In F-region ionospheric depletion experiments involving the explosive release of reactive molecular vapour such as H2O, CO2 and N2, a depletion core of >5 km diameter is formed within a few seconds of the release, where the plasma density is depleted by more than a decade. This paper examines the depletion core formation mechanism in the Lagopedo and Waterhole experiments. It is found that the ''snowplow'' model is inconsistent with observation. The core region is shown to be caused primarily by rapid three-body ion recombinations during the initial, self-continuum phae of the expansion. An analytic model is derived to estimate the extent of plasma depletion in the core region due to three-body H3O+ ion recombination reaction, and the estimate is compared with experiment.