Viscoelastic behaviour of basaltic lavas

The rheological properties of basaltic lavas from Etna, Hawai′i and Vesuvius have been investigated at temperatures between ∼500 and 1150°C using a small-strain oscillatory shear. The viscoelastic response of the lavas to small, forced, sinusoidal torques (<10−3 N m) at frequencies between 0.002 and 20 Hz was measured. A purely viscous regime was only approached during experiments with Hawai′i samples. These experiments indicated that at temperatures between ∼1070 and 1130°C, strain rate-independent viscosities (>109 Pa s) could be measured at strain rates less than ∼10−2–10−1 s−1. At 800°C, temporal variations in complex shear modulus and internal friction suggest that, over durations of up to 120 h, structural adjustments were occurring within some of the samples. This time-varying behaviour of lava samples may be attributed to the slow closing (healing) of microcracks and small pore spaces, resulting in the apparent stiffening of lava samples under annealing. Thus, those parts of lava flows that undergo slow cooling will have more elastic properties. Regions that cool faster possess smaller shear moduli and higher internal friction due to thermal microcracking and less cohesion between crystals and the bulk glassy matrix.