Accurate rheological data for silicate melts and crystal-melt, melt-vapor, and crystal-melt-vapor suspensions of both natural compositions and simple binary and ternary systems are useful in constraining theories of momentum transport in magmatic systems and theories of melt structure. Data addressing these problems are presented in a series of papers of which this is the first. A high-torque wide-gap concentric cylinder viscometer has been constructed and used for characterization of the rheometric properties of rhyolitic magma under conditions of varying temperature (1100¿--1350¿C) and shear rate (0.05--13 s-1) at 105 Pa total pressure. Two natural rhyolitic starting materials were used in these experiments. Four independent temperature-shear rate cycles were performed on composition RlA (~76% SiO2) and two independent cycles on composition R2A (~74% SiO2). Each run had a small (3--8 vol%) fraction of vapor present during viscometric testing. A power law model of the form &tgr;=m (T) &ggr;˙n(T) was found to fit the data.

The temperature dependencies of m and n are m (T)=a exp (b/T) and n (T)=c+dT, where a, b, c, and d are constants for fixed melt composition and bubble content. The viscosity function is given by &eegr;=a exp (b/T)&ggr;˙c-1 +d T. Representative values of the constants for composition R2A are a=5.579¿0.034¿10-7 Pa s-n, b=41491¿9.5 K, c=3.039¿ 0.003 and d=-1.516¿0.002 K-1. The power law index coefficient n increases as temperature increases in the range 1100¿--1350¿C due to both volumetric expansion of vapor bubbles with increasing temperature and because of the effects of high shear rate on vapor bubble shapes. The Capillary number (Ca=&ggr;˙&eegr;a/ϕ), the ratio of stress tending to distort bubbles and stress tending to maintain sphericity, is high for these experiments, indicating substantial deviations from bubble sphericity. The activation energy for viscous flow, defined according to ∂ ln&eegr;/∂(1/T) =b-dT2 ln&ggr;, depends strongly and monotonically on shear rate. The Weissenberg effect, a pheonomenon indicative of the presence of nonzero normal stresses has been observed. Melt climbs along the inner rotating rotor due to a positive first normal stress difference defined according to &tgr;ϑϑ-&tgr;rr=-&PSgr;1 &ggr;˙r&THgr;2, where -&PSgr;1 is the first normal stress cofficient. Based on a plot of climb height Ch versus rotor angular frequency ω, -&PSgr;1 is found to be ~2.5¿103 Pa s2. Viscoelastic effects may be important in certain high shear rate flows of petrologic significance in which the Deborah number, defined according to De=&psgr;1&ggr;˙/2&eegr;, is of order unity. To our knowledge, this is the first reported measurement of a normal stress coefficient for magma.