Bull Volcanol (2002) 64:363–371 DOI 10.1007/s00445-002-0213-6
R E S E A R C H A RT I C L E
C. Buisson · O. Merle
Experiments on internal strain in lava dome cross sections
Received: 9 March 2001 / Accepted: 8 March 2002 / Published online: 18 June 2002 © Springer-Verlag 2002
Abstract Simple experiments have been conducted to study the strain evolution in lava dome cross sections. A viscous fluid is injected vertically from a reservoir into a feeding conduit. Silicone putty is used as analogue magma. Two-dimensional experiments allow the assessment of the internal strain within the dome. Particle paths are symmetrical on either side of a central line passing through the feeding conduit and display parabolic trajectories. The highest strain zone is located above the extrusion zone. In cross sections, stretch trajectories show a remarkable concentric pattern, wrapping around the extrusion zone of the analogue magma. To the lateral margins, a triple junction of stretch trajectories defines an isotropic point in the strain field. In the main central part of the dome, an intermediate zone of reversed sense of shearing is caused by a change in the sign of the velocity gradient with respect to that in the upper and lower zones. Knowledge of this evolving strain pattern can provide a better understanding of the evolution of natural domes. Also, it can help to unravel the kinematic history of ancient domes partly removed by erosion. Keywords Analogue modelling · Domes emplacement · Kinematic evolution · Particle paths · Stretch trajectories
Editorial responsibility: D. Dingwell C. Buisson (✉) Laboratoire Magmas et Volcans, Observatoire de Physique du Globe, CNRS – Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France e-mail:
[email protected] e-mail:
[email protected] Tel.: +33-4-73346721, Fax: +33-4-73346744 O. Merle Laboratoire Magmas et Volcans, Observatoire de Physique du Globe, CNRS – Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand, France
Introduction Studies of dome growth are abundant (e.g. Williams 1932; Christiansen and Lipman 1966; Cole 1970; Huppert et al. 1982; Murase et al. 1985; Fink and Manley 1987; Swanson et al. 1987; Anderson and Fink 1990; Duffield and Dalrymple 1990; Swanson and Holcomb 1990; Dadd 1992; Miller 1994; Fink and Bridges 1995; Nakada et al. 1995). These mostly deal with the mechanical aspects of lava dome emplacement. These studies have brought much information about the evolution of height vs. diameter, flow vs. extrusion rate, and the rheology of the magma during emplacement, which constitute an invaluable source of data. Huppert et al. (1982) were the first to present analogue models concerning lava dome growth. They used a simple Newtonian fluid flowing radially under its own hydrostatic pressure on a horizontal surface. They compared experimental results with the growth of the St Vincent Soufriere dome. Over the past decade, several studies have described experiments of lava domes. Blake (1990) introduced non-Newtonian effects using Bingham slurries of kaolin and water. According to observed changes in aspect ratios, Blake proposed four classes of domes: upheaved plugs, pelean, low and coulee. Experiments using kaolin slurry injected in to cold water (Griffiths and Fink 1993, 1997) or gum resin (Lejeune 1995) have allowed a better understanding of the cooling effect on lava dome evolution. Fink and Bridges (1995) emplaced polyethylene glycol (PEG) wax into cold sucrose to study the impact of eruption history and cooling rate on lava dome growth. These studies have dealt with the geometrical aspects of domes, including the evolution of the height/diameter ratio with time and the varying textures at the surface of the dome during cooling. They have stressed the physical constraints leading to dome collapse and explosion, and have investigated their consequences on the dynamics of eruptive processes. Field data on the evolving strain within endogenous lava domes are scarce. This is mainly for two reasons:
364 Table 1 List of experiments carried out in this study
Experiment
Type
Effusion rate (cm3 h–1)
Duration (h)
Marker type
Material
1 2 3 4 5 6 7 8
3-D 3-D 3-D 3-D 3-D 2-D 2-D 2-D
0.26 0.54 0.78 1.04 1.30 0.26 1.30 0.26
80 72.5 60 60 55 80 56 80
Tinted wood particles Tinted wood particles Tinted wood particles Tinted wood particles Tinted wood particles Tinted wood particles Tinted wood particles Tinted wood particles
