On Convective Style and Vigor in Sheet-like Magma Chambers

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Abstract
The well known absence of magrnatic superheat is held here to be a direct reflection of the ease and efficiency of large Rayleigh number (Ra) convection in evacuating all convectable heat from the magma. Magmatic temperature is thus continually buffered at or below the convective liquidus where the temperature difference driving convection is vanishingly small and the governing Ra is also always small regardless of body size. It is further held here that for bodies where side wall cooling is of lesser importance, the more common perception of magma chambers is of cooling from above and below where both the initial, isothermal (i.e. isodensity), and final (solid) states are dynamically stable and that convection is necessarily a transient process connecting these states. Extensive theoretical and experimental studies of cooling from above show that regardless of boundary conditions transient, small Ra convection is independent of layer thickness. Instead, convection is driven by formation of a thin, cool, and dense sublayer along the top boundary, and the characteristic length scale of the governing Rayleigh number, which is time-dependent, is the sublayer thickness (d<<L). All dynamic features of the flow, including heat transfer rely on this length scale and not body thickness; virtually any sheet-like magmatic body appears infinitely thick to such convection. Because Ra is small, this transient stage persists for most, if not all, of the period of solidification to mush, whence the body is dynamically dead.

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