Rankine models for time‐dependent gravity spreading of terrestrial source flows over subplanar slopes

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Abstract
Geological mass flows extruding from a point source include mud, lava, and salt issued from subsurface reservoirs and ice from surface feeders. The delivery of the material may occur via a salt stock, a volcanic pipe (for magma and mud flows), or a valley glacier (for ice). All these source flows are commonly skewed by a superposed far‐field velocity vector imposed by the topographic slope and thus develop plumes having a wide range of shapes. The morphological evolution of the perimeter of the plumes (in plan view) can be simulated by varying the key parameters in a simple analytical flow description on the basis of Rankine equations. Our model systematically varies the strength of the point source relative to the downslope far‐field velocity of its expelled mass. The flow lines are critically controlled by the relative speed of the two rates, which can be concisely expressed by the dimensionless Rankine number (Rk, introduced in this study). For steady flows, plume widths can be expressed as a function ofRk. The viscosity of the rock, mud, or lava mass involved in the gravity flow affectsRkand thus the appearance of the plumes. For unsteady source strength,Rkbecomes time dependent and the plume width varies over time. The model flow shapes suggest that the plume shapes of natural gravity flows of terrestrial surface materials (mud, lava, salt, and ice) commonly express fast initial flux of the source, followed by an exponential decline of the source strength. Flows having initially higherRkbut otherwise equal life cycles create broader plumes. Peaks in the source flux due to magmatic pulsing during the eruption cycle can explain the formation of pillow lavas. Rather than instantaneously reaching full strength before declining, some natural source flows start by swelling slowly, leading to the creation of unique plume shapes like a flying saucer.

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