Viscoelastic behaviour of basaltic lavas
Journal of Volcanology and Geothermal Research, 132 (2-3):
99--113 (30.04.2004)DOI: 10.1016/S0377-0273(03)00340-8
Аннотация
The rheological properties of basaltic lavas from Etna, Hawaii and
Vesuvius have been investigated at temperatures between 500 and 1150C
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 Hawaii samples.
These experiments indicated that at temperatures between 1070 and
1130C, strain rate-independent viscosities (>10 9 Pa s) could be
measured at strain rates less than 10 -2 - 10 -1 s-1. At 800C, 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.