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The mechanical properties of solidi¢ed polyethylene glycol 600, an analog for lava crust
S.A. Soule �, K.V. Cashman Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403-1272, USA
Received 29 May 2002; received in revised form 20 November 2002; accepted 24 November 2002
The strength of lava crust and the role of that crust in determining lava flow dynamics and morphology are of fundamental importance in modeling lava flow behavior. Great improvements to flow models have been made by combining theoretical analysis with the results from analog experiments using polyethylene glycol (PEG 600) wax, which solidifies just below room temperature and can be used to simulate solidifying flows. However, a paucity of experimental data on the mechanical properties of solid PEG 600 has limited the quantitative interpretation of the role of crust in these experiments. We have, therefore, conducted experiments to measure the mechanical properties (tensile strength, shear strength, and Young’s modulus) of solidified PEG 600 at temperatures and strain rates similar to those employed in analog experiments. The mechanical properties of PEG 600 can be modeled by a power law function of temperature, with the tensile strength exhibiting an additional dependence on strain rate. At intermediate temperatures, marked changes in the temperature dependence of the Young’s modulus and in the character of failure surfaces indicate a transition from brittle to ductile behavior. We find that the strength of solid PEG 600 at experimental temperatures and strain rates exceeds that inferred for the crust on active laboratory flows by four orders of magnitude. This discrepancy is best explained by the presence of a thin visco-elastic layer separating the solid surface from the fluid interior of PEG 600 flows, a model that has been suggested to explain the behavior of some basaltic lava flows. 6 2003 Elsevier B.V. All rights reserved.
Keywords: lava £ow; crust; PEG 600; rheology; tensile strength
Crust is an important parameter of lava £ow emplacement that is both e¡ected by and a¡ects physical processes during £ow. For example, the rate of heat loss from crusted and crust-free lava
di¡ers dramatically (Crisp and Baloga, 1990, 1994), resulting in unique thermal (and resulting rheological) histories for lava transport systems with or without continuous surface crusts. How- ever, complex feedback mechanisms among rates of £ow cooling, crystallization, and £ow advance rate directly in£uence the mechanical stability of the crust. Understanding these complexities have required that careful observations of active £ows be supplemented by analog experiments per- formed under controlled conditions (e.g. Hulme,
0377-0273 / 03 / $ ^ see front matter 6 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-0273(03)00237-3
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E-mail addresses: firstname.lastname@example.org (S.A. Soule), email@example.com (K.V. Cashman).
VOLGEO 2674 20-11-03
Journal of Volcanology and Geothermal Research 129 (2004) 139^153
Available online at www.sciencedirect.com
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1974; Blake, 1990; Fink and Gri⁄ths, 1990, 1992, 1998; Stasiuk et al., 1993; Gri⁄ths and Fink, 1992a,b, 1993, 1997; Gri⁄ths, 2000; Blake and Bruno, 2000). Many of these simulations have used polyethylene glycol (PEG 600) as an analog to lava because it freezes near room temperature (19^20‡C) and can thus be used to investigate solidifying £ows. However, the physical properties of solidi¢ed PEG 600 are poorly understood, lim- iting interpretation of the nature and role of the crust in £ow emplacement. Here we present exper- imental measurements of the tensile strength, shear strength and Young’s modulus of solid PEG 600 at temperatures (4^13‡C) and strain rates (0.005^0.05 s31) relevant to PEG 600 £ow experiments. We then use the results to constrain the role of solid crust in £ow emplacement, both in the laboratory and, by inference, in lava £ows and domes.
It has long been recognized that morphologic and dynamic di¡erences among basaltic lava £ow types can be related to variations in the rheology of £ow interiors and the strength and thickness of surface crusts. Dana (1849, 1888) ¢rst interpreted PaPa formation to result from solidi¢cation and subsequent crustal breakage at the £ow front and hypothesized that the size of PaPa clinkers re£ected the thickness of the cooled crust at the time of their genesis. Since then, numerous studies have addressed the relative importance of crustal strength and internal £ow rheology in the devel- opment of di¡erent surface morphologies and em- placement styles. Flow crusts may deform by duc- tile folding (Fink and Fletcher, 1978; Fink, 1980; Gregg et al., 1998) or brittle failure at length scales that re£ect both the lava rheology and the thickness of the thermal boundary layer at the time of failure (e.g. Wentworth and Macdonald, 1953; Rowland and Walker, 1987; Cashman et al., 1999), resulting in a spectrum of surface mor- phologies from smooth pahoehoe to blocky lava £ows. Kilburn (1981, 1990, 1993, 1996) has gen- eralized £ow models to argue that the distinct £ow advance styles of pahoehoe and PaPa are de- termined by rates of crustal growth vs. rates of crustal disruption, with PaPa crusts experiencing near continuous failure and pahoehoe crusts fail- ing episodically.
