At Kohala volcano, Hawaii, the amount of source partial melting decreases rapidly toward the end of magmatic activity. The decreasing amounts of partial fusion lead to progressively higher concentrations of incompatible trace elements in lavas erupted during the waning stages of volcanism. In addition to the geochemical variations that characterize the transition from tholeiitic to alkalic basalt production, there is a pronounced decrease in the eruption frequency observed for the younger lavas. A model of magma generation by viscous dissipation can duplicate the relationships between geochemistry and age of the lavas. A reduction in the shear stress that produces deformation causes both a decrease in the amount of mantle partial melting and an increase in the eruption interval.