Academic journal article Journal of Geoscience Education

The Viscosities of Foods as Analogs for Silicate Melts

Academic journal article Journal of Geoscience Education

The Viscosities of Foods as Analogs for Silicate Melts

Article excerpt

ABSTRACT

The viscosities of maple syrup, 0.32 ± 0.03 Pa s, molasses, 0.87 ± 0.09 Pa s, ketchup, 9.8 ± 1.0 Pa s, and smooth peanut butter, 1.2 ± 0.9 × 10^sup 5^ Pa s, were measured at 25 °C and 1 bar pressure for comparison with the viscosities of natural silicate melts to aid in teaching the importance of viscosity in igneous processes to students. Viscosity measurements were made at a single stress state for each of these materials, which are probably all thixotropic, and measurement accuracy is only to within a factor of 3, nevertheless these values are adequate and useful for teaching purposes. The viscosities of most common natural silicate melts are bracketed by the measured viscosity of peanut butter, which is near the viscosity of a rhyolitic or granitic melt + 1.8 wt% H2O at 800 °C, and of ketchup, which is near the viscosity of an anhydrous, molten tholeiitic basalt at 1200 °C. The effectiveness of using food to teach melt viscosities was evaluated by a survey of two student groups, a test group who had been taught silicate melt viscosities using food examples in two courses and a control group who had been taught silicate melt viscosities in their first course without the use of food examples and in their second course with the use of food. The results of this survey indicate that the test group retains their knowledge of silicate melt viscosities better than the control group and demonstrate that food viscosities are a useful teaching tool.

INTRODUCTION

Viscosity is one of the most important variables in the transport of magmas and the eruption of lavas (Hess, 1989; Clemens and Petford, 1999). Low viscosity melts of basaltic composition are rapidly extracted from their source regions (McKenzie, 1985). Basaltic compositions typically form thin lava flows with a low ratio of the height to length or width and can travel 100's of kilometers away from their vents (Basaltic Volcanism Study Project, 1981). These low viscosity melts allow rapid volatile loss, which often results in relatively quiescent volcanic eruptions (MacDonald, 1972; Hess, 1989). On the other hand, high viscosity melts such as those of rhyolitic (equivalent to granitic) composition are slowly removed from their source regions; in some cases they cannot be completely removed and remain trapped where they cool to form migmatites (e.g. McKenzie, 1985; Sawyer, 1994). High-viscosity rhyolitic lava flows are characterized by high ratios of the height to length or width and often travel no more than a kilometer from their vents (McDonald, 1972). The high viscosities of rhyolitic melts inhibit volatile loss and can lead to violent Plinian eruptions (Jaupart and Tait, 1990). Additionally, melt viscosity is related to chemical diffusion and thus to rates of crystal growth and dissolution (Baker, 1993).

Most students qualitatively comprehend the concept of high and low viscosity liquids based upon their personal experience with common objects such as water and maple syrup. However, the S.I. units of viscosity measurement, Pascal seconds (Pa s), are unfamiliar to most people, which results in a poor quantitative understanding of magma viscosity. Furthermore, many magmas have viscosities so many orders of magnitude above water, the most common liquid with which students are familiar, that a quantitative comparison between the viscosities of water and magma is almost meaningless.

In order to more successfully engage the students and provide them with everyday examples of liquids whose viscosities are similar to those of silicate melts, we began an experimental program to measure the viscosities of maple syrup, molasses, ketchup, and smooth peanut butter. As we will show, these common North American foods have viscosities that are similar to those of natural magmas. All of the viscosity measurements described below were made with household items or discarded laboratory equipment; many of them are suitable for demonstrations in the classroom. …

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