It is critical to note that as the plume penetrates into the propellant mass, the granules preferentially screen out heavier (still reacting) molecules and reacting solids. This happens for two main reasons: first, generally, such components are necessarily moving at a lower velocity, so those will naturally tend to lag behind the lightest molecules; second, such components simply cannot ricochet around the intervening particles as efficiently as lighter molecules (more massive components give up a greater percentage of momentum to intervening granules at each interaction – conservation of energy and momentum). Hence, at the instant the plume reaches plume penetration depth, the plume tip will contain essentially no material that will continue to react (and thereby generate more heat) while the plume base will contain a large amount of such material. Continuing reaction within the plume (which grades from nothing at tip to maximum at base) locally dramatically expands plume volume. Of coarse, since this is a dynamic event, tip penetration depth and base reactions are concurrent.
In addition to reaction-related expansion, the plume will continue to undergo decompressive expansion but this cools the plume and hence reduces available ignition energy. However, a second source of additional energy is the nascent combustion of ignited granules, which occurs first within those granules proximal the flash hole.