A theoretical model of cooling viscous gravity currents with temperature‐dependent viscosity

Abstract

Gravity current theory has applications to any geophysical phenomena involving the spreading of fluid on a horizontal interface. Many geological gravity currents (e.g., lava flows and mantle plume heads) are composed of cooling fluid with temperature‐dependent viscosity. An axisymmetric gravity current theory accounting for these thermo‐viscous effects is thus presented and explored here. Unlike isoviscous gravity currents [Huppert, 1982], cooling variable‐viscosity currents do not conserve shape and can undergo a somewhat exotic evolution. For large viscosity contrasts between cold and hot fluid, a constant volume, initially hot, domed current collapses rapidly into a flat plateau with a steep edge. Gravity currents ejected at a constant volume flux from a central conduit also spread with a flattened plateau shape. Continuously fed currents that have a large hot initial volume develop an outwardly propagating interior plateau. Regardless of initial state, continuously‐fed, variable‐viscosity currents grow primarily by thickening; this contrasts signficantly from isoviscous currents which grow almost entirely by spreading.