Postemplacement dynamics of basaltic intrusions in the continental crust

Laboratory experiments document the postemplacement behavior of mafic intrusions that spread at a density interface and founder as they become denser than their surroundings due to cooling and crystallization. All else being equal, the larger the intrusion volume, the farther the intrusion can spread and the smaller its aspect ratio is. The final aspect ratio is a function of a single dimensionless number analogous to the Rayleigh number of thermal convection. Once it is denser than its surroundings, the intrusion becomes unstable and may founder in two different regimes. At aspect ratios larger than about 0.4, the “teardrop” regime is such that the intrusion thickens in a central region, developing the shapes of a funnel and a pendant drop. At lower aspect ratios, another regime is observed, with thickening of the intrusion at the leading edge and thinning in a central region. The thick outer ring in turn becomes unstable into a set of teardrops and leads to an irregular horizontal outline. In one variant called the “jellyfish” regime, the thin central region develops a number of downwellings and upwellings in a Rayleigh‐Taylor‐like pattern. These instabilities may get arrested due to cooling as the intrusion and encasing rocks become too strong to deform. One would then be left with a funnel‐shaped residual body or a wide irregular one with thick peripheral lobes and a thinner central region. These different patterns can be recognized in upper crustal mafic intrusions.