V13E-01 INVITED
The Stability of Lava Lakes
Lava lakes may exhibit complex cycles of filling and draining on time-scales of hours to weeks. Such activity is recorded at Pu`u `O`o, Hawai`i. Other examples, e.g. Erta Ale in Ethiopia, do not display these draining episodes; instead an ‘equilibrium' system is observed in which the lake may persist for years. We present the results of a theoretical and experimental investigation of lava lakes and identify behavioural regimes distinguished by system geometry and gas content. Laboratory analogue modelling shows that even a simple conduit-lake system, driven by a constant gas flux, can display steady-state equilibrium or cyclic behaviour, depending on the rate of gas flux. A theoretical approach to the same system captures this range of behaviour. By testing the stability of an ‘equilibrium' lake to small perturbations, we show that the degree of stability is controlled by the ratio of conduit to lake areas, and the gas volume fraction at the top of the conduit. Despite the simplicity of the modelled system, a rich spectrum of behaviour is found. The model predicts that a stable system must drain over time. This implies that lakes which exhibit ongoing degassing for years or decades (e.g. Erta Ale) either must have an exogenous supply of gas bubbles from depth, or an effective conduit convection mechanism must exist.
V13E-02
Lava Lake Filling, Draining, and Density-Driven Crustal Foundering: Kilauea Iki 1959
The 1959 eruption of Kilauea formed an approximately 46x106 m3 lava lake during 17 episodes of Hawaiian fountaining. The vent location, on the steep walls of the Kilauea Iki pit crater, resulted in interplay between fountaining, pit filling and drain back from the lake into the vent. Previous studies of the Kilauea Iki lava lake have highlighted the post-eruption cooling and crystallization history, but little has been published regarding the processes of lake growth. The lava lake margins preserve features associated with filling, drainage and density-driven crustal foundering during the final eruptive episodes. Features associated with lake filling include relics of numerous vertically accreted layers preserved at the lake margins. The thickness of these onion-skin-like layers relates to variations in lake depth, the length of time a certain level was maintained, and cooling of the lake lava against the crater wall. Horizontal lava shelves are the relics of crusts formed as a result of surficial cooling during periods when the lake remained stable at a particular height for a period of time. Morphology, thickness, spacing and vertical placement of these shelves relate to the length of time a particular level was maintained, lava accretion style, as well as drainage velocity and cooling rates. Shelves were formed in both static environments (where crust standing in a fixed location thickened with time) and dynamic environments (where crust rolling along the lake edge accreted to the wall in a snow-ball-like fashion). Complete crustal overturn was observed repeatedly during the eruption and is a common process in all lava lakes, yet the initiating occurrence of crustal foundering is rarely preserved in situ. The northern margin of the 1959 lake reveals plates of lava plunging into the lake core and permits a detailed interpretation of the mechanics involved in crustal foundering during lake drainback. Density differences between (1) a lava lake crustal plate and (2) overriding and underlying material are several hundred kg/m3. These differences are substantial and lead to inevitable foundering of lake crust and eventually to complete crustal overturn.
V13E-03
Solitary Waves, Magma Migration and Dome Building Eruptions at Mt. St. Helens, Washington
Solitary waves have first-order attributes that include shape and volume conserving packets of fluid that migrate with characteristic wavelengths, amplitudes, wave numbers, and pulse durations. To ascend through dike-like, magma-filled fractures or sub-circular conduits, the solitary wave pulse duration is directly proportional to the conduit wall region viscosity and inversely proportional to the density contrast that drives the flow. Solitary waves are produced by the collapse of conduit wall rocks following the passage of a magma batch. The 1980-current eruptions at Mt. St. Helens display a variable time-series in their erupted volumes, as well as lava dome \(or spine\) heights / volumes and vent flow rates. Inter-eruption repose periods, however, have often shown broad regularity over extended periods. The rhythmic ‘beat' of eruptive episodes within a long-lived series and their roughly regular repose periods arises directly from the solitary wave migration mechanism. Composite domes are suggested to be the products of solitary wave incremental additions of dacite, as in the 1980-1983 composite dome resulting from at least 9 such solitary wave-controlled additions. The 18 May 1980 dacite cryptodome may now be interpreted as a composite of several solitary wave-based intrusions leading to the climatic eruption volume. Domes may be either solitary or composite but are built up of one or several batches of evolving magma that ascend individually from the 8 to 1 km depth storage reservoir as solitary waves. Analytical calculations of wave speed, wave length, batch volume, parcel shapes and repose periods reveal the dependence on material properties appropriate for Mt. St. Helens intrusions and dome-building eruptions. Predicted solitary wave volumes and flow rates are in good agreement with observed values for dacitic dome and spine-building eruptions from 1980-1986 and from 2004-2007. Conduit dimensions are inferred to vary over the range R=2 to R=20 m. Magma viscosities minimally span the inferred range 7E6 to 2E8 Pa s, appropriate for dacites of Mt. St. Helens composition, crystallinity, and water content at P and observed &\ computed T. Higher viscosities may prevail in the cooled carapace of the dome / spine. Second-order effects that modify pathways and compositions include heat loss to conduit wall rocks, and progressive crystallization episodes along conduit walls. Shallow vesiculation is a secondary effect superposed on a fundamental theme dominated by solitary wave-based magma transport.
