Mapping real time growth of experimental laccoliths: The effect of solidification on the mechanics of magmatic intrusion

The dynamics of solidification front growth along the margins of magmas have been widely found to be fundamental in controlling magma transport and emplacement. In this vein, the role of solidification fronts in determining the basic growth mechanics of laccoliths are investigated here in a series of scaled experiments using two contrasting magma analogs, water and molten wax that are injected into a visco-elastic gelatin based crustal analog. In non-solidifying, water-style experiments, intrusion is a relatively simple process. In contrast, wax magma emplacement displays a vast palette of compelling behaviors: propagation can slow, stop, and reactivate, and the directionality of lateral growth becomes much more variable. Small flow deviations in water-based intrusions are likely the product of flow instabilities, the result of injecting a viscous fluid along an interface (similar to Hele-Shaw cell experiments). However, the much more complex emplacement style of the wax experiments is attributed to solidification at the leading edge of the crack. The overall effects of solidification during emplacement can be described by a non-dimensional parameter measuring the relative competition between the rates of crack propagation and solidification at the crack leading edge. In this context, laccolith growth mechanics can be separated into three distinctive characteristic stages. Namely, I: A thin pancake style sill initially emanating radially from a central feeder zone, II: As solidification stalls magma propagation at the leading edges, enhanced thickening begins, forming a true, low aspect ratio laccolith, and III: As stresses accumulate, tears and disruptions readily occur in the solidified margin causing fresh breakouts, thus reactivating lateral growth into new lobes. The competitive combination of these latter stages often leads to a characteristic pulsatile growth. The unexpected richness of these results promises to add fundamentally to the basic understanding of laccolith growth mechanisms, and also adds key observations to growth processes to be sought in field studies that have been hitherto suspected but rarely observed. Despite seemingly simple shapes, the growth history of laccolithic intrusions is likely quite complex.