V54B-01
Fragmentation and Cataclasis of Lava Domes: Field Evidence of Conduit-Margin Faulting and Cryptodome Unloading at Mount St. Helens
Structures and textures preserved in dome rocks reveal much about ascent history, seismicity, and dynamics of eruptions. The current eruption of Mount St. Helens (MSH) produced dacite spines mantled by fault gouge and breccia. Flow-banded spine interiors attest to early degassing and ductile deformation; micro-textures and structures in the spine margins indicate entirely brittle shear, rock breakage, grain-flow and gas-escape along fractures. Paleomagnetic pole positions and demagnetization data constrain cataclasis to the sub-vertical volcanic conduit at temperatures above 500°-570°C. Low water content of matrix glass and presence of tridymite require nearly complete decompression-driven solidification at depths <1 km, coincident with the eruption's seismogenic zone. 1-3 m thick cataclastic breccia of spine margins contains multiple Reidel shears in a conjugate set formed by shear between the vertically extruding spines and conduit walls. This breccia is overlain by a thin (<10 cm) outer mantle of finely comminuted gouge with 1-3 mm-thick, surface-parallel layers of slickenside-bearing ultracataclasite, forming through-going fault planes. Slickenside lineations and direction indicators are consistent with upward transport of the spines. These relations document two dominant modes of brittle failure in the spine margins, similar to the brittle S-C fabrics seen in tectonic fault zones. The Reidel shears represent limited-slip planes (S-shears), which are inclined relative to the primary bounding fault planes (C-surfaces). We infer that the Reidel shears formed as multiple, domino-like episodes of fracture, prior to transfer of slip to the bounding C-surfaces. Because the depth of deformation is the same as the depth of the seismogenic zone, and because there are two distinct modes of brittle fracture (S and C fabrics) as well as two distinct types of earthquakes (volcano-tectonic and longer-period hybrids) it is logical to infer that these structures are sources for many of the earthquakes at MSH. Additionally, grain-flow textures in the bounding ultracataclasite fault zones suggest that observed periods of aseismic extrusion were accommodated by creep on these faults. In contrast, Pine Creek age (~2.5 ka) dome dacite exposed over a large area at the mouth of the MSH crater is pervasively fractured over paleo-depths of >300m. Like the dacite of the current eruption, this rock has a microcrystalline groundmass, indicative of extensive shallow crystallization and subsurface solidification. However, the extent and character of fragmentation is unrelated to shear along conduit margins. Instead, a multi- stage history of brittle deformation consists of: 1) early 10-20cm thick subhorizontal zones of sandy cataclasite that repeat at intervals of meters to tens of meters within pervasively shattered dacite, 2) early cross-cutting high- angle faults with slickensides and <1mm pseudotachylite seams, and 3) later cross-cutting planar faults. We suggest that early cataclasis was produced by rapid unloading of the still-hot dome during a Pine-Creek age sector collapse and that the early high-angle faults represent boundaries of large mega-blocks that had begun to detach within the source dome. Together with mapped Pine Creek avalanche deposts nearby (Hausback, 2000), these features suggest that a transition from avalanche to still-intact Pine-Creek age cryptodome is exposed in the mouth of the MSH crater.
V54B-02 INVITED
Mount St. Helens Fault Gouge - Textural Constraints on Deformation Mechanisms
An unusual feature of the 2004–2006 eruptive activity of Mount St. Helens (MSH) has been the continuous growth of successive spines that are mantled by a zone 1-3 m thick composed of breccia, cataclasite and fault gouge. Individual fragments are holocrystalline, indicating that crystallization of the ascending magma preceded gouge formation. Here we examine both the transformation of solid dacite to cataclasite, and the further textural modifications that accompany deformation and disaggregation of cataclasite to form gouge, by detailed analysis of 8 oriented samples. We then compare textural features of the gouge with those of gouge from creeping and stick-slip segments of the San Andreas fault to better constrain the deformation mechanisms by which holocrystalline magma ascends to the surface. Initial disruption of solid dacite occurs by distributed dilation (porosity increase) and fracture. Fractures tend to follow phenocryst margins but also traverse the matrix and some larger crystals. Further deformation produces cataclasite with a wide range of grain size, a fine-grained (< 10μm) matrix, and rounding of larger fragments (commonly armored by powdered matrix). The outer, unconsolidated gouge zone contains finely comminuted shear zones (slickensides); crystals and lava fragments adjacent to these zones are shattered into small angular fragments that are entrained into shear trains to create a well-developed foliation. By comparison, microstructures in gouge from a creeping segment of the San Andreas fault record distributed shear deformation and porosity reduction without accompanying grain breakage; strain is accommodated by compaction, grain rotation, and possibly grain sliding and/or rolling. In contrast, fault gouge formed during the 1906 slip event contains deformation bands characterized by grain comminution and strongly localized slip, as reflected in abundant fractured and broken grains, and preferred grain orientations ~ 30° counterclockwise from the fault. From these observations we suggest that particle rolling and sliding may dominate when the grain size variation is large; this type of behavior seems to characterize shear in much of the MSH cataclasite. Extreme shear localization in the gouge, with accompanying extensive grain shattering and crushing, may reflect the onset of stick-slip behavior.
