Chapter 6 - Pascal’s Principle, a Simple Model to Explain the Emplacement of Laccoliths and Some Mid-crustal Plutons

Abstract

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. 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 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. Using Pascal’s principle, the magma pressure required for inflation is similar to the magma pressure determined for dome growth at Mt. St. Helens. This model is applied to mid-crustal plutons which expand from dykes. The Birch Creek pluton of California is used as an example of an inflating pluton based on detailed mapping of internal contacts and fabrics which reveals the transition from initial sheeting to final inflation.