Dynamics and geometry of thermal plumes in the laboratory and the mantle

Abstract:

Mantle plumes are thought to be the cause of hot spots and intraplate volcanism. Despite their importance in geophysics, our understanding of their dynamics is limited; the classical head-and-tail structure known from flow-visualisation work has not been observed in tomographic studies; significant uncertainty remains about the degree to which plumes entrain ambient material from the mid-mantle; and there is no accepted definition of the plume geometry. This work addresses these problems by providing quantitative, 3D measurements of the velocity fields surrounding a thermal plume using stereoscopic Particle-Image Velocimetry, as well as local temperature measurements. The Rayleigh number range in the experiments is 0.7 - 2 x 10⁶. The concept of the ‘vortex ring bubble’ is introduced, which provides a quantitative means of defining the geometry of the plume. This definition is shown to be theoretically and experimentally robust, and is easily applied to numerical and experimental data. The definition allows the degree of entrainment of ambient material to be quantified. It is also shown that this boundary definition separates the fluid that has risen from the heater from the ambient material that has gained heat through conduction. In the Earth’s mantle, the Prandtl number is very large (~10²³) and the effects of conduction are negligible. Our new plume-geometry definition is shown to be a close approximation of the geometry of a plume that would occur in the mantle. This shape does not contain the classical head-and-tail structure - in contrast to the widespread conception of plumes - but in agreement with tomographic studies.