
Johnson, J.B.,Lees, J.M.,Gerst, A., Sahagian, D., Varley, N., 2008.Long-period earthquakesand co-eruptive dome inflation seen with particle image velocimetry. Nature 456,377381.
Keam, R.F., 1988. Tarawera: the volcanic eruption of 10 June 1886, p. 472. Published bythe author, Auckland, New Zealand.
Kennedy, B., Spieler, O., Scheu, B., Kueppers, U., Taddeucci, J., Dingwell, D.B., 2005.Conduit implosion during Vulcanian eruptions. Geology 33, 581584.
Knoche, R., Dingwell, D.B., Seifert, F.A., Webb, S.L., 1994. Non-linear properties ofsupercooled liquids in the system Na2OSiO2. Chem. Geol. 116, 116.
Larsen, J.F., Denis,M.H.,Gardner,J.E.,2004.Experimental study ofbubble coalescence inrhyolitic and phonolitic melts. Geochim. Cosmochim. Acta 68, 333344.
Lavalle, Y., Hess, K.-U., Codonnier, B., Dingwell, D.B., 2007. Non-Newtonian rheological
law for highly crystalline dome lavas. Geology 35, 843
846.
11B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing andexplosivity at Tarawera volcano, New Zealand, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.08.028
http://dx.doi.org/10.1016/j.epsl.2010.08.028http://dx.doi.org/10.1016/j.epsl.2010.08.0288/3/2019 B.M. Kennedy et al- Time-and temperature-dependent conduit wall porosity: A key control on degassing and explos
12/12
Lavallee, Y., Meredith, P.G., Dingwell, B.D., Hess, K.-U., Wassermann, J., Cordonnier, B.,Gerik, A., Kruhl, J.H., 2008. Seismogenic lavas and explosive eruption forecasting.Nature 453, 507510.
Liu, Y., Zhang, Y., Behrens, H., 2005. Solubility of H2O in rhyolitic melts at low pressuresand a new empirical model for mixed H2OCO2 solubility in rhyolitic melts. J.Volcanol. Geoth. Res. 143, 219235.
Long, W., Friedman, I., 1968. The refractive index of experimentally hydrated rhyoliteglass. Am. Mineral. 53, 17541756.
Matthews, S.J., Gardeweg, M.C., Sparks, R.S.J., 1997. The 1984 to 1996 cyclic activity ofLascar Volcano, northern Chile; cycles of dome growth, dome subsidence,degassing and explosive eruptions. Bull. Volcanol. 59, 7282.
Melnik, O., Sparks, R.S.J., 2002. Modeling of conduit flow dynamics during explosiveactivity at Soufrire Hills Volcano, Montserrat, West Indies. In: Druitt, T.H.,Kokelaar, B.P. (Eds.), The Eruption of the Soufrire Hills Volcano, Montserrat, from1995 to 1999: Geological Society of London Memoir, 21, pp. 307 317.
Michaut, B.D., Sparks, R.S.J., 2009. Ascent and compaction of gas rich magma and theeffects of hysteretic permeability. Earth Planet. Sci. Lett. 282, 258267.
Michol, K.A., Russell, J.K., Andrews, G.D.M., 2008. Welded block and ash flow depositsfrom Mount Meager, British Columbia, Canada. J. Volcanol. Geoth. Res. 169,121144.
Mueller, S., Scheu, B., Spieler, O., 2008. Permeability control on magma fragmentation.Geology 36, 399402.
Mungall, J.E., Bagdassarov, N.S., Romano, C., Dingwell, D.B., 1996. Numerical modellingof stress generation and microfracturing of vesicle walls in glassy rocks. J. Volcanol.Geoth. Res. 73, 3346.
Nairn, I.A., 2002. Geology of the Okataina Volcanic Complex. Inst. Geol, Nuc. Sci,geological map 25, 1 sheet +150p. Lower Hutt, NZ.
Nairn, I.A., Shane, P.R., Cole, J.W., Leonard, G.J., Self, S., Pearson, N., 2004. Rhyolitemagma processes of the VAD 1315 Kaharoa eruption episode, Tarawera volcano,New Zealand. J. Geotherm. Volcanol. Res. 131, 265294.