9 10 11 12 13 14 15 16 17 18 19 20 21
2-D 2-D 2-D 2-D 2-D 2-D 2-D 2-D 2-D 2-D 2-D 3-D 2-D
0.26 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 0.26
80.5 53 7 7.75 22 7.5 7 7 6 7.75 8 23.25 46
Small coloured sticks Small coloured sticks 5-mm grid 5-mm grid 5-mm grid 5-mm grid 5-mm grid 3-mm grid 3-mm grid 3-mm grid 3-mm grid 3- and 5-mm grid 3-mm grid
Transparent silicone Transparent silicone Transparent silicone Transparent silicone Transparent silicone Transparent silicone Transparent silicone Transparent silicone Non-lubricated walls Transparent silicone Transparent silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone Pink silicone
either the dome was active and direct observations of the deforming interior was not possible or the dome was cooled and erosion reveals only the final stage of deformation. This is why little is known about their overall internal strain. To address this question, simple experiments have been conducted with a viscous fluid, injected vertically and flowing on a rigid planar base. The scaled models reveal the internal strain patterns that can be expected in viscous endogenous domes. They make it possible to determine particle paths, velocity gradients, sense of shearing and the attitude of the flattening plane.
Experimental procedure In our experiments, silicone is poured into a reservoir and simulates viscous magma (Fig. 1). Pushed up by a piston, the silicone is vertically extruded through a feeding conduit of 6 mm in diameter and flows on a rigid planar base. The velocity of the piston is governed by a computer-controlled stepper motor and the silicone comes out of the feeding conduit at a rate ranging from 0.26–2.6 cm3 h–1 (Table 1). In three-dimensional experiments, the silicone spreads radially onto a horizontal base exhibiting a symmetrical deformation pattern around an axis corresponding to the feeding conduit. In two-dimensional experiments, the viscous fluid is confined between two walls of glass 6 mm apart corresponding to the width of the feeding conduit. These 2-D models make it possible to study internal strain and motion. When using transparent silicone, particle paths within the dome are followed by the means of small elongated pieces of wood previously incorporated into the reservoir. When using pink silicone, a square grid of
Fig. 1 Experimental device – the feeding conduit is 6 mm wide
carbon is printed on the lateral side once the extruding silicone has risen up enough to exhibit an hemi-elliptical shape of a few centimetres high (i.e. 5 or 6 h after the beginning of the experiment; Fig. 2b, c). The grid is emplaced by removing one of the two walls for a few seconds. At the same time, the glass is lubricated with a soap mixture in order to avoid boundary effects. The lubricated walls cause the silicone to glide freely and no boundary effect was observed. Because of this experimental procedure, two-dimensional experiments only reveal the evolution of mature domes because the first stages of deformation cannot be investigated. During experiments, successive photographs of the deforming grid or particle trajectories are taken through
365
Fig. 2a–c Photographs of two-dimensional experiments. a, b Experiment 15 at an injection rate of 2.6 cm3 h–1. The deformation (bottom) of initially undeformed square grid (top) allows computation of strain anywhere in the model except in the lower central zone of newly injected material. c Deformation of a grid with smaller square and circle elements in experiment 21 after a duration of 41 h, 30 min after grid emplacement.
the glass to follow the evolving strain pattern. Experiments have been conducted at room temperature and have lasted from 6 to 80 h. Our experiments are intended to be geometrically, kinematically and dynamically similar to natural domes (Hubbert 1937; Ramberg 1981; Merle and Borgia 1996). As the model uses an isothermal Newtonian fluid, the only criterion that is relevant for justifying the experiments as analogues of the volcanic flows is that the Reynolds numbers are small in both cases. The Reynolds number is : where H and R are the height and the radius of the dome. The Reynolds number varies from 10–9 to 10–11 in experiments and is about 5×10–11 in nature. These very small values (Re
