Viscosity characteristics of selected volcanic rock melts

A basic experimental study of the behavior of magma rheology was carried out on remelted volcanic rocks using wide gap viscometry. The complex composition of magmatic melts leads to complicated rheologic behavior which cannot be described with one simple model. Therefore, measurement procedures which are able to quantify non-Newtonian behavior have to be employed. Furthermore, the experimental apparatus must be able to deal with inhomogeneities of magmatic melts. We measured the viscosity of a set of materials representing a broad range of volcanic processes. For the lower viscous melts (low-silica compositions), non-Newtonian behavior is observed, whereas the high-silica melts show Newtonian behavior in the measured temperature and shear rate range (T=1423K-1623K, γ=10^-2s^-1-20s^-1). The non-Newtonian materials show power-law behavior. The measured viscosities η and power-law indexes m lie in the intervals 8Pasη≤210³Pas, 0.71≤m≤1.0 (Grímsvötn basalt), 0.9Pas≤η≤350Pas, 0.61≤m≤0.93 (Hohenstoffeln olivine-melilitite), and 8Pas≤η≤1.510⁴Pas, 0.55≤m≤1.0 (Sommata basalt). Measured viscosities of the Newtonian high-silica melts lie in the range 10⁴Pas≤η≤310⁵Pas.