Newman, S., Stolper, E.M., Epstein, S., 1986. Measurement of water in rhyolitic glasses:calibration of an infrared spectroscopic technique. Am. Mineral. 71, 15271541.
Nichols, A.R.L., Wysoczanski, R., 2007. Using micro-FTIR spectroscopy to measurevolatile contents in small and unexposed inclusions hosted in olivine crystals.Chem. Geol. 242, 371384.
Okamura, S., Nakamura, M., Nakano, T., Uesugi, K., Tsuchiyama, A., 2010. Sheardeformation experiments on vesicular rhyolite: implications for brittle fracturing,degassing, and compaction of magmas in volcanic conduits. J. Geophys. Res. 115.
Proussevitch, A.A., Sahagain,D.L.,Kutolin, V.,1993.The stabilityof foamsin silicic melts.J. Volcanol. Geoth. Res. 59, 161178.
Quane, S., Russell, J.K., Freidlander, B., 2009. Timescales of compaction in volcanicsystems. Geology 37, 471474.
Rosseel, J.-P., White, J.D.L., Houghton, B.F., 2006. Complex bombs of phreatomagmaticeruptions: role of agglomeration and welding in vents of the 1886 Rotomahanaeruption, Tarawera, New Zealand. J. Geophys. Res. 111, 124.
Rust, A.C., Cashman, K.V., 2004. Permeability of vesicular silicic magma: inertial andhysteresis effects. Earth Planet. Sci. Lett. 228, 93107.
Rust, A.C., Cashman, K.V., Wallace, P., 2004. Magma degassing buffered by vapor flowthrough brecciated conduit margins. Geology 32, 349352.
Saar, M.O., Manga, M., 1999. Permeabilityporosity relationship in vesicular basalts.Geophys. Res. Lett. 26, 111114.
Sable, J.E., Houghton, B.F., Wilson, C.J.N., Carey, R.J., 2006. Complex proximalsedimentation from Plinian plumes: the example of Tarawera 1886. Bull. Volcanol.69, 89103.
Sable, J.E., Houghton, B.F., Wilson, C.J.N., Carey, R., 2009. Eruption mechanisms duringthe climax of the Tarawera 1886 basaltic Plinian inferred from microtexturalcharacteristics of deposits. In: Thordarson, T., Larsen, G., Rowland, S.K., Self, S.,Hoskuldsson, A. (Eds.), Studies in Volcanology: The Legacy of George Walker, pp.129154.
Sahetapy-Engel, S., Harris, A., 2009. Thermal structure and heat loss at the summitcrater of an active lava dome. Bull. Volc. 71, 1528.
Shane, P., Ingram, N., 2002. D values of hydrated volcanic glass: a potential record ofancient meteoric water and climate in New Zealand. NZ J. Geol. Geophys. 45,453459.
Shane, P., Smith, V.C., Nairn, I., 2008. Millennial timescale resolution of rhyolite magmarecharge at Tarawera volcano: insights from quartz chemistry and melt inclusions.Contrib. Mineral. Petrol. 156, 397411.
Smith, R., Sammonds, P.R., Kilburn, C.R.J., 2009. Fracturing of volcanic systems.Experimental insights into pre-eruptive conditions. Earth Planet. Sci. Lett. 280,211219.
Sparks, R.S.J., 2003. Dynamics of degassing. Geol. Soc. Sp. Pub. 213, 522.Stasiuk, M.V., Barclay, J., Carroll, M.R., Jaupart, C., Sparks, R.S.J., 1996. Degassing during
magma ascent in the Mule Creek vent (USA). Bull. Volcanol. 58, 117
130.Stolper, E., 1982. Water in silicate glasses: an infrared spectroscopic study. Contrib.Mineral. Petrol. 81, 117.
Stolper, E., 1989. Temperature dependence of the speciation of water in rhyolitic meltsand glasses. Am. Min. 74, 12471257.
