The characteristic size of two types of intrusions identified beneath Kīlauea's East Rift zone are uniquely estimated by combining time constraints from fractional crystallization and the rates of magma solidification during cooling. Some intrusions were rapidly emplaced as dikes, but stalled before reaching the surface, and cooled and crystallized to feed later fractionated eruptions. More specifically, using the observed time interval between initial emplacement and eruption of fractionated lava, whose degree of fractionation is estimated from petrologic mixing calculations, the extent of solidification or cooling needed to produce this amount of fractionation can be directly inferred. And from the known erupted volumes the spatial extent or size of this fractionated volume can be analytically related to the full size of the source body itself. Two examples yield dike widths of 82 and 68 m. Other intrusions remain close to the east rift magma transport path and are observed to last for decades or longer as viable magma bodies that may participate in feeding later eruptions. The thickness of semi-permanent reservoirs near the East Rift Zone magma transport path can be estimated by assuming a resupply rate that is sufficiently frequent to restrict cooling to