V13E-04
Simulations of Stiffness and Yield Stress Growth in Crystal Networks
Magmas experience a change in rheology from Newtonian to Bingham flow upon developing a space-spanning crystal network. The crystal volume fraction at which this transition occurs can be predicted with percolation theory; however, the manner in which the yield stress grows with increasing crystal number densities is less-well understood. This presentation discusses a simple numerical model that aims to predict the growth of yield stress in bonded crystal assemblies above (and close to) the percolation threshold. The crystal assemblies are represented by soft-core interpenetrating cuboid particles, whose mechanical properties are reproduced in a network model. The model is used to investigate the influence of particle shape and alignment anisotropy on the yield stress of crystal networks with particle volume fractions above the percolation threshold. The predictions of the model are compared with results obtained from a critical path analysis. Good agreement is found between a characteristic stiffness obtained from critical path analysis, the growth in assembly stiffness predicted by the model (both of which have approximately cubic power-law exponents) and, to a lesser extent, the growth in yield stress (with a power-law exponent of 3.5). The effects of preferred particle alignment and the presence of bubbles on the yield stress are also briefly discussed.
V13E-05
A strategy for interpretation of magnetic susceptibility in lava flows: Application to the Tiretaine lava flow (Chaîne des Puys, France)
Anisotropy of Magnetic Susceptibility (AMS) is a convenient method for finding strain and thus emplacement information on lava flows. However, interpretation of AMS parameters requires caution and several factors need verification to avoid misunderstandings. Verification can be divided into 2 groups: 1) what minerals create AMS? 2) how AMS behaves? 1) For the magnetic minerals, thin sections are needed for estimating concentrations, sizes, shapes and compositions. Then Curie temperature determination will indicate if magnetites are the main AMS carrier. First Order Reversal Curves (FORCs) are then performed to find the magnetic type: multi domain (MD) or single domain (SD). 2) To find AMS behaviour, a comparison between AMS ellipsoid and microlite fabric ellipsoid shows how AMS is acquired and if it is controlled by the silicate template. An example was taken on a 5m-high vertical profile from a lava flow of the Tiretaine valley. Results show that AMS is carried by interstitial titanomagnetite, with a 5 to 50 μ m size. The FORCs show that AMS is mainly due to MD magnetites. The comparison between the magnetic and microlite fabric shows that AMS fabric is controlled by the silicate template. Magnetites are therefore late formed, i.e. during the late stages of emplacement, when significant cooling had occurred. Moreover kmax is parallel to the downflow direction and thus kmax is significant in term of maximal elongation axis. The degree of anisotropy profiles can not be used as a tool for estimating lava flow deformation as titanomagnetites cover a large size and shape range, and so the magnetic fabric is assumed to be stabilized. However, degree of anisotropy is not constant and displays several breaks linked with changes in the kmax dip. This indicates several horizontal interfaces. Compartments between interfaces can display kmax dip greater than 45°, greater than the incremental elongation axis during simple shear deformation. As a consequence pure shear may be contributing. As the pure shear is assumed to be constant in the lava profile, variations of simple shear intensity will allow kmax dip greater than 45°. The lava flow section should thus be considered to be undergoing general pure shear, but compartmentalised into different simple shear bands. A way to explain such difference of the simple shear amount is the existence in the lava flow of layers with different viscosity. A more viscous lava will undergo less simple shear than a more fluid one.