Takeuchi, S., Nakashima, S., Tomiya, A., 2008. Permeability measurements of natural andexperimental volcanic materialswith a simple permeameter:toward an understandingof magmatic degassing processes. J. Volcanol. Geoth. Res. 177, 329339.
Takeuchi, S., Tomiya, A., Shinohara, H., 2009. Degassing conditions for permeable silicicmagmas; implications from decompression experiments with constant rates. EarthPlanet. Sci. Lett. 283, 101110.
Tuffen, H., Dingwell, D.B., Pinkerton, H., 2003. Repeated fracture and healing of silicicmagma generate flow banding and earthquakes? Geology 31, 10891092.
Tuffen, H., Smith, R., Sammonds, P.R., 2008. Evidence for seismogenic fracture of silicicmagma. Nature 453, 511514.
Tuffen, H., Owen, J., Denton, J., 2010. Magma degassing during subglacial eruptions andits use to reconstruct palaeo-ice thicknesses. Earth Sci. Revs. 99, 118.
Voight, B., Sparks, R.S.J., Miller, A.D., Stewart, R.C., Hoblitt, R.P., Clarke, A., Ewart, J.,Aspinall, W.P., Baptie, B., Calder, E.S., Cole, P., Druitt, T.H., Hartford, C., Herd, R.A.,
Jackson,P., Lejeune, A.M., Lockhart,A.B.,Loughlin, S.C., Luckett, L.,Lynch, R.,Norton,G.E., Robertson, R., Watson, I.M., Watts, R., Young, S.R., 1999. Instability and cyclicactivity at Soufriere Hills Volcano, Montserrat, British West Indies. Science 283,11381142.
Webb, S.L., Dingwell, D.B., 1990. The onset of non-Newtonian rheology of silicate melts.A fiber elongation study. Phys. Chem. Minerals 17, 125132.
Westrich, H.R., Eichelberger, J.C., 1994. Gas transport and bubble collapse in rhyoliticmagma: an experimental approach. Bull. Volcanol. 56, 447458.
Wooster, M.J., Kaneko, T., 1997. Satellite thermal analyses oflava dome effusion rates atUnzen Volcano, Japan. J. Geophys. Res. 103 (B9), 20,93520,947.
Wright, H.M.N., Cashman, K.V., Rosi, M., Cioni, R., 2007. Breadcrust bombs as indicatorsof Vulcanian eruption dynamics at Guagua Puchincha volcano, Ecuador. Bull.Volcanol. 69, 281300.
Wright, H.M.N., Cashman, K.V., Gottesfeld, E.H., Roberts, J.J., 2009. Pore structure ofvolcanic clasts: measurements of permeability and electrical conductivity. EarthPlanet. Sci. Lett. 280, 93104.
Wysoczanski, R., Tani, K., 2006. Spectroscopic FTIR imaging of water species in silicicvolcanic glasses and melt inclusions: an example from the Izu-Bonin arc. J.Volcanol. Geoth. Res. 156, 302314.
Yamashita,S., 1999.Experimental study of the effect of temperature on watersolubilityin natural rhyolite melt to 100 MPa. J. Pet. 40, 14971507.
Yoshimura,S., Nakamura,M., 2008. Diffusive dehydrationand bubble resorption duringopen-system degassing of rhyolitic melts. J. Volcanol. Geoth. Res. 178, 7280.
Yoshimura S., Nakamura M., in press, Fracture healing in a magma: an experimentalapproach and implications for volcanic seismicity and degassing. J. Geophys. Res.doi:10.1029/2009JB000834.
Zhang, Y., Belcher, R., Ihinger, P.D., Wang, L., Xu, Z., Newman, S., 1997. New calibrationof infrared measurement of dissolved water in rhyolitic glasses. Geochim.Cosmochim. Acta 61, 30893100.
Zhang, Y., Xu, Z., Zhu, M., Wang, H., 2007. Silicate melt properties and volcaniceruptions. Revs. Geophys. 45, 127.
12 B.M. Kennedy et al. / Earth and Planetary Science Letters xxx (2010) xxxxxx
Please cite this article as: Kennedy, B.M., et al., Time-and temperature-dependent conduit wall porosity: A key control on degassing and