Magma chamber growth models in the upper crust: A review of the hydraulic and inertial constraints

ARAGÓN, EUGENIO; DEMARTIS, MANUEL; CONIGLIO, JORGE E.; PINOTTI, LUCIO P.; WEINBERG, ROBERTO F.; D´ERAMO, FERNANDO J.; TUBÍA, JOSÉ MARÍA

Geoscience Frontiers

China University of Geosciences

Año: 2019 vol. 10 p. 1211 - 1218

1674-9871

Finite volumes of magma moving in confinement, store hydraulic potential energy for the generation,control and transmission of power. The Pascal?s principle in a hydraulic jack arrangement is used tomodel the vertical and lateral growth of sills. The small input piston of the hydraulic jack is equivalent tothe feeder dike, the upper large expansible piston equivalent to the magmatic chamber and the inertialforce of the magma in the dike is the input force. This arrangement is particularly relevant to the case ofsills expanding with blunt tips, for which rapid fracture propagation is inhibited. Hydraulic modelsconcur with experimental data that show that lateral expansion of magma into a sill is promoted whenthe vertical ascent of magma through a feeder dike reaches the bottom contact with an overlying, flatrigid-layer. At this point, the magma is forced to decelerate, triggering a pressure wave through theconduit caused by the continued ascent of magma further down (fluid-hammer effect). This pressurewave can provide overpressure enough to trigger the initial hydraulic lateral expansion of magma into anincipient sill, and still have enough input inertial force left to continue feeding the hydraulic system. Thelateral expansion underneath the strong impeding layer, causes an area increase and thus, further hydraulicamplification of the input inertial force on the sides and roof of the incipient sill, triggeringfurther expansion in a self-reinforcing process. Initially, the lateral pressure increase is larger than that inthe roof allowing the sill to expand. However, expansion eventually increases the total integrated forceon the roof allowing its uplift into either a laccolith, if the roof preserves continuity, or into a pistonbounded by a circular set of fractures. Hydraulic models for shallow magmatic chambers, also suggestthat laccolith-like intrusions require the existence of a self-supported chamber roof. In contrast, if theroof of magmatic chambers loses the self-supporting capacity, lopoliths and calderas should be expectedfor more or less dense magmas, respectively, owing to the growing influence of the density contrastbetween the host rock and the magma.