Non-Newtonian effects in silicate liquids and crystal bearing melts

High-silica volcanic systems are considered to be the most
devastating. Their highly viscous properties create a high
pressurised non- fluent system which consequently relaxes the
stress mostly by exploding through the brittle regime. Even if an
explosion is avoided and the magma fl ows, it often generates lava
domes at the top of the volcano; which, patiently, accumulate
magmas that will rush down the slopes once the yield stress is
crossed. Thus, such volcanoes have an explosive nature and often
generate catastrophic pyroclastic flows. The modelling of magma
ascent inside eruptive conduits is commonly based on fl uid
mechanic principles. The difficulty of the approach is however not
as much driven by the physical equations of the numerical model as
the variability of the parameters of the magma itself. It is well
established that the rheology of magma strongly depends on the
temperature, the stress, the strain, the chemical composition, the
crystal and the bubble contents. In other words, magmatic modelling
involves a set of movement equations which call for a
comportmental/rheological law. The movement equations give roughly
equivalent results through the different models; however magma
rheology is poorly controlled. The deformation of highly
crystallised dome lavas is key to understanding their rheology and
to fixing their failure onset. It is thus essential to adequately
understand magma rheology before performing complex numerical
models. Here we focus on the well studied Unzen volcano, in Japan,
which had a recent period of activity between 1990 and 1995. The
dome building eruptions in Unzen generate repeated dome failure and
pyroclastic ows. They vary in character and behaviour from effusive
domes to brittle pyroclastic events. Since then, the Unzen Scientic
Drilling Project, initiated in April 1999, drilled through the
volcano and sampled the eruptive conduit. This provides us with
rare original samples for study and characterisation. The
physico-chemical properties of these valuable samples were
determined with an array of devices. Of these a large uniaxial
deformation press, which can operate at high load (0 up to 300KN),
and temperature (25-1200°C), will be of utmost importance. This
press deforms the samples under known parameters and allows us to
determine the viscosity of the melt. In this study we investigate
the stress and strain-rate dependence on several glasses and Mt
Unzen dome lavas. Their rheology has been determined for
temperatures from 900 to 1010°C and stresses from 2 to 120 Mpa (60
MPa for crystal bearing melt) in uniaxial compression . This survey
aims to distinguish the Non-Newtonian effects perturbing magmatic
melts, also known as indicators of the brittle field. Towards our
experiments we observed three majors viscosity decrease types: Two
were dependant and typical of the solid fraction (Shear thinning
& Time weakening effect). The first is instantaneous and on the
whole recovered during stress release. The second is time dependent
and non-recoverable. The third and last effect observed is
attributed to the melt fraction and its self heating under stress
(Viscous Heating Effect). We extensively investigated this last
eect on pure silicate liquids and crystal bearing melts. Our
findings suggest that most of the Non-Newtonians effects observed
in silicate melts are linked to a self heating of the sample and
can subsequently be corrected with the temperature without
involving other laws than a pure viscous material. Moreover we
observed that this self heating reorganise the energy distribution
within the sample and by localising the strain may favours the
formation of shear banding and the apparition of 'hot cracks'.
Crystal bearing melts exhibit two more Non-Newtonian eects. The
first one, the shear thinning, is typical of that observed in
previous experiments on crystal-bearing melts. On crystal free
melts, this viscosity decrease is observed at much lower magnitude.
We infer that the crystal phase responds elastically to the stress
applied and relaxes once the load is withdrawn. The second one, the
time weakening effect, appears more complex and this regime depends
on the stress (and/or strain-rate) history. We distinguish four
different domains: Newtonian, non-Newtonian, crack propagation and
failure domains. Each of these domains expresses itself as a
dierent regime of viscosity decrease. Due to stress localisation,
cracking appears in crystal-bearing melts (intra-phenocryst and/or
the in the melt matrix) earlier than in crystal-free melts. For low
stresses, the apparent viscosity is higher for crystal-bearing
melts (as predicted by Einstein-Roscoe equations). However, while
the stress (or strain rate) increases, the apparent viscosity is
decreasing to that of the crystal-free melt and could be even lower
if viscous heating effects are involved. Consequently, we emphasise
that any numerical simulation performed without taking into account
the strain-rate dependencies described above would overestimate the
apparent viscosity by orders of magnitude. The magma dynamics will
appears slower than in reality. Exaggerating the viscosity of a
volcanic dynamic system would overestimate the time range available
for a potential evacuation of the red zones. Applying a more
realistic rheology would improve the early warning tools and
improve the safety of the population surrounding volcanic systems.