Strain localisation during dome-building eruptions

Volcanic landscapes often present advantages for people who inhabit
the surrounding areas, but the increasing numbers of people
threatened by potential activity increases as these settlements
grow. It is thus of vital importance to glean as much information
as possible by monitoring active volcanoes (including seismicity,
ground deformation, gas flux and temperature changes). Although
volcanic behaviour can be difficult to predict, precursory
information can often be identified retrospectively (once an
eruption begins) to help link antecedent behaviour to eruption
attributes. Likewise, eruption relics can be used to identify
processes in pre-eruptive magma. Additionally, a huge amount of
information may be gathered through experimentation on rock and
magma samples. This study combines field and analytical studies of
natural samples from Volcán de Colima (Mexico), Mount St. Helens
(USA) and Soufrière Hills (Montserrat) with high-temperature magma
deformation experiments to investigate the processes involved with
magma ascent during dome-building eruptions (Figure S-1). The study
of conduit-dwelling magma is of the utmost importance for
understanding transitions from effusive to explosive eruptions. Of
primary interest is the rheology of highly crystalline magmas that
make up the magma column. Rheology is integrally linked to the
composition and textural state (porosity, crystallinity) of magma
as well as the stress, temperature and strain rate operative during
flow. Many studies have investigated the rheology of multi-phase
magmas, but in Chapter 2 this is notably linked to the evolution of
the physical properties of the magmas; tracing the changes in
porosity, permeability, Poisson’s ratio, Young’s modulus during
strain dependent magmatic flow. Especially at high strain rates
mechanical degradation of the magma samples may supersede magmatic
flow and crystal rearrangement as the dominant form of deformation,
resulting in lower apparent viscosities than those anticipated from
magmatic state. This leads to an evolution of the fracture network
to form inhomogeneous distribution of the permeable porous network;
with damage zones cutting through areas of densification. In a
conduit setting this is analogous to the formation of a dense,
impermeable magma plug which would prohibit degassing through the
bulk of the magma. Degassing may or may not proceed along conduit
margins, and the plug formation could lead to critical
overpressures forming in the conduit and result in highly explosive
eruption. During the multi-scale process of strain localisation it
is also probable that another previously unforeseen character acts
upon magma rheology. Chapter 3 details the first documentation of
crystal plasticity in experimentally deformed multi-phase magmas.
The extent of the crystal plasticity (evidenced by electron
backscatter diffraction (EBSD)) increases with increasing stress or
strain, and thus remnant crystals may be used as strain markers.
Thus it seems that crystal-plastic deformation plays a significant
role in strain accommodation under magmatic conditions. Indeed
plastic deformation of phenocrysts in conduit magmas may be an
important transitional regime between ductile flow and brittle
fracture, and a time-space window for such deformation is envisaged
during the ascent of all highly-crystalline magmas. This phenomenon
would favour strain localisation and shear zone formation at
conduit margins (as the crystal-plastic deformation leads the magma
toward brittle failure) and ultimately preferentially result in
plug flow. During volcanic eruptions, the extrusion of
high-temperature, high-viscosity magmatic plugs imposes frictional
contact against conduit margins in a manner that may be considered
analogous to seismogenic faults. During ascent, the driving forces
of the buoyant magma may be superseded by controls along conduit
margins; where brittle fracture and sliding can lead to formation
of lubricating cataclasite, gouge or pseudotachylyte as described
in Chapter 4 at Mount St. Helens. Within volcanic systems,
background temperatures are significantly higher than the geotherm
permits in other upper-crustal locations, whereas confining
pressures are much lower than in high-temperature, lower-crustal
settings: thus via their exceptional ambient P-T conditions,
volcanic systems represent unique environments for faulting. This
can result in the near-equilibrium melting and slow
recrystallisation of frictional melt, which hinders the development
of signature pseudotachylyte characteristics. Thus frictional
melting may be more common than previously thought. Indeed Chapter
5 documents a second occurrence at Soufrière Hills volcano. Here,
the formation is linked to repetitive seismic “drumbeats” which
occurred during both the eruption at Mount St. Helens and at
Soufrière Hills. Strain localisation, brittle rupture, sliding and
the formation of shear bands along the conduit margin can have
important implications for the dynamics of eruptions. Specifically,
the capability of degassing via the permeable porous network may be
strongly influenced by the formation of pseudotachylyte, which has
almost no porosity. Based on the findings in chapters 4 and 5, a
series of high-velocity rotary shear (HVR) experiments were
performed. In Chapter 6 the results of these experiments
demonstrate the propensity for melting of the andesitic and dacitic
material (from Soufrière Hills and Mount St. Helens respectively)
at upper conduit stress conditions (