Millimeter-scale modal layering and the nature of the upper solidification zone in thick flood-basalt flows and other sheets of magma
Water flooding the surface of the Holyoke flood-basalt flow of Connecticut and Massachusetts was probably responsible for its entablature developing quench textures to depths of 80 m beneath the flow's surface. Despite the rapid cooling from above, the entablature-colonnade boundary is displaced above the center of the flow, which can only mean that crystal mush sank from the roof zone to the floor of the flow during solidification. The entablature of the Holyoke flow exhibits a subtle, quasi-horizontal, millimeter-scale modal layering, with sheets of ophitic pyroxene-plagioclase clusters alternating with discontinuous sheets of residual liquid. The layering is believed to have formed by repeated nucleation of pyroxene-plagioclase clusters in a rapidly advancing thermal boundary layer. The alternating layers of crystals and residual liquid produced a mush with planes of weakness along which the dense crystal mush could separate from the roof. The layering parallels the cuspate entablature-colonnade boundary, which must therefore be a primary magmatic feature rather than simply where downward and upward propagating fractures met. The modal layering has been found in other flood-basalt flows, but also in the roof zone of the Palisades sill. This texture may therefore be common in the roof zones of many sheet-like bodies of magma.