V54B-03 INVITED
The Effect of Strain Localization on the Rheology of Crystal-Rich Magmas
The rheology of magmas containing more than 40 vol. % of crystals is of particular interest to constrain the physical conditions of magmas during the extrusion of volcanic domes and necks. In this study we have conducted series of simple shear experiments on synthetic crystal-bearing hydrous rhyolitic material and on natural rocks from the Monte Nuovo eruption (Phlegrean Fields, Italy) in a Paterson-type internally heated rock-deformation apparatus. The experiments covered a wide range of crystallinities (50-80 vol. %) and strain rates (10-6-10-3 s-1). All experimentally deformed samples lack yield strength but they exhibit a tendency to non-Newtonian and Binghamian behavior at relatively high (>10-4 s-1) strain rates. The experiments have been performed under temperature and strain rate conditions where the melt phase behaves Newtonian. Moreover, in the range of strain rates applied, viscous heating is unable to account for the observed shear thinning effects (decrease of viscosity with increasing strain rate). Consequently, the non-Newtonian effects must be due to the presence of crystals in the magma. In experiments were shear weakening was indeed observed, the samples exhibit macroscopic localization features that are oriented at around 30° with respect to the shear plane. In all the other experiments, microstructural observation of cross-sections of the samples revealed melt-enriched planes and crystals alignments with an orientation identical to the localization features observed in the samples that experienced shear thinning. Numerical simulations have been conducted that were designed to reproduce the experiments and were, therefore, computed with identical stress and strain rate conditions as the experiments. Samples from both, the experiments and the numerical simulations reveal the generation of melt-enriched bands oriented at around ±30 ° with respect to the direction of flow. The melt enrichment induces localization of the strain in these regions and leads to a geometrical redistribution of melt and particles that is responsible for the complex rheological behavior of crystal-rich magmas.
V54B-04
A Fracture-Mechanical Model of Crack Growth and Interaction: Application to Pre-eruptive Seismicity
A greater understanding of the physical processes occurring within a volcano is a key aspect in the success of eruption forecasting. By considering the role of fracture growth, interaction and coalescence in the formation of dykes and conduits as well as the source mechanism for observed seismicity we can create a more general, more applicable model for precursory seismicity. The frequency of volcano-tectonic earthquakes, created by fracturing of volcanic rock, often shows a short-term increase prior to eruption. Using fracture mechanics, the model presented here aims to determine the conditions necessary for the acceleration in fracture events which produces the observed pre-eruptive seismicity. By focusing on the cause of seismic events rather than simply the acceleration patterns observed, the model also highlights the distinction between an accelerating seismic sequence ending with an eruption and a short-term increase which returns to background levels with no activity occurring, an event also observed in the field and an important capability if false alarms are to be avoided. This 1-D model explores the effects of a surrounding stress field and the distribution of multi-scale cracks on the interaction and coalescence of these cracks to form an open pathway for magma ascent. Similarly to seismic observations in the field, and acoustic emissions data from the laboratory, exponential and hyperbolic accelerations in fracturing events are recorded. Crack distribution and inter-crack distance appears to be a significant controlling factor on the evolution of the fracture network, dominating over the effects of a remote stress field. The generality of the model and its basis on fundamental fracture mechanics results makes it applicable to studies of fracture networks in numerous situations. For example looking at the differences between high temperature fracture processes and purely brittle failure the model can be similarly applied to fracture dynamics in the edifice of a long repose volcano and a lava dome.
