Abstract

To better understand the processes which occur in magma chambers and in liquefied sediments, we conduct laboratory experiments of gravitational instability of a close packed granular medium in a liquid. First we consider a case in which a granular medium overlies a liquid layer. Here the lowermost thin layer of a granular medium dilates and becomes mobile to form a rheological boundary layer (RBL), from which narrow granular plumes grow downwards. We measure the wavelength and the growth rate of the instability and constrain the thickness of the RBL to be only about twice the particle size, and its packing fraction as Φ = 0.42, indicating dilation. When the granular medium is thicker, we find that the granular plumes organize themselves into convection cells. Importantly, the ascent velocity of the boundary between the granular medium and the liquid layer is approximately constant regardless of the thickness ratio of the two layers. A scaling in which the packing fraction of the RBL governs the ascent velocity is found to explain the results well. Next we extend the first experiment and model magma chamber roof melting and subsequent particle settling. An experimental cell consists of a lower layer of a thermally convecting molten wax, and an upper layer comprising of a mixture of particles and solid wax. As the interstitial solid wax of the upper layer melts, the particles become mobile and settle downwards. From experiments with different particle sizes, we find that when the particle is smaller than 0.1 mm, cycles in which melting starts and stops, occur spontaneously. Melting stops due to the formation of a stable stratification in the melt layer which suppresses the vertical heat transfer. Melting resumes by the convective overturning because the cell is heated from below. A dimensionless Buoyancy number is found to explain the critical particle size well. As a third experiment, we consider the instability caused by liquefaction. Here an experimental cell consists of a water-immersed granular medium which is size-graded into 2 layers such that the upper layer has a smaller permeability. When the cell is shaken vertically, liquefaction occurs and the water temporarily accumulates at the 2 layer interface resulting in a gravitational instability. From experiments in which we vary the acceleration and frequency of shaking, we find that there is a critical acceleration for instability to occur, and that this acceleration is minimum at a frequency of about 100 Hz. We show that the frequency dependence can be interpreted from a combined condition of energy and jerk of shaking exceeding their respective critical values.

Full Text
Paper version not known

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

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.