Greater depth corresponds to past times when the Mauna Kea volcano was more nearly centered over the plume. The new core hole has penetrated already to a depth of about 3 km, where the lava represents the time when Mauna Kea was closest to the center of the plume. The chemical compositions of the lavas in the drill core can also be used to help identify which volcano they came from. The diagram below shows the abrupt change in barium concentration within lavas from the upper 2000 feet of the new HSDP drill core. The Ba concentrations are especially high in the uppermost several flows of the Mauna Kea section because they are alkalic basalts.
Alkalic basalt is erupted in the waning stages of shield building in all of the volcanoes.
The alkalic basalts form because near the end of its life, the volcano drifts off of the plume, and near the edge of the plume the amount of melting is lower than it is over the middle of the plume.