Properties keywords: Newtonian, low viscosity
Analogue keywords: magmatic intrusions, density currents, ash columns
Common names: saline solution, brine, H2O-NaCl
General Information: Water is one of the simplest analogue materials. It is often combined with salt to alter its density and viscosity. It is non-toxic, very low cost and very readily available.
Properties
Water is an incompressible Newtonian fluid with a viscosity of 10-3 Pas and a density of 1000 kg m-3, which can change with temperature. It is solid below 0°C. Salt water properties depend on the concentration of salt in the solution. Density can be up to 1160 kg m-3 and viscosity up to 17-3 Pas. The freezing point of a 22 wt.% salt water solution is -19.2°C.
Water and salt water can be easily dyed to enable tracking through experiments. They also have different refractive indices, enabling detailed observation of fluid flow.
Applications
Water is often used in analogue experiments of magma intruding into the crust, where water is the magma analogue and gelatine is the elastic crust (see Gelatine page). Several authors have used water to study dyke propagation, for example, fissure propagation from a magma reservoir under pressure (Takada, 1990; McLeaod and Tait, 1999; Menand and Tait, 2002), the effect of volcanic edifice instability on the generation of dyke swarms and rift zones (Walter and Troll, 2003), the effect of density and rigidity contrasts on the formation of dykes and sills (Kavanagh et al., 2006; Ritter et al., 2013) and the development of dyke paths in a volcanic rift system (Tibaldi et al., 2014).
Keryvn et al. (2009) used different materials to study the different methods of dyke propagation, i.e. through shear failure and tensional hydraulic fracturing, by using golden syrup in sand and water in gelatine, respectively. Many experimental studies have added salt to the gelatine in order to create the correct density contrast between the model materials for adequate scaling to natural systems (Kavanagh et al., 2006; Ritter et al., 2013). Tibaldi et al. (2014) also found that colouring the water with coffee resulted in the best tracking of the intrusion when using gelatine as the host media.
Salt water
By combining salt water with fresh water it is possible to study the dynamics of density-driven currents e.g. pyroclastic and ash flows, by adding particulate matter to salt water and observing the interactions between dense, turbulent flows (salt water + particles) and the air (fresh water). Experiments of this type have shown the implications of sedimentation from particle-laden plumes for the generation of pyroclastic flows from ash column collapse and provided information on the dynamics of turbulent flows (Huppert et al., 1986; Carey and Sigurdsson, 1988; Woods et al., 1998).
Stratified mixtures of salt and fresh water can represent the density-layered atmosphere and enable the study of buoyantly rising ash columns by injecting water and particle mixtures into the layered water. This has been used to understand the effect of density stratification on ash cloud stability, on lateral spreading of ash columns and of gas/ash separation within the cloud (Holasek et al., 1996; Carazzo and Jellinek, 2012; Carazzo and Jellinek, 2013). However, Carazzo and Jellinek (2012) noted that, whilst particle aggregation is an important process occurring in volcanic ash clouds, experiments with water resulted in no particle aggregation.
Limitations and tips for use
A complication with using water-gelatine analogues is the lack of a density contrast between the two materials. This requires that an internal fluid pressure is imposed in order to drive buoyancy (e.g. Kervyn et al., 2009).
Gelatine is often used because water cannot be injected into granular media when simulating magmatic intrusions, because water will percolate through the grains, especially in the case of silica grains, as surface tension increases percolation through this hydrophilic material (Galland et al., 2015).
Finally, due to sedimentation of the particulate matter within, salt water currents lose turbulent energy and velocity faster than their volcanic counterparts, meaning that distance travelled is shorter in the analogue material (Gray et al., 2006).
References
Carazzo G and Jellinek AM (2012), A new view of the dynamics, stability and longevity of volcanic clouds. Earth and Planetary Science Letters 325–326: 39–51
Carazzo G and Jellinek AM (2013) Particle sedimentation and diffusive convection in volcanic ash-clouds. Journal of Volcanology and Geothermal Research 118: 1420-1437
Carey NS and Sigurdsson H (1988) Experimental studies of particle laden plumes. Journal of Geophysical Research 93, B12: 314-328
Galland O, Holohan E, van Wyk de Vries B, and Burchardt S (2015) Laboratory modelling of volcano plumbing systems: a review. Advances in Volcanology. Springer Berlin Heidelberg. 1-68
Gray TE, Alexander J, and Leeder MR (2006) Longitudinal flow evolution and turbulence structure of dynamically similar, sustained, saline density and turbidity currents. Journal of Volcanology and Geothermal Research 111: 1-14
Holasek RE, Woods AW, and Self S (1996) Experiments on gas-ash separation processes in volcanic umbrella plumes. Journal of Volcanology and Geothermal Research 70: 169-181
Huppert HE and Sparks RSJ (1980) Restrictions on the compositions of mid-ocean ridge basalts: a fluid dynamical investigation. Nature 286: 46-48
Huppert HE, Turner JS, Carey SN, Sparks RSJ, and Hallworth MA (1986) A laboratory simulation of pyroclastic flows down slopes. Journal of Volcanology and Geothermal Research 30: 179-199
Kavanagh JL, Menand T, Sparks RSJ (2006) An experimental investigation of sill formation and propagation in layered elastic media. Earth and Planetary Science Letters 245(3–4): 799–813
Kervyn M, Ernst GGJ, van Wyk de Vries B, Mathieu L, and Jacobs P (2009) Volcano load control on dyke propagation and vent distribution: insights from analogue modeling. Journal of Geophysical Research 114(B3): B03401
McLeod P and Tait S (1999) The growth of dykes from magma chambers. Journal of Volcanology and Geothermal Research 92: 231– 245
Menand T and Tait S (2002) The propagation of a buoyant liquid-filled fissure from a source under constant pressure: an experimental approach. Journal of Geophysical Research 107(B11): 2306
Rittter MC, Acocella V, Ruch J, and Philipp S (2013) Conditions and threshold for magma transfer in the layered upper crust: Insights from experimental models. Geophysical Research Letters 40: 6043-6047
Takada A (1990) Experimental study on propagation of liquid-filled crack in gelatin: shape and velocity in hydrostatic stress condition. Journal of Geophysical Research 95 (B6): 8471–8481
Tibaldi A, Bonali FL, and Corazzato C (2014) The diverging volcanic rift system. Tectonophysics 611: 94–113
Walter TR and Troll VR (2003) Experiments on rift zone evolution in unstable volcanic edifices. Journal of Volcanology and Geothermal Research 127(1–2): 107–120