EXPERIMENTS ON THE EVOLUTION OF LACCOLITH MORPHOLOGY: THE MATURATION FROM ELLIPTICAL TO CIRCULAR SHAPED INTRUSIONS

Laccoliths are shallow crustal magma reservoirs that grow by mechanical deflection of the overburden. In plan view most laccoliths are near circular in shape, which is to say that their long axis is approximately that of their short axis. However, very elliptical laccoliths exist, yet no good explanation has been provided to account for these. The presence of elliptical laccoliths in Nature seems to contradict the fact that magmas grow in the path of least resistance. We present here the results from analog experiments that produce scaled down laccolith intrusions that utilize layered gelatin as host-rock analog and molten paraffin as magma analog. The transparent nature of gelatin allows for observations during the growth of the intrusion, and so the morphology of the laccolith can be tracked through time. We have found that the ellipticity of experimental laccoliths is not constant during growth. Experimental laccoliths tend to be the most elliptical early in their formation and mature into more circular intrusions as growth proceeds. Young elliptical laccoliths are tightly aligned with the long axis of the feeder dike—indicating that there is a connection between the feeder system and the morphology of the overlying laccolith. This connection fades with growth. Additionally, the size of the feeder dike itself controls rates of laccolith maturation. With small feeder dikes, laccoliths mature very quickly, and with increasing length of the feeder dike, the transition from elliptical to circular laccolith proceeds more slowly. These intimate observations of experimental laccolith growth histories illustrate that the formation processes of laccoliths in Nature is likely much more complex than previous models have suggested.