APPLYING PASCAL'S PRINCIPLE TO THE MAGMATIC INFLATION OF LACCOLITHS AND PLUTONS

The common ratios between the length and thickness for many plutons and laccoliths suggests that there is a scale independent mechanism controlling their shape, and therefore the mechanism of emplacement (McCaffrey and Petford, 1997; Cruden and McCaffrey, 2001). It is also widely accepted that intrusions are constructed from magma pulses, and these pulses are often sheet-like. Using Pascal’s principle of transmitting fluid pressures equally everywhere, a simple two stage model for the emplacement of shallow and mid-crustal intrusions is developed. The initial pulse is sheet-like and expands, increasing the force until the force is equal to the overlying lithostatic load or outlying horizontal stress, similar to a hydraulic press. Examples of shallow crustal intrusions which inflated from sheets are described from the Henry Mountains of Utah and the Shonkin Sag Laccolith in Montana. These inflated intrusions have very circular outer margins which may be due to the circle having the greatest area to outer contact ratio which diminishes the amount of rock that has to be fractured in order for uplift to occur. Using Pascal’s principle, the magma pressure required for inflation is determined to be similar to the magma pressure determined for dome growth at Mt. St. Helens. This model is applied to mid-crustal plutons which expand from dikes. The Birch Creek pluton, emplaced into the mid-crust in eastern California, is described because the transition from initial dike-like to inflation stage has been mapped in detail (Barton, 2000).