  As differentiated planetesimals cool, their cores can solidify from the
outside-in, as evidenced by paleomagnetic measurements and cooling rate
estimates of iron meteorites. The details of outside-in solidification and fate
of residual core melt are poorly understood. For a core primarily composed of
Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward
core growth would likely be achieved by growth of solid FeNi dendrites. Growth
of FeNi dendrites results in interconnected pockets of residual melt that
become progressively enriched in sulfur up to a eutectic composition of 31 wt
percent sulfur as FeNi continues to solidify. Here we show that regions of
residual sulfur-enriched FeNi melt in the core attain sufficient excess
pressures to propagate via dikes into the mantle. Thus, core material will
intrude into the overlying rocky mantle or possibly even erupt onto the
plantesimals surface. We refer to these processes collectively as
ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are
more likely on bodies with mantles less than 50 km thick. We show that
intrusive ferromagmatism can produce pallasites, an enigmatic class of
meteorites composed of olivine crystals entrained in a matrix of FeNi metal.
Ferrovolcanic eruptions may explain the observations that Psyche has a bulk
density inconsistent with iron metorites yet shows evidence of a metallic
surface composition.