While the de¢nition of crust is commonly me- chanical (e.g. Wright and Okamura, 1977), changes in the mechanical properties of solidify- ing lava are directly linked to temperature and consequently the thermal structure and history of a lava £ow. For this reason, models linking £ow dynamics to cooling rates predict many as- pects of £ow morphology using a single dimen- sionless parameter, 8, which represents the rela- tive importance of quenching and advection timescales (Fink and Gri⁄ths, 1990, 1992; Grif- ¢ths and Fink, 1993). Also important, however, are both the rheology of the £uid interior (Hulme 1974; Gri⁄ths and Fink, 1998; Cashman et al., 1999) and the development of a visco-elastic boundary layer between the £uid interior and brittle surface (Hon et al., 1994). For this reason, we use our measurements to examine the relative importance of these three thermo-mechanical layers during £ow emplacement.
2.1. PEG 600
Polyethylene glycol is a crystalline, thermoplas- tic, water-soluble polymer that is commercially available in a wide range of molecular weights depending on the number of monomers (n),
OH� ð�CH2 � CH2 �O�Þn �H;
in the polymeric chain (Bailey and Koleske, 1976). This study uses PEG 600 (commercially produced as Carbowax 600 by the Union Carbide Corp.), the same material used in prior lava £ow simula- tions (e.g. Fink and Gri⁄ths, 1990; Blake and Bruno, 2000; Gregg and Fink, 2000). PEG has numerous industrial applications and as a result, a signi¢cant body of literature exists on the material. However, most studies are restricted to the properties of liquid, not solid, PEG (see Harris and Zalipsky, 1997 for a review). Measurements of the mechanical properties solid PEG exist only for compounds with molecular weights much greater than those used in £ow experiments (PEG ns 4000; Al-Nasassrah et al., 1997).
VOLGEO 2674 20-11-03
S.A. Soule, K.V. Cashman / Journal of Volcanology and Geothermal Research 129 (2004) 139^153140
2.2. Sample preparation
We prepare PEG 600 samples by heating the wax above its solidi¢cation temperature (Ts) and casting it in molds of the desired shape. For ten- sile strength measurements, the mold consists of two polystyrene cocktail cups with bottoms re- moved, placed end-to-end and separated by a rub- ber o-ring (Fig. 1a). The o-ring, which is removed prior to testing, creates a necked region to be fractured with a known cross-sectional area. We allow ¢lled molds to equilibrate to experimental temperatures for V12 h. The mold for direct shear tests is formed by two V15-cm lengths of polyvinyl chloride (PVC) pipe, again separated by a rubber o-ring (Fig. 1b). A 2.5-mm-deep thread cut on the inside of each pipe prevents the solid PEG 600 from sliding during the tests. Volume loss of the PEG 600 during freezing results in holes through the center of the cylinder that occa-
sionally penetrated the necked fracture region. Hole formation is limited, but not eliminated, by insulating the tops of the cylinders during cooling. We discard test results for samples with extreme volume loss.
We determine the solidi¢cation temperature of the PEG 600 with a concentric cylinder viscome- ter. Temperature is decreased in 0.5‡C increments until the viscosity increases dramatically (Fig. 2), indicating the onset of solidi¢cation. To ensure homogeneity in physical properties, we measure the freezing point of small amounts of the wax (V3 liters) prior to use, and discard the PEG 600 after one use.
2.3. The apparatus
We conduct tests within a thermally regulated chamber at temperatures of 4‡C, 7‡C, 10‡C, and 13.3‡C, nearly spanning the range of temperatures in PEG 600 £ow experiments (e.g. Fink and Grif- ¢ths, 1990; Blake and Bruno, 2000). A DC servo motor with a linear actuator produces constant strain at rates of 0.005, 0.01, 0.03, and 0.05 s31, covering the range of strain rates achieved in sim- ulations (Gri⁄ths and Fink, 1993).
For tension, two split ring clamps with inner surface