V54B-05 INVITED
The SPASM Model of Seismogenic Solid-state Extrusion at Mount St. Helens
The 2004-2007 eruption of Mount St. Helens (MSH) has been characterized by nearly steady extrusion of a gouge- coated plug of solid dacite and by millions of small (<M2) earthquakes with foci <1 km beneath the extrusion site. For more than a year these earthquakes occurred so regularly (roughly once a minute) that they were dubbed "drumbeats." A recently derived mathematical model I call SPASM (for Seismogenic Plug of Ascending, Solidifying Magma) formalizes the hypothesis that the drumbeat earthquakes resulted from repetitive stick-slip motion along the subsurface margins of the extruding plug (Nature 244, 439-443, 2006). The model assumes that extrusion is driven by steady discharge of magma from a deep reservoir, and it assesses whether this steady discharge favors stable, steady extrusion at the surface, jerky extrusion characterized by stick-slip cycles, or some alternative behavior such as runaway acceleration or progressive plugging of the conduit. Analysis of the model equations shows that the basic requirements for sustained stick-slip behavior are simple and few: a magma ascent rate comparable to the plug accretion rate, a magma compressibility much greater than that of solid rock, and extrusion resistance provided by gravity and rate-weakening friction along the lateral margins of the solidifying plug. Data show that these requirements were likely met during the first 15 months of the 2004- 2007 MSH eruption. Indeed, with use of reasonable parameter values, the SPASM model yields predictions of repetitive stick-slip cycles with periods and amplitudes commensurate with occurrence of drumbeat earthquakes at MSH. The model also predicts, however, that several phenomena could gradually reduce the tendency for stick-slip behavior (as implied, for example, by diminishing drumbeats observed at MSH in 2006-2007). These phenomena include a decline in magma ascent rate, increase in plug-mass growth rate, and decline of plug- boundary friction or the rate-weakening effect. Changes in plug or conduit geometry could also affect extrusion dynamics, but such changes are inherently multidimensional and cannot be assessed with the current version of the SPASM model.
V54B-06 INVITED
From Magma Fracture to a Seismic Magma Flow Meter
Seismic swarms of low-frequency events occur during periods of enhanced volcanic activity and have been related to the flow of magma at depth. Often they precede a dome collapse on volcanoes like Soufriere Hills, Montserrat, or Mt St Helens. This contribution is based on the conceptual model of magma rupture as a trigger mechanism. Several source mechanisms and radiation patterns at the focus of a single event are discussed. We investigate the accelerating event rate and seismic amplitudes during one swarm, as well as over a time period of several swarms. The seismic slip vector will be linked to magma flow parameters resulting in estimates of magma flux for a variety of flow models such as plug flow, parabolic- or friction controlled flow. In this way we try to relate conceptual models to quantitative estimations which could lead to estimations of magma flux at depth from seismic low-frequency signals.
V54B-07
Dome surges, long period earthquake generation, and pyroclastic eruptions at Santiaguito Dome, Guatemala
Sudden horizontal surges of the 104 m2 Caliente dome at Santiaguito (Guatemala) are likely responsible for the frequent (1-2 per hour) long period (LP) earthquakes that have been consistently observed at this volcano for years. Dramatic dome surface movements, in which portions of the dacite/andesite dome are accelerated from rest to 4 m/s during a few tenths of a second (greater than 30 m/s2), were captured using a high resolution video camera from a vantage point 1200 m above, on the Santa Maria summit. During each event the surge was observed to propagate outward from the central "vent" and reach the crater periphery (more than 100 distant) after 1 to 2 s. This "deformation front" was observed to propagate at a velocity too slow for elastic waves and too quickly for buoyancy waves and appears to represent a static displacement. Assuming a conservatively thin dome thickness of 101 m and a laminar horizontal dome flow, the entire surge involves a moment gain of about 109 kg m/s during a time scale of about 1 s. An impulsive force of this magnitude can be expected to impart significant elastic energy to the surrounding country rock. Indeed, this source appears to be the cause of the corresponding LPs, which possess dominant frequencies of 0.5 to 2 Hz, and which were recorded by our temporary local network of six broad-band seismometers. Precursory low-amplitude seismicity (leading up to the LP) and explosive degassing (coincident with the LP) provide additional evidence linking the LPs to the observed acceleration events affecting the dome. We postulate that precursory seismicity reflects incremental failure of the dome and that the main LP and dome surge occurs only when cracks propagate up through the shallow dome itself. Explosive gas venting occurs when pathways have been opened throughout the 104 m2 surface extent of the dome, which has been violently accelerated. Resealing of these fissures is suggested by the termination of pyroclastic emissions over the course of a few tens of seconds. The episodic nature of these events, which cycle from quiescence, to precursory seismicity, to surge / LP / pyroclastic emission, to quiescence again on a time scale of 30-60 minutes suggests repeated stress accumulation due to a consistent influx of gas-charged magma of more than 104 m3 per day. This consistent flux of magma is also suggested by the long duration (more than 80 years) of continuous effusion of Santiaguito dome dacite. Further exploration of these relations between observed dome motion, precursory seismicity, LPs, and degassing events in the context of an episodic eruptive sequence may provide new insights regarding the mechanisms of eruption in volcanic domes in general.