Understanding silicic volcanism: Constraints from elasticity and failure of vesicular magma

Volcanic eruptions are one of the most spectacular and dangerous
natural phenomena. Volcanic activity can either be effusive,
dominated by quiescent emission of lava or explosive, dominated by
the eruption of pyroclastic material. A rapid transition between
these two regimes is possible. possible. On a broad scale, a clear
distinction of the eruption style can be drawn by magma
composition. Large-scale basaltic eruptions are mostly effusive,
whereas large-scale silicic eruptions are mostly explosive. Thus
silicic volcanism possesses a severe hazard potential and can have
devastating effects on human population. This work comprises
experimental investigations of the fragmentation behavior of porous
magma as well as of the propagation of elastic waves within this
material. The analyses were conducted on samples from Unzen
Volcano, Japan as well as samples from Soufrière Hills Volcano,
Montserrat, West Indies. The elastic wave velocities of differently
porous Unzen dacite samples were investigated with the use of a
cubic multi-anvil press. The main results of this study show that
porosity (density) and texture affect the elastic properties of
samples at a given temperature. In particular, it can be stated:
(1) Seismic velocities increase with pressure due to compaction and
closing of microcracks. The Vp anisotropy decreases with pressure
for the same reason. (2) Increasing the temperature also leads to
higher elastic wave velocities and lower anisotropies. This must be
highlighted as the inverse behavior is documented for the majority
of rocks. The effect may be linked to reduction of pore volume and
further closing of microcracks due to reduction of cooling
tensions. At 600 °C mean Vp values of 4.31 - 5.64 km/s and mean Vs*
values of 2.20 - 3. 32 km/s could be determined. (3) The velocity
anisotropy can be linked to the texture of the samples: Those with
a high anisotropy show a pronounced shape-preferred orientation of
phenocrysts and microcrystals, sometimes in addition to layering
within the groundmass of the sample. Since the crystals are
typically aligned parallel to walls of volcanic conduits, the
velocity normal to the walls is likely to be reduced. The data
allows better estimates of the properties of silicic volcanic rocks
at shallow depths within volcanoes, e.g. at conduit walls. These
estimates are vital for computation of conduit models as well as
the modelling of volcano seismic data, and may lead to an improved
analysis of precursor phenomena in volcanic areas. The physical
properties of magma within volcanic conduits and domes are crucial
for modelling eruptions. This study comprises a detailed
investigation of the fragmentation behavior (threshold and
propagation speed) of differently porous sets of dacitic and
andesitic samples derived from Unzen Volcano, Japan and Soufrière
Hills Volcano, Montserrat, West Indies. The experiments were
performed with a shock-tube based fragmentation apparatus and
pertain to the brittle fragmentation process. The results show a
strong influence of the open porosity and the initial pressure on
the fragmentation behavior. The speed of fragmentation follows a
logarithmic relationship with the pressure difference, the
fragmentation threshold an inversely proportional power-law
relationship with increasing porosity. In this study fragmentation
speed values ranging from 15 - 150 m/s were observed for applied
pressure differences of up to 40 MPa and open porosities from 2.5 -
67.1 %. The expansion of the pressurized gas in the vesicles
largely provides the energy driving the fragmentation process. The
fragmentation speed results of all analyzed samples show a close
relationship to the energy density (fragmentation energy
standardized to volume). A logarithmical increase of the
propagation speed was observed with the energy density as soon as
the energy threshold of 2.0 x 0.5 J/m³ was exceeded. The
fragmentation speed is independent from the origin and composition
of the samples, proving the governing role of the energy to the
initiation a propagation of fragmentation process. Different
fragmentation mechanisms were discussed and the layer-by-layer
fragmentation due to vesicle bursting is concluded to be the main
process responsible for the disintegration of vesicular rocks. The
increased importance of fracturing due to the passing of the
unloading wave after a rapid decompression could be proved for low
porous samples. Further the influence of the sample’s permeability
on the fragmentation behavior was evaluated. It could be shown that
a high permeability hinders the initiation of a fragmentation and
reduces the propagation speed of this process at a certain energy
density. The fragmentation results were applied to the dome
collapse events and Vulcanian events of the 1990-1995 Unzen
eruption and the 1997 Vulcanian events at Montserrat. Large blocks
with layers of various porosity were observed at the block-and-ash
flow deposits of Unzen Volcano and support the model that a dome
and dome lobes consist of areas of differing porosity. In addition,
the samples gained from Montserrat, allow to postulate a porosity
gradient within a volcanic conduit, with low porous magma close to