V13E-06 INVITED
The measurement and implications of short-term lava flux variability
Lava effusion rate is a critical parameter for flow models, with particular control over the potential maximum flow length attainable. However, effusion rate (or volume flux) can be extremely difficult to measure accurately in the field and is known to vary over a wide range of timescales. Here, we describe the application of computer vision and oblique photogrammetric techniques to both visible and thermal images of active aa flows in order to investigate distal flow processes at Mount Etna, Sicily, during the 2004-2005 eruption. Ground-based photogrammetric surveys were carried out (using a standard digital SLR camera) to produce repeated topographic datasets for calculation of volumetric lava flux at the flow fronts. Significant variations of the magma flux were detected (between ~0.05 and 0.35 m3s-1), and pulses of increased flux were visible in the distal channel region on timescales of several hours. The pulses are believed to result from more frequent flux changes which were observed in the vent region. They must thus also reflect the importance of some down- flow pulse coalescence process as well as short-period variations in effusion rate at the vent. Estimates of lava effusion rate can also be made from ground-based thermal data. The effect of the observed flow variations on the thermal data is described and the implications discussed in terms of observation frequency and distance. The lava flux at the vent was estimated to be around an order of magnitude larger than that at the flow fronts, suggesting that processes such as degassing, inflation and overflows were taking place in the unobserved medial part of the flow. Consequently, when considering the advance and stopping processes for these individual flow-fronts, it must be assumed that they were fed by a highly unsteady flux, which was volumetrically significantly lower than that at the vent.
V13E-07
The Influence of Crustal Thickness and Slope on the Surface Morphology of Active Lava Flows: an Experimental Approach
Many of the surface features that develop on `a`a and blocky lava flows relate to internal dynamics during flow emplacement, but it can be difficult to infer the precise relationships between morphology and dynamics from observations of flows either during or after their emplacement. Experiments using PEG have greatly improved our understanding of the behaviour of lavas with relatively thin crusts. Here we describe an alternative approach (similar to that of Lescinsky and Merle (2005), GSA Special Paper 396, p.136) in which the crust plays a significant role in flow development. Our experiments investigated the effect of crustal thickness and slope on the morphological development of channelised distal flows. The materials used were high viscosity (104 Pa s) silicone gel to simulate the still-fluid lava, and a mix of sand and plaster to represent the cohesive brittle crust and the confining levees. Experiments were conducted on an inclined board with a reservoir constructed at one end. Silicone was released from the reservoir through a sliding gate, where it encountered a seed flow consisting of a silicone sheet topped with a crust of known depth and constrained by levees. The models therefore represented the influx of fresh lava into a channel. Sequential digital images taken over the course of each experiment allowed marker points on the flow surface to be tracked, and these data were used to construct surface velocity maps. Several experiments were recorded using stereo imagery, allowing changes in the surface relief to be monitored. The insights from these quantitative techniques, combined with morphological observations, are used to illustrate the effect of the crust on the flow dynamics, and to show the response of the brittle crust to the movement of the viscous flow interior. An overview of the experimental techniques and results will be presented, together with an assessment of how the observed model morphologies can be related to features observed in the field.
V13E-08
An Examination of the Morphology and Petrology of the Ring Creek Lava Flow, South-West British Columbia, Canada
The Ring Creek lava flow is the youngest event in a series of Quaternary volcanic activity in Mount Garibaldi Volcano of south-west British Columbia, Canada, occurring between 10.7 – 9.3 ka. The lava flow erupted from parasitic Opal Cone and extends south 6.5 km before making a sharp turn and continues another 11.5 km. At this turn, the flow width reaches approximately 2.5 km. Examination of outcrops along the southern flow margin yield thickness estimates in excess of 200 m. The centre of the flow contains lava blocks up to 2 m in diameter, linear ridges parallel to flow direction ranging from 9 – 11 m in amplitude with a 26m peak to peak distance, and pressure ridges perpendicular to flow direction ranging from 4 – 13 m height with 21 – 35 m wavelength. Flow front breccia approximately 200 m in thickness is observed at the toe of the flow. The presence and magnitude of these features coupled with the mineral assemblage (pl+hbl+bt ± pyx ± mt) are typical of high viscosity flows. The Ring Creek lava flow is anomalous in that it was able to travel 18 km down-valley. This apparent contradiction is even more striking when compared to other flows on Mount Garibaldi with similar mineralogy, which are typically short thick flows or domes. This suggests that a factor other than viscosity has played an important role in determining the behaviour of the Ring Creek lava flow. Our investigation will address alternative governing factors of the Ring Creek flow, how they relate to other flows, and the implications of this anomalous activity for volcanic hazards.