Abstract
The effects of crystal nucleation and growth on the viscosity of andesitic magmas are investigated at 1 atm. Pressure. The data are used to describe the rheological evolution of andesitic magmas and to derive empirical equations to model the crystallization dynamics. Viscosity experiments were performed at temperatures of 1473, 1483, and 1493 K with shear rates of 0.5 s−1 by using wide-gap concentric cylinder viscometry. We focused on eruptible magmas (i.e., crystal contents well-below ~50%) by following the time evolution of crystal nucleation and growth of a natural andesitic magma, while continuously monitoring the changes in viscosity. At near-equilibrium conditions, the basaltic-andesite contains 6, 13, and 25 area % crystals at 1493, 1483, and 1473 K, respectively, resulting in a viscosity increase of ca. 1 log unit. We show that the timescale of viscosity increase is mainly dictated by the delay time preceding crystallization and markedly decreases with increasing undercooling (from 22 to 42 K) and shear rates (from 0.1 to 1 s−1).The plagioclase growth rates estimated from our data are in the order of ~3–5 × 10−6 cm/s, which is much faster than the rates estimated for crystallization in static conditions. We conclude that the effect of the shear rate must be taken into account in modelling magmatic and volcanic processes, especially when transient changes in viscosity need to be estimated. Our results are important for understanding the dynamics of lava flows where deformation plays a significant role in promoting crystallization.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.