the conduit walls. A layered composition of a dome and dome lobes,
respectively, may lead to the fragmentation of single layers,
followed by the collapse of the overlaying sections. These events
could catalyze gravitationally induced dome collapse events leading
to vigorous pyroclastic flows and / or trigger a sector collapse
followed by an Vulcanian event. A porosity gradient at the magma in
the conduit leads to a concave shape of the fragmentation surface
and facilitates lateral fragmentation of dense magma close to the
conduit walls. Conduit implosion may to occur during most explosive
eruptions and is likely to influence the cessation or pulsation of
the eruption. The slow magma ascent and extrusion rate at Unzen
resulted in relatively dense extruded magma, as the magma could
almost completely degas during the ascent. The low porosity of this
magma causes a high fragmentation threshold of most material, which
is too high for unassisted fragmentation. Therefore dome collapse
events were the most abundant events of the 1990-1995 activity of
Unzen Volcano, leading to numerous block-and-ash flows. Also the
fragmentation-amplified collapse of dome lobes or parts of the dome
are reasonable. This accounts especially for the long lasting
collapse events with vigorous pyroclastic flows at the early stage
of the eruption in June 1991, which were followed by minor
Vulcanian events. Nevertheless a larger explosive event would have
been possible, triggered by a landslide or a sector collapse of the
dome. Similarly to Unzen, the first phase of activity of the recent
eruption of Montserrat is characterized by numerous dome collapse
events leading to violent pyroclastic flows. As the magma extrusion
rate was quite high during this phase, the extruded magma was
higher vesiculated compared to the Unzen magma and thus a more
violent evolution of the eruption activity took place. Large dome
collapse events frequently caused Vulcanian events, and even two
cycles of Vulcanian activity from August to September 1997
occurred. The calculations of the fragmentation depth, reached by
this explosions yielded about 1500 m, based on the laboratory
gained fragmentation speeds, which is in good agreement numerical
models and observations. In silicic volcanic systems the conduit
seems not to be sharply defined. The conduit walls are more to be
seen as a kind of transition zone between a hot, ductile and
vesicular magma within the conduit and the host rock. The rocks
forming this transition zone are assumed to be quite hot, but
presumably below glass transition and react therefore solely
brittle. Furthermore these rocks should be quite dense, compared to
magma in the conduit, and heavily fractured due to the high shear
strains this zone is presumably exposed to. The transition zone is
less likely affected by a fragmentation event. Their rocks (magma
as well as host rocks) are too dense to fragment due to pore
pressure. The needed pressure difference is unrealistically high,
for example an overpressure of 18 MPa would be needed to initiate
the fragmentation of rocks with a porosity of 7.5 %. Nevertheless
this material may be found in the deposits of explosive events, due
to processes like conduit wall erosion or as remnant of dense
lenses within more porous areas. Indeed, also fragmentation may
take place, the most likely process fragmenting even this dense
material is by lateral fragmentation, which may occur from a
certain depth on behind a fast propagating fragmentation of highly
vesicular magma at the center of the conduit. The style and
progression of an eruption is depending on the properties of this
vesicular magma. If its fragmentation threshold can be exceeded, an
explosive event may take place. Otherwise the magma is extruded
quiescent in a dome forming eruption. The transition zone bears
important implications on the one hand for the explosive event as a
lateral fragmentation of a certain area may cause cessation or
pulsation of the event, on the other hand for the propagation of
seismic signals related to the eruption. Within this transition
zone as well as the nearby host rock a decisive change of
temperature and porosity can be supposed. This leads to a
significant shift of the elastic wave velocities within this zone,
sometimes resulting in trapped waves within this zone as observed
for Montserrat. Especially the abnormal velocity increase with
increasing temperatures has to be mentioned. Thus implications for
an overall view of a volcanic system are provided in this study,
with the transition zone as the common link. The properties of the
elastic wave velocities account for the for host rock as well as
the transition zone and bear vital constraints for the
interpretation and modelling of volcano seismic data. The results
of the fragmentation experiments are applicable for dome rocks, the
vesicular interior of a conduit as well as the transition zone and
contain important implications for the modelling of conduit
processes. Together the results of this study may contribute to a
refined understanding of processes typical for silicic volcanism.
This may allow an improved analysis of precursor phenomena in
volcanic areas and consequently provide important constraints to
the hazard and risk management.