Non‐Newtonian rheology of igneous melts at high stresses and strain rates: Experimental results for rhyolite, andesite, basalt, and nephelinite

Abstract

The stress‐strain rate relationships of four silicate melt compositions (high‐silica rhyolite, andesite, tholeiitic basalt, and nephelinite) have been studied using the fiber elongation method. Measurements were conducted in a stress range of 10–400 MPa and a strain rate range of 10−6 to 10−3 s−1. The stress‐strain rate relationships for all the melts exhibit Newtonian behavior at low strain rates, but non‐Newtonian (nonlinear stress‐strain rate) behavior at higher strain rates, with strain rate increasing faster than the applied stress. The decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt. The decrease in viscosity observed at the high strain rates of the present study ranges from 0.25 to 2.54 log10 Pa s. The shear relaxation times τ of these melts have been estimated from the low strain rate, Newtonian, shear viscosity, using the Maxwell relationship τ = ηs/G∞. Non‐Newtonian shear viscosity is observed at strain rates ( trueε˙=time−1 ) equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time. Brittle failure of the fibers occurs 2 log10 units of time above the relaxation time. This study illustrates that the occurrence of non‐Newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate. High‐silica rhyolite melts involved in ash flow eruptions are expected to undergo a non‐Newtonian phase of deformation immediately prior to brittle failure.

Evaluating Igneous Activity at Yucca Mountain

“…With continued ascent, the water-saturated magma becomes a glassy, highly viscous (~10 8 pascal seconds) mass of reduced mobility (Figure 2). This immobility is reflected in the high viscosity that regulated flows from nearby cinder cones associated with wet basalt (which flowed approximately 1 kilometer over 1-2 months, as estimated by analog flows), and it is consistent with the experimentally determined values of dry basalt glass [e.g., Webb and Dingwell, 1990]. This viscosity limits the distance that magma can travel in repository drifts (less than ~10 meters per day), reducing the number of waste packages affected; and magma in this state quenches rapidly on all that it touches, including waste packages.…”

Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling

“…The contrast depends on bubble volume fraction and is relevant only for late stages of magma rise near the vent. At high strain rates many silicic melts exhibit nonlinear viscoelasticity [Webb and Dingwell, 1990]. However, relatively low strain rates are associated with bubble growth considered in the present study, so Newtonian rheology is considered appropriate.…”

Conditions for detection of ground deformation induced by conduit flow and evolution

“…The threshold criterion used is either a given overpressure within the magma below the plug [Lensky et al, 2008] or the condition for brittle failure of the magma [Collier and Neuberg, 2006]. The onset of brittle failure has been identified to occur when the product of the melt viscosity and the shear strain rate is larger than the shear strength of the melt (s s ) [Webb and Dingwell, 1990], values being estimated between 10 7 and 10 8 Pa for pure glass [Tuffen et al, 2003;Tuffen and Dingwell, 2